Main issues in research and practice of environmental protection for water conservancy and hydropower projects in China

Ang Chen*,Mio Wu,Ki-qi Chen,Zhi-yu Sun,Chen Shen,Peng-yun Wng

aInstitute of Hydropower and Environment Research,China Three Gorges Corporation,Beijing 100012,China

bSchool of Civil Engineering,Tsinghua University,Beijing 100084,China

cCollege of Hydrology and Water Resources,Hohai University,Nanjing 210098,China

dAppraisal Center for Environment and Engineering,Ministry of Environmental Protection,Beijing 100012,China

Main issues in research and practice of environmental protection for water conservancy and hydropower projects in China

Ang Chena,b,*,Miao Wuc,Kai-qi Chend,Zhi-yu Suna,Chen Shenb,Peng-yuan Wanga

aInstitute of Hydropower and Environment Research,China Three Gorges Corporation,Beijing 100012,China

bSchool of Civil Engineering,Tsinghua University,Beijing 100084,China

cCollege of Hydrology and Water Resources,Hohai University,Nanjing 210098,China

dAppraisal Center for Environment and Engineering,Ministry of Environmental Protection,Beijing 100012,China

In this paper,we generally summarize the main issues in the operational period of water conservancy and hydropower projects in China over the past several decades.First,the adverse impacts of these projects since the technicalguidelines were proposed in 2006 are analyzed. Then,combined with projects and experience from 2006 to 2014,the four main issues are summarized:(1)There existmany questions in the design and construction of fi shways,which are useful for fi sh migration,and the migration effects are not as expected.(2)Temperature stratifi cation affecting the downstream fi sh is the major impact of temperature,and alters fish spawning in the reproduction season.(3) Ecologicalbase flow has been one of the primary questions of the last30 years in China,the greatestrelated difficulty being quantifi cation of the amount and fl ow process necessary to satisfy fish life history.(4)Fish habitat protection and restoration are popular topics in recentyears with the development of river ecosystem restoration.Fish habitat loss due to the impacts of dam construction and habitat fragmentation has become more and more serious.These four issues are now the main difficulties in water projectmanagement,and interactwith one another to bear combined effects on river ecosystems.The issues of eco-hydraulic consideration in the design period are the key factors.Finally,future priorities for research and practice of environmental protection for water conservancy and hydropower projects in China are proposed.The main purpose of this paper is to enhance the scientifi c research,monitoring,and assessment of operating effectiveness.

Fishway;Temperature stratification;Ecological base flow;Fish habitat;Environmental protection

1.Introduction

China has exploitable hydropower resources of 542 million kW·h,and thus ranks highest in the world.Nevertheless,the installed capacity only accounts for 46%of the total exploitable amount(Li et al.,2015c).As is wellknown,the primary energy resource used in China is coal,and it is diffi cult to alter its primary status in the short term(Chang et al.,2010).The 12th Five-Year Plan states that,“On the condition that the ecological environment is protected and resettlements of the local people affected are properly handled,China will actively develop hydropower”(Hong et al.,2013).Hydropower is still the main driving factor supporting national economic development in the coming period.The distribution of developable hydropower potential across provinces is uneven(Li,2012),due to the fact that hydropower projects in China are mostly implemented in the southeastern mountain areas where the natural environment is more sensitive,and the public holds varying opinionsregarding hydropower development,especially regarding its impacts on the environment,such as the direct or indirect detrimental impacts on river ecosystems through alteration of the water fl ow pattern and restructuring of natural habitats (Chen et al.,2015).The development of hydropower must mitigate impacts on river ecosystems in order to alter current energy structures.The protection of fish species is attracting greater attention,as hydraulic power and the functions of rivers have been modifi ed through dam construction activities,and the impacts of the changes on the fi sh fauna must be assessed for biodiversity conservation and fi sheries management(Welcomme et al.,2006).Fish require different circumstances to complete breeding,feeding,wintering,and other activities,and the direct impacts of water conservancy and hydropower projects on fi sh are the obstructions and habitatfragmentation,with the naturalriver ecosystem being divided into discrete units(Zhao et al.,2008).Many spawning sites of migratory fi sh are lost,and their growth and reproduction are affected.In order to reduce the splintering of the river ecosystem,technological and engineering measures have been taken to ensure the effectiveness of the fi shways and habitat restoration.However,this cannot signifi cantly mitigate the effects of large dams(Orr et al., 2012).For example,fi sh have been seriously affected by large hydropower projects in the Yangtze River Basin,and the dams have cut off migration passages and have separated habitats.Some fish have been threatened and migrations have decreased due to the changes in connectivity between rivers and lakes(Yi et al.,2010b).In the Lancang-Mekong River Basin,the fi sh assemblages have changed greatly since the beginning of operation of the Xiaowan Dam(Li et al.,2013).Even for small-scale hydropower projects,the degradation of downstream ecosystem services caused by the periodic drying up of the river constitutes the largest portion of the ecological impact(Pang et al.,2015).

Some factors have indirect impacts on fi sh,including hydrologic regime alteration,infl uence of discharged cold water,minimum fl ow,and damage to the hydro turbine. Both the direct and indirect impacts on fi sh are among the main issues of environmental impact assessment of water conservancy and hydropower projects due to dam construction.In order to mitigate the impactof dam construction,fi sh engineering protection measures under environmental impact assessment must be integrated with the main project, and the protection measures must be taken after the operation begins.There are two main kinds of fi sh migration systems:the upstream and downstream fish migration systems.The upstream fi sh migration systems include fishways, fi sh ladders,fi sh barges,and so on.However,the downstream fi sh migration systems commonly pass through hydro turbines,which severely harm the downstream fi sh(Katopodis and Williams,2012).Compared with the upstream passage of fi sh,the downstream passage has mostly been ignored. This harms fi sh by causing them to pass over the spillway and through the turbines.Over the past decade,many studies have described the injury mechanisms associated with the turbine passage,the response of various fi sh species to these mechanisms,and the probability of survival through specific dams under certain conditions(Richmond et al.,2014). Additionally,even if the fi sh successfully move upstream and downstream,the separated downstream habitats may move away from their original locations as a result of erosion,and the cold deep water released due to thermal stratifi cation in the reservoir and the unnatural flow regime will also contribute to the habitat changes,especially the insufficient minimum discharge fl ow(Chang et al.,2011).

The Environmental Protection Law(EPL)for trial implementation in the People's Republic of China,promulgated in 1979,provided the foundation for the environmental impact assessment(EIA)(Jin,2015).After more than 20 years,the Law of the People's Republic of China on Environmental Impact Assessmentwas passed in 2002,and came into force in 2003.Over the past several decades,the EIA system has been expanded with consideration of dam construction activities,in China and other Asian countries(Erlewein,2013).As mentioned in the EIA Law,2002,post-project environmental appraisalfor dam projects has been proposed in practice(Chen et al.,2014a).The new Environmental Protection Act took effect on January 1,2015.The ecological protection demands for water conservancy and hydropower projects,especially the impacts of large dam construction on fish,were proposed in the EIA(Liu et al.,2013).In the EIA practice of water conservancy and hydropower projects,we found that there have been four major issues in water conservancy and hydropower projects over the past ten years,namely the design and construction of fi shways;thermal stratifi cation in deep reservoirs;the minimum released fl ow or otherwise designated ecologicalbase flow;and fish habitatrestoration.We collected as many related research results as possible,to summarize the issues in research and practice of environmentalprotection for water conservancy and hydropower projects in China,and we propose future directions in each field.

2.Fishways

2.1.Fishway status

In aquatic systems,in-stream structures,such as dams, weirs,and road crossings,can actas barriers to fish movement, particularly upstream movement,along waterways.A fi shway, also known as a fish passage,fish-pass,or fish ladder,is a type of structure built on or around artifi cial and natural barriers (such as dams,locks,and waterfalls)to facilitate the natural migration of diadromous fish(Chen etal.,2014b;Keller etal., 2012).There are various types of fi shways,including Denil fishways,pools and weir fi shways,vertical slot fishways,and natural fishways(Katopodis and Williams,2012).At present, the major types of fishways in China are verticalslot fi shways and natural fishways.

The firstfi shway in the world was built in 1662,in France, as described by Chen et al.(2012).The fi rst law to forbid the construction of weirs which would limit fish passing duringthe spring was passed in 1709,and then,although such laws or regulations were not promulgated,fi shways were sometimes built(Katopodis and Williams,2012).In the 19th century,many countries,such as Norway and Ireland,proposed laws,acts,or regulations to protect fi sh by requiring the construction of fishways at dams or otherwise impassable barriers.In the 20th century,the U.S.Federal Power Act of 1920(passed in 1920),which was amended in 1935,prescribed fi shways at all hydropower plants,and by the early 1960s over 200 fi shways were in place.Then,the Endangered Species Act of 1973 and other acts focused on the further effectiveness of fishways.In the 21st century,related laws and technologies were strengthened,requiring efforts to be made to improve the effectiveness of fi shways in Canada and the European continent,and natural fi shways became the main concern.Fishway designs have been evaluated to examine the effi ciency in physical models or numerical models(Foulds and Lucas,2013;Gustafsson et al.,2013).

The fi rst fishway facility builtin China was in the Qililong Reservior on the Fuchunjiang River in 1958,and,according to statistics,there were more than 40 fishways constructed from 1958 to 1980(Wang and Guo,2005).Because it is difficult to assess the effectiveness of the fishways,a lag phase occurred from 1980 to 2000 and no fishways or other facilities were built even in large hydropower projects like the Gezhouba Dam.With the increasing development and evaluation of fishways throughout the world,China began to restore fi shway construction in 2000,and researchers began to pay more attention to fishway design with both numerical simulations and physical experimental methods(Mao et al.,2012).This has required additional fishway engineering measures in new and reconstructed projects in the EIA process,in order to maintain biodiversity and mitigate impacts on the fi sh.According to assessment reports,since 2000,fishways or other facilities have been built in a total of 27 large water conservancy and hydropower projects.

2.2.Issues involved in fishway design

Recenteco-hydraulic studies on fishways have mainly been performed in the U.S.,Canada,and Europe.Such studies have connected fish,fishways,and the correlation between movementof fi sh and turbulence(Santos et al.,2012).Compared to global achievements related to fi shways,in China there are currently three main difficulties in fishway design:

(1)Modeling limitations in upstream passage simulation

Even though vertical slot fi shways are widely used,many difficulties remain in ensuring the effectiveness of fi shways. At present the major issue is the interplay between hydraulic and biological variables,and the hydrodynamic properties of the fi shway must meet the requirements of the fish species (Puertas et al.,2012;Bian and Sun,2013).One of the main factors that must be considered in the design is swimming performance,and there are still other factors such as the depth and fl ow velocities.Recently,fishway projects established in China have faced several diffi culties,the main ones being theoretical limitations on fish physiology habits, emulation of foreign designs and empirical formulas,and lack of experimental verification or authentication.Fishway hydraulics may present barriers to fi sh in three places(the entrance,interior,and outlet)as the entrances and outlets of fi shways are the places where the fl ow is distributed at the interface between the water fl ow in the fi shways and other structures,and the pool creates dissipation diffi culties related to the following hydraulic characteristics:

(a)Entrance fl ow characteristics

Flow and velocity distribution for fi sh at the entrances of fi shways is the key to the success of the fi shways.There are many factors that must be considered in the design stage, such as the biological habits of target species,long-term series of hydrological data,geographical location,terrain features,and other environmental factors.Quantitative tests have been performed to encourage European eels to pass through fishways,and the results have shown that a strong edge effect infl uences the route choices(Piper et al.,2012). Four major criteria were compared in the Han River in Korea to determine the best positions of fishways(Baek and Kim, 2014).Using lighting and other complementary measures to induce fi sh to enter fishways is the mostly widely used method in China.However,the fact is that it is difficult to fi nd the entrance for most fi sh and thus many fish gather below the dam,indicating the deficiency of the design.

(b)Internal fl ow characteristics

The fl ow characteristics of internalpools mainly depend on the specifi c geometry,such as the slope,internalfl ow velocity, water depth,and other relevant factors(Bermu′dez et al., 2010).Fishway internal flow characteristics are hydraulic conditions thatattractthe fi sh readily,and allow them to enter, pass through,and exit safely with minimum costs in time and energy.According to research performed outside China,there is a linear relationship between the discharge and average depth in a pool,and the turbulence is important for evaluating the biological efficiency of the design.This is because the fi sh become fatigued,and high turbulence levels confuse the fi sh when they are finding their way through the pools(Mao et al., 2012).Previous research has shown that there are minor differences in the turbulence distributions for different baffl e types.The differences in the turbulence distributions are due to the variations in flow created by each baffl e(Morrison etal., 2009;Yagci,2010;Rodríguez et al.,2006).The turbulence distributions,in essence,are the traditional hydraulic dissipation diffi culty,so we need to take a different energy dissipation approach within a predetermined length of fishways, and create suitable physical conditions.There have been an insufficient number of hydrodynamic studies performed regarding the pools of fi shways,but with the results and fi ndings,it is possible to determine the slope,the size of the pools depending on the targetspecies and temperature,and the discharges that are available or suitable for these structures.

(c)Outlet fl ow characteristics

The outlet flow parameters mainly seek to prevent the fi sh from falling back downstream because of the high flowvelocity,or to preventthe fish from getting lost in the pooldue to the hydrostatic force in the reservoir(Alvarez-Vˊazquez et al.,2011).The development of fishways in some locations in the Pacific Northwest goes back nearly half a century,and has been greatly successful in allowing the large,strongswimming,and highly motivated pre-spawning adult salmonids and non-native American shad to pass through(Schilt, 2007).However,important challenges remain in China against the backdrop of increasingly widespread hydropower development.The alteration of water level also affects the status of the fi shway outlets,especially because of the operation of cascade reservoirs(Sui et al.,2013).The fishway outlets in China are often clogged by sand,driftwood,or large stones,and the power generation benefi tis the firstobjective of the cascade reservoir operation,but the fl ow status in the fi shway outlets is rarely considered.

(2)Threats of downstream passage

There are as of yet no effective measures for solving the downstream issues of fi shways.As a rule,downstream migrants do not use fishways.Downstream options in other countries usually include turbines,juvenile bypass systems, spillways,sluiceways,and other routes on the surface,or transporting fi sh directly by barge or truck(Schilt,2007).To avoid fish injuries,the most commonly used method is to prevent fi sh from entering turbines using racks,while at the same time accessing spill gates,and allowing them to proceed downstream at small-to medium-sized hydroelectric plants (Calles et al.,2012).However,the main downstream route for fi sh in China directly passes through the hydro turbines or spillways,which in recent years has resulted in significant harm to fish.Most fish in China are multiple-spawning fi sh.In contrast,fi sh like salmon and trout die after spawning in other countries such as the U.S.and Canada.This signifies that we must ensure the safety of parental fish passing through fishways for species protection and the sustainable development of fisheries.

(3)Determination of multi-species hydraulic parameters

Velocity is relatively insensitive to the variations of discharge.Traditional fi shway designs mainly aim at one or two major kinds of fi sh,in accordance with a certain velocity, water depth,and other indicators of fi sh types.However,for some types of fi sh,due to the fish's different sizes,habits,and movement,the following two major problems exist in China:

(a)Velocity range determination

Calculation and determination of the velocities that allow different fish to pass through fishways is a technical diffi culty. Too high of a velocity results in direct reduction in fi sh that pass through fi shways,due to the increased energy demands, and too low of a velocity fails to encourage the fish to enter the pool(Puertas et al.,2012).The minimum velocity matching the swimming ability of one fish must take other fi sh into consideration.

(b)Multiple fi shway necessity

On the other hand,whether or not it is necessary to build more than one fishway on account of the differentfi sh species must also be discussed.Most of the projects carried out over the past ten years in China have involved construction of only one fi shway,and some projects have involved other fish passages like lifts,though the effects have not been ideal.There still remain large challenges such as the fact that fish ladders and lifts have been evolving for a long time and are effective for many species,but the fi sh species in China are very different from those in other countries,and the hydraulic characteristics for species in China require further study.

2.3.Issues involved in management system

The main issues involved in the management system include the following:

(1)Issues in adaptive management

Multidisciplinary studies of the management of fi shways include hydrology,hydraulics,and ecology,and these must be considered in the construction of fi shways.Related laws and regulations must also be considered in management.At the same time,the effectiveness assessment of fi shways is a broader study of whether there is evidence that fi sh populations are benefiting from the fi shways(Dan et al.,2014). Establishing an adaptive management mechanism for fishways can ensure their normal operation and maintenance. The adaptive management cycle of fi shways includes six main steps:(1)assessment of the problem and specifi cation of the objectives;(2)modeling of existing knowledge;(3) assessment of the potential management options;(4)implementation of the chosen options;(5)monitoring of the changes;and(6)review and adjustment of the actions (Humphries and Walker,2013).The adaptive managementof fishways lacks the monitoring and review period,which is important to improving the efficiency of fishways.Furthermore,the monitoring often requires a long period of time. The life of the fi shways should be continuously monitored in order to maintain the functionality of the fi shways,but in China this is also an unsubstantial step.

(2)Insuffi cient understanding of related concepts

At present,the main problem is the misunderstanding of fishways.Fishways in China were originally designed for precious migratory fi shes to help their feeding migration, reproductive migration,and wintering migration.At the same time,fishway construction in China realized the goal of aquatic protection,for the protection of both precious and endemic fi sh,or connectivity protection of benthic organisms and organic materials.The fishway construction was initially based on the techniques used in Europe and the U.S.The fishway effectiveness in China has not been ideal,because the fishways were not designed for the local fish species,and did not consider their habits and behavior characteristics.During the fi shway efficiency evaluation and review,not only should the target of the original fish be assessed,but the integrity of aquatic organisms and the river connectivity should also be comprehensively evaluated.

(3)Lack of fundamental surveys

Fishways should be monitored in real time to verify the efficiency of the fishways and identify the existing problems offish passage design.Atthe same time,itwillbe indispensable in the design and development of future fi shways to gather technical and biological information.A lack of survey and monitoring data within the regional ecological environment is the most common problem.To effectively evaluate the fi shway performance,studies must directly address clearly defi ned passage objectives(Roscoe and Hinch,2010).China has more than 3800 kinds of endemic fi sh species on record,but the behavioralstudies offi sh are mainly based on Chinese sturgeon and four major Chinese carp,while there have been fewer studies on the behavior and swimming ability of the vast majority of fish.Another difficulty is that the requirements of fishways and other facilities are generally presented in the EIA period,during which both the operation time and monitoring time are too short,so itis diffi cult to ensure a complete monitoring cycle in the design period and during work related to fish resources surveys,fi sh behavioral testing,and model testing. Earlier watershed planning in China had no requirements for fishway facility construction,and the monitoring and managementlacked supervision afterthe reconstruction offi shways.In addition,with the development of artifi cial propagation and other facilities,it is not easy to distinguish the effects of fishways from those of other facilities.

(4)Lack of sound legal norms

Many legislatures require the construction of fi shways (Farnham,1904).Developed countries have published laws, regulations,or guidelines to improve the effectiveness of fishways,such as Fishway Guidelines for Washington State.In Africa there have been guidelines such as Guidelines for the Planning,Design and Operation of Fishways in South Africa. Atpresent,fi shway construction in China is less prevalent,and there are rarely specific guidelines to follow.There are two major regulations for fi shway design and management:the Executive Summary of Aquatic Biological Resource Conservation in China,issued by the State Council in 2006,and the Technical Guide for Environmental Impact Assessment of River Ecological Flow,Cold Water,and Fish Passage Facilities for Water Conservation Construction Projects(Trial), issued by the State Environmental Protection Administration in 2006.This shortage of fishway guidelines has greatly limited the construction and development of fishways,and there is an urgent need for the composition and publication of guidelines for fi shway design for water conservancy and hydropower projects,fishway design regulations,etc.The mechanism of fishway compensation has not been established yet,and the costs of fishway operation,management,and maintenance vary with different companies.The hydropower loss in the operation of fi shways has no relationship with the duty of the companies,so there is no enthusiasm for the companies to strengthen the management.

3.Temperature stratification

3.1.Impacts of cold water release

The differences in temperature between reservoir water and a naturalriver channelwillimpactthe river ecosystem,and the aquatic ecosystem,water quality,and crops will be influenced by the cool water released from the reservoir(Yang et al., 2012;Fan et al.,2009).In northern China,the minimum fl ow of many reservoirs is too low to maintain the life cycle for downstream fish(Zhang et al.,2006).

The deep water of a stratifi ed reservoir is typically lowtemperature and anoxic.The cold water released to the downstream river ecosystem willhave an infl uence on fish and crops(Yang et al.,2012;Milstein and Feldlite,2015).After the construction of dams,the annual regulation,uncompleted annual regulation,and seasonal regulation,are formulated for them.The water temperature changes in the vertical direction during the operation of dams,and the trends are approximately as follows:Isothermal distribution occurs in winter,and the temperature of the release fl ow is higher than the temperature of the natural environment.The surface water temperature is higher and the bottom water temperature is lower than the water temperature at the same location in spring and summer, and the temperature of the release fl ow is lower than the natural water temperature.From the typical examples collected around the world,the temperature of the release flow is as much as 10°C lower than the naturalwater temperature in spring and summer.Numerical models are commonly used in hydro-environmental simulations for water quality,water temperature,and hydraulic characteristic recognition(Wang et al.,2014),but there are still problems related to thermal stratification because of the impacts on the fi sh.

High water temperatures in summer have led to increased juvenile fish stress in fish passage facilities(Politano et al., 2008).Fish are also threatened by the cold water at the deep bottom.There are severaladverse impacts of dissolved oxygen and chemicalcomposition changes due to the cold waterrelease on the downstream fi sh(Zhang et al.,2015a).As a result,the composition of the fish fauna has changed.Research has shown that temperature stratification is one of the dominant factors infl uencing the verticaldistribution and fish avoidance behavior (Sajdlovˊa et al.,2015).In general,the fi sh spawning period is from April to August,during which the lowest endured temperature of spawning fish is generally 18°C.Though the increased water temperature in winter has some advantages,the released warmwaterdirectly leads to the extension ofthe growth and breeding season of the fi sh,reduction of the growth time of the juveniles,and decrease of the growth rate.The impactofthe cold water release on spawning sites is another limitation,as some spawning sites disappear during the construction,their scale becomes smaller,and they migrate during the operation period.The scale ofthe spawning sites downstream,fartheraway from the dam,may expand,but,in general,the totalnumber of fi sh spawning sites is reduced after the dam construction.

3.2.Difficulties in multi-level intake practices

Difficulties in multi-level intake practices mainly include the following:

(1)Lack of sufficient water intake measures

Design,construction,operation,and management experience from around the world suggestthatthere are no technicallimitations to setting the water intake.However,construction has become more difficultdue to the topographic and geologic conditions,and surface water release increases the head loss in the reservoir operation period.Theoretically,each multi-level intake measure has certain effects on the recovery of water temperature,but,in fact,the effectiveness depends on the fi tting of the recovered water temperature and the growth of the critical temperature of aquatic organisms.

Furthermore,the lack of consideration of downstream aquatic organisms in conventionalreservoir designs has led to fi sh reduction and ecosystem integrity destruction in many areas.To mitigate the adverse effects of cold water release downstream and to protect the river ecosystem,multi-level intake measures must be considered in the design of deep water reservoirs.

(2)Lack of sufficient monitoring data of release flow temperature

Compared with the empirical discriminant method,numerical models have many advantages,such as higher accuracy and wider range of application.However,numerical models also have some limitations,i.e.,the models must go through calibration and validation to meet the accuracy demands for forecasting.Most numerical studies on temperature dynamics in reservoirs are based on one-or two-dimensional models.Over the past two decades,the models for reservoir water simulation have developed from one-dimensional(1D) to three-dimensional(3D)(Lee et al.,2013;Han et al.,2000; Diao et al.,2015),and today the most commonly used models are EFDC,MIKE 21,MIKE 3,and FLUENT.There are also some new models,such as a holistic water depth simulation (HWDS)and time to empty(TE)model for small reservoirs (Alia et al.,2015).Therefore,when using the numerical models,it is preferable to add field observations or laboratory measurements to ensure the model accuracy.However,it is very diffi cult to monitor the temperature of the released cold water,and the lack of monitoring temperature data,especially the lack of temperature data in deep-water reservoirs where multi-level intake buildings have been constructed,limits the development of numerical models.

(3)Limitations of empirical discriminant method

The empirical discriminant method is based on observed data from many reservoirs,and the discrimination results have been reasonable.Nonetheless,this method is not suitable for some narrow-deep reservoirs in mountains,due to the neglect of the morphology,climatic conditions,operation mode,and so on.Due to the limitations of the empirical discriminant method,for important projects the preferable choice is a more sophisticated numerical method.

(4)Limitations of validation of water intake measures

For large reservoirs with thermal stratification,the hydraulic characteristics and hierarchical structures are more complex.A study on a large dam showed that the temperature decreased rapidly in both the surface and bottom zones from June through October(Ma et al.,2015).Another example is the Fenhe Reservoir,where the simulated temperature of discharged water is consistent with the measured data,and the difference in temperature between the discharged water and the natural river channel under the current operating conditions is less than 3°C(Fan et al.,2009).However,although multi-level intake measures have been designed,the temperature of the surface water still cannot be determined,and the measures require further research and validation.

4.Ecological base flow

4.1.Origins and impacts of ecological base flow

The impact of ecological base flow on fish is currently a major issue.The ecologicalbase flow sustains fish through life stages and the river course.Many cutoff rivers have been created after construction and the beginning of operation of dams,in particular diversion hydropower stations.Insuffi cient ecological base flow can cause a reduction in habitat availability and fi sh reproduction,the mostserious impactfalling in the fi sh spawning period,which is the most important period in fi sh life history.Dams greatly affect river hydrology,primarily through changes in the timing,magnitude,and frequency of high and low flows(Magilligan and Nislow,2005). For both large and smallrivers in China,dam construction has modified hydrologic regimes on a nationwide scale.Since the Three Gorges Dam(TGD)wentinto regular operation in 2006, its impacts on the flow regime in the middle and lower Yangtze River reaches have received attention worldwide,and the overall impact of dams on the flow regime have included the decrease of high-flow magnitudes and,at the same time, the increase of low-fl ow magnitudes over time(Gao et al., 2013;Zhang et al.,2015b).To assess the effects of dam operation,a number of hydrologic indices were recognized to describe the different characteristics of fl ow regimes,and the most commonly used indicators are the indicators of hydrologic alteration(IHA)(Gao et al.,2009).

With the intensification of human activities in river ecosystems,ecological base fl ow studies have become active once again,aimed at the concepts,methods,practices,or guidelines,and legal systems.The effectiveness of fl ow releases mainly depends on how they are managed(Mackie et al.,2013).However,due to the diversifi cation of river ecosystem services,in China,there are still many diffi culties in ecological base fl ow research,or,more accurately, ecological fl ow demand research.For example,the definitions and implications have yet to be unified,and the implications have expanded greatly(Chen and Zhao,2011).At the same time,calculation methods have not been formed, and it is diffi cult to transplant the research results,due to the signifi cantly different characteristics of regional watersheds (Deitch and Mathias Kondolf,2012).The insufficiency of a legal system is the third factor that limits the development of ecological flow demand,along with ecological protection measurements,especially in the design and planning period of water conservancy and hydropower projects.

The greatest diffi culty in ecological fl ow demand research is setting the threshold value against the background ofsignifi cant differences in different regions,and different functions associated with the fl ow values for ensuring the ecosystem services are not clearly defined.Presently,the most urgent task is to set the criteria for ecologicalbase fl ow in China.This will provide technical support for decision making in studies.The study of ecological base fl ow is relatively mature in developed countries,while in China it is still insufficient.Specific criteria have been developed to ensure the minimum released fl ow from dams in most countries.China has also promulgated a series of guidelines or regulations,including the Technical Guide for Environmental Impact Assessment of River Ecological Water,Cold Water,and Fish Passage Facilities for Water Conservation Construction Projects(Trial),published in 2006,which states that the ecological base fl ow is 10%of the annual average fl ow.Since then,a number of related guidelines have been promulgated over the past several decades,which have enriched the system of laws and regulations.However,in the operation and implementation processes,the Technical Guide is stillthe main basis for determining the ecologicalbase fl ow in the planning,design,and operationalperiods of mostwater conservancy and hydropower projects.Although it is convenient and simple to execute operations described in the Technical Guide,lack of consideration of the temporal characteristics of ecological base fl ow has led to the appearance of many drying areas in river courses as well as excessive discharged fl ows,which are not conducive to the sustainable development of river ecosystems.

4.2.Difficulties in research and practice

Over the past decade,the main diffi culties related to ecological base flow may be summarized as follows:

(1)Lack of united concepts and calculation methods

There are many concepts or defi nitions related to the ecological base fl ow around the world,including environmental flow,instream flow,minimum flow,minimum acceptable fl ow,ecological and environmental water demand, sensitive ecological water demand,and minimum ecological flow(Carvajal-Escobar,2008).There are also many differences between different concepts,and the implications continue to expand,thereby satisfying mostecosystem service functions.Studies have combined hydro-ecological response model outputs and nonmarket economic values(Akter et al., 2014).It is becoming increasingly diffi cult to determine the threshold value of ecological base flow.The development progress of ecologicalbase flow outside China began from the minimum flow for maintaining shipping and fi shing,evolved then to suffi cientfl ow for maintaining the normal life cycle of aquatic organisms,then to suffi cient fl ow for preserving the river and landscape patterns,and fi nally to fl ow that maintains the sustainable development of the river ecosystem.In China, ecological base flow research began from research on the minimum fl ow in the northwestern region in the 1970s.Later, studies turned to the Yellow River and Yangtze River basins. After nearly 40 year of development,the sub-regional ecological flow results had been obtained.

At the same time,it is difficult to select a suitable method from various calculation methods for determining the ecological base fl ow.There are many methods for calculation, but they usually require accurate long-term hydrological flow records,which are sometimes unavailable(Alcˊazar et al., 2008).Under this condition,the hydrological models are important for runoff simulation(Olsen et al.,2013).In summary,there are currently more than 200 methods,which can be divided into the categories of hydrological methods,hydraulic methods,habitat methods,and synthesis methods (Tharme,2003).All of the methods are established on the basis of specifi c regions,and similarities of the natural environmental and community compositions in different regions play a very important role in the successful application of the methods(Li et al.,2009).Hydrological methods are most widely used,including the Tennant method and RVA method utilized in China and other countries(Gippel,2001;Babel et al.,2012;Gippel and Stewardson,1998).The habitat and synthesis methods are mostly used in the U.S.,the U.K.,and other developed countries,while most of these methods have also been introduced to China.The ecological limits of hydrologic alteration(ELOHA),which is a new framework for developing regional ecological base flow standards,was proposed in 2007(Poff et al.,2010).Aside from the previously introduced methods,researchers in China have developed several methods suitable for rivers in China,including the hydrological index method,ecological hydraulic method,and water quality numerical model method.

(2) Significant differences in temporal and spatial characteristics

The greatest diffi culties of ecological base fl ow research are the significant regional and functional differences of rivers,and the fact that the threshold values of the ecological base flow in different regions differ according to river ecosystem services.The lack of regional guidelines protecting the ecological values of rivers,especially when the fl ows mustbe determined at many locations in large basins,has led to the establishment of very simplistic criteria to fix the flow needs in rivers(Alcˊazar and Palau,2010).The threshold value is also affected by the types and functions of different reservoirs.The diversity of sub-region characteristics in China has led to differences in the ecological base fl ow in different sub-regions,including not only differences in the ecologicalbase flow between the northern rivers and southern rivers in China,but also differences in the ecological base fl ow between the upstream and downstream sections of the same river.

Most of the calculation methods used in China have taken the distribution process of runoff into consideration,and the threshold value of the ecological base flow has been a single value,or two threshold values in two different periods according to the Tennant method.Compared to rivers without protected aquatic organisms,the ecological base fl ow requirements that meet different conservation objectives in different periods vary greatly for rivers with protected aquatic organisms.Although there have been some studies that take the operation rules into consideration,itis stillvery difficulttouse one set of rules for different rivers with different aquatic organisms,periods,and fl ow alterations.

(3)Inadequate system of laws and regulations

The ecological base fl ow has been guaranteed by specifi c criteria in most developed countries,such as Water Law and Fisheries Law,published by France in 1992,which stipulates 10%of the measured runoff as the minimum instream fl ow, and thatthe runoff series should be based on atleast10 years of data(Bethune et al.,2005).The U.S.,Canada,Australia, and other countries have also published ecological fl ow standards.In addition,there are countries that have not yet recommended a standard(Benetti et al.,2004).International experience regarding setting the threshold value is valuable. China has promulgated a series of regulations and guidelines that lack legal protection,so there is a greater arbitrariness in the actual implementation process.Another problem is the possibility of ecological base fl ow.The water-supply guaranteed rate for residential areas is generally above 95%,and for industrial areas it is above 90%.Currently,the released ecologicalbase flow is based on the threshold value, in most cases when the inflow is greater than the perspective threshold value,but turns into the inflow under other conditions.Therefore,the current ecological base fl ow standard needs to be revised.

5.Fish habitat restoration

5.1.Fish habitat protection in China

River restoration is currently a major issue in many countries,and the assessment of fi sh habitats in a water body is a basic approach for strategically planning ecological conservation and EIA(Maeda,2013).Fish habitats include spawning grounds,feeding grounds,wintering grounds,and migration routes,with the spawning grounds being the most important and sensitive places for fish to complete their propagation process.The mechanical characteristics of fi sh habitats in the breeding season,such as the suitable water depth and flow velocity,are unique.The fi sh habitats also require suitable and stable sediment,along with appropriate water temperature and dissolved oxygen.During feeding,they require suitable sediment to satisfy the organisms and sufficient organic supply. They must also provide protection sites,shelter,diversity of fl ow types,and shoals.

There have been few studies on the hydraulic characteristics of fish habitats in China.One of the major studies aimed at protecting the four major Chinese carp and Chinese sturgeon, by describing different hydraulic parameters.There is no hydraulic index system for fi sh habitats,and the commonly used indicators are water depth,fl ow velocity,and Froude number (Fr).Most of the methods are based on fuzzy habitat suitability and the modeling of different river scales(O'Hanley et al.,2013;Marsili-Libelli et al.,2013;Yi et al.,2014). Studies on fish habitats are generally seen from one of two aspects:the distribution of various fish habitat locations in the river,and the distribution of flow from the surrounding water (Li et al.,2015a,2015b).The infl uence of hydraulic characteristics on habitat conditions is mainly divided into direct and indirect effects.The direct effects involve the hydraulic indices,which affect fish over their life period;for example there is a signifi cant correlation between the spawning time and water level.The indirect effects are wider, and the hydraulic conditions affect not only the oxygen content and water temperature,but also the terrain of the habitat. In the U.S.,the hydrology and geomorphology of large rivers reflect the pervasive influence of an extensive water control infrastructure,including more than 75000 dams(Graf,2006). In addition,the altered terrain also has effects on hydraulic indices.

5.2.Difficulties in fish habitat restoration

Diffi culties in fi sh habitat restoration mainly include difficulties in fi sh habitat research and management.

(1)Diffi culties in fish habitat research

Hydrology is a primary controlfor the ecologicalquality of river systems,through its infl uence on the fl ow,channel geomorphology,water quality,and habitat availability.Although there have been numerous studies on the hydraulic conditions of fi sh habitat,some problems remain.Research on fish habitat protection and restoration is an interdisciplinary endeavor involving a variety of hydraulic parameters and terrain conditions.Reservoir operation results in downstream river channel erosion,which can detrimentally alter the physical habitat.Restoring stream habitatconnectivity is also one of the objectives(O'Hanley etal.,2013).For example,the Gezhouba Dam and Three Gorges Dam are the two largest hydraulic projects in the middle reaches of the Yangtze River,and have changed the physical habitat and reduced the connectivity of the river(Yi et al.,2010a).Studies on water temperature have been limited to several endangered species or species with high economic value,while the majority of common species have not received attention.Yet another diffi culty is that there are different water temperature requirements for different life cycles,while there are few studies on the water temperature requirements of different kinds of fish.The traditional river geomorphological indicators are defined from the perspective of an abiotic environment and do not meet the fi sh habitat requirements.Specifi c research on landscape patterns has not been conducted.

Research on suitable fish habitat demands is still weak,and the methods of technical assessment of fish habitat quality remain under discussion.The lack of monitoring data restricts the development of habitat models.Effective methods requiring fewer data and less expertise can be suitable for the assessment of restoration potential,and monitoring of rehabilitation activities and excessive data requirements for assemblage information render many current assessment models expensive and limit their wide use(Zhao et al.,2015). Fish habitatrestoration also meets the ecological requirements and conforms to the natural river erosion trends.Therefore,it is restricted by basic techniques,which lead to obviously artificial landscape patterns,especially in downstream river courses and natural rivers.

(2)Difficulties in fish habitat management

Diffi culties in fish habitat management include the following:

(a)Lack of protection and management funds

Government agencies and stakeholders agree that river restoration is necessary for sustainable development. However,many projects involving river restoration have failed,and river ecosystems continue to deteriorate(Choi and Choi,2015).The infrastructures of the planned fi sh habitats in China are inadequate mainly due to the lack of management funds.This affects the capacity of basic construction and management of endemic and unique fi sh in the habitat area.At the same time,lack of government funds also leads to the interruption of routine patrolling and monitoring.

(b)Lack of concern for potential habitats

Fish habitats are gradually shrinking due to the delays in habitat protection and restoration,coupled with frequent economic activities in the habitats.In addition,there are no agencies for specific fish habitat protection,and very few stations for fish protection.There have only been special surveys or observations.

6.Conclusions

As hydropower development in China continues,the demands of environmental protection have become stricter than before due to the fragile river ecosystem in regions under exploitation.Therefore,river ecosystems and fishery resources must be protected,in order to allow research and facilitation of fishways,multi-level water intakes,ecological base flow,and fi sh habitat restoration.Because new practical problems have arisen over the past several decades,studies and practices on the monitoring of basic data and assessment of operating results must be enhanced.Therefore,we put forward the following four prospects integrated with the main problem:

(1)Fishway effi cacy is affected by many factors,requiring full observation and survey of fi sh species,as well as their physical characteristics,habits,and distribution.On the basis of analysis of hydrology and hydraulic characteristics,the physicalmodeland numericalmodelcan be used to determine the fishway type and size of the various fi shway elements. Through adaptive management and integrated operational observation,the fi shway effectiveness,fi shway type,and operational management can be improved.

(2)The traditional method of water release leads to the decrease of downstream water temperature,which in turn affects the downstream environment and aquatic organisms. The adverse impacts on the downstream river course can be reduced by taking multi-level water intake measures,and by controlling the temperature of the released water.The key point is to reinforce the monitoring of released flow water, which can provide basic data for the numerical models.In the planning and design of high dams,multiple objectives,such as project safety,economic benefits,operability,and reliability, must be taken into consideration to achieve the maximum socio-economic benefits and fulfill the needs of water temperature in downstream river courses.

(3)There are still no specifi c methods or standardized indices forecologicalbase fl ow in China,which results in some diffi culties in the EIA of water conservancy and hydropower projects.The currently used method,which stipulates 10% annualaverage fl ow as the ecologicalbase flow threshold value, mustbe refi ned.In addition,an index system ofecologicalbase fl ow and regionalization of the limitline mustbe developed in the future,and a reasonable assessment standard must be established by filtering the representative factors and taking the ecological base flow characteristics into account.Most importantly,the ecological base flow sub-region,grading standard, and regionalization method of the red-line constraint zoning can be proposed to draft a nationwide ecological base flow constraintzoning map,which can provide technicalsupportfor determining the ecological base fl ow threshold values in the EIA of water conservancy and hydropower projects.

(4)There is an urgent need to launch continuous monitoring of fish habitats in the near future,and to perform research on the correlation between biological behavior and hydrology,and hydraulics characteristics.It is imperative to further study the habitat assessment and numerical simulation technology,in order to carry out post-environmental impact assessment and fish habitat management.

Akter,S.,Grafton,R.Q.,Merritt,W.S.,2014.Integrated hydro-ecologicaland economic modeling of environmental flows:Macquarie Marshes, Australia.Agric.Water Manag.145,98-109.http://dx.doi.org/10.1016/ j.agwat.2013.12.005.

Alcˊazar,J.,Palau,A.,Vega-García,C.,2008.A neural net model for environmental flow estimation at the Ebro River Basin,Spain.J.Hydrol. 349(1-2),44-55.http://dx.doi.org/10.1016/j.jhydrol.2007.10.024.

Alcˊazar,J.,Palau,A.,2010.Establishing environmental flow regimes in a Mediterranean watershed based on a regional classifi cation.J.Hydrol. 388(1-2),41-51.http://dx.doi.org/10.1016/j.jhydrol.2010.04.026.

Alia,S.,Ghosh,N.C.,Mishra,P.K.,Singh,R.K.,2015.A holistic water depth simulation model for small ponds.J.Hydrol.529(3),1464-1477.http:// dx.doi.org/10.1016/j.jhydrol.2015.08.035.

Alvarez-Vˊazquez,L.J.,Martínez,A.,Vˊazquez-Mˊendez,M.E.,Vilar,M.A., 2011.The importance of design in river fishways.Proc.Environ.Sci.9, 6-10.http://dx.doi.org/10.1016/j.proenv.2011.11.002.

Babel,M.S.,Dinh,C.N.,Mullick,M.R.A.,Nanduri,U.V.,2012.Operation of a hydropower system considering environmental flow requirements:A case study in La Nga river basin,Vietnam.J.Hydro Environ.Res.6(1),63-73. http://dx.doi.org/10.1016/j.jher.2011.05.006.

Baek,K.O.,Kim,Y.D.,2014.A case study for optimalposition of fishway at low-head obstructions in tributaries of Han River in Korea.Ecol.Eng.64, 222-230.http://dx.doi.org/10.1016/j.ecoleng.2013.12.044.

Benetti,A.D.,Lanna,A.E.,Cobalchini,M.S.,2004.Current practices for establishing environmental flows in Brazil.River Res.Appl.20(4), 427-444.http://dx.doi.org/10.1002/rra.758.

Bermu′dez,M.,Puertas,J.,Cea,L.,Pena,L.,Balairˊon,L.,2010.Influence of pool geometry on the biologicalefficiency of vertical slotfi shways.Ecol. Eng.36(10),1355-1364.http://dx.doi.org/10.1016/j.ecoleng.2010.06.013.

Bethune,S.,Amakali,M.,Roberts,K.,2005.Review of Namibian legislation and policies pertinent to environmentalfl ows.Phys.Chem.Earth Parts A/ B/C 30(11-16),894-902.http://dx.doi.org/10.1016/j.pce.2005.08.036.

Bian,Y.H.,Sun,S.K.,2013.Study on hydraulic characteristic of fl ow in the vertical slot fishway.J.Hydraul.Eng.44(12),1462-1467(in Chinese).

Calles,O.,Karlsson,S.,Hebrand,M.,Comoglio,C.,2012.Evaluating technical improvements for downstream migrating diadromous fish at a hydroelectric plant.Ecol.Eng.48,30-37.http://dx.doi.org/10.1016/ j.ecoleng.2011.05.002.

Carvajal-Escobar,Y.,2008.Environmental flow regime in the framework of integrated water resources management strategy.Ecohydrol.Hydrobiol. 8(2-4),307-315.http://dx.doi.org/10.2478/v10104-009-0024-x.

Chang,F.,Tsai,W.,Wu,T.,Chen,H.,Herricks,E.E.,2011.Identifying natural flow regimes using fish communities.J.Hydrol.409(1-2),328-336. http://dx.doi.org/10.1016/j.jhydrol.2011.08.029.

Chang,X.L.,Liu,X.H.,Zhou,W.,2010.Hydropower in China at presentand its further development.Energy 35(11),4400-4406.http://dx.doi.org/ 10.1016/j.energy.2009.06.051.

Chen,A.,Sui,X.,Wang,D.S.,Liao,W.G.,Tao,J.,2014a.A brief review on post-project environmental appraisals of dam projects and recommendations for China.In:The 3rd International Conference on Energy and Environmental Protection.Trans Tech Publications,Xi'an, pp.3451-3456.

Chen,H.,Zhao,Y.W.,2011.Evaluating the environmental flows of China's Wolonghu wetland and land use changes using a hydrological model,a water balance model,and remote sensing.Ecol.Eng.222(2),253-260. http://dx.doi.org/10.1016/j.ecolmodel.2009.12.020.

Chen,K.Q.,Chang,Z.N.,Cao,X.H.,Ge,H.F.,2012.Status and prospection of fish pass construction in China.J.Hydraul.Eng.43(2),182-188.http:// dx.doi.org/10.13243/j.cnki.slxb.2012.02.012.

Chen,K.Q.,Tao,J.,Chang,Z.N.,Cao,X.,Ge,H.,2014b.Difficulties and prospects of fishways in China:An overview of the construction status and operation practice since 2000.Ecol.Eng.70,82-91.http://dx.doi.org/ 10.1016/j.ecoleng.2014.04.012.

Chen,S.Q.,Chen,B.,Fath,B.D.,2015.Assessing the cumulative environmental impact of hydropower construction on river systems based on energy network model.Renew.Sustain.Energy Rev.42,78-92.http:// dx.doi.org/10.1016/j.rser.2014.10.017.

Choi,B.,Choi,S.,2015.Physicalhabitatsimulations ofthe DalRiverin Korea using the GEP model.Ecol.Eng.83,456-465.http://dx.doi.org/10.1016/ j.ecoleng.2015.06.042.

Dan,B.,Justin,O.,Matthew,J.,2014.Evaluating the effectiveness of the Dights Falls fishway in the Yarra River,Melbourne Australia.In:Vietz,G., Rutherfurd,I.D.,Hughes,R.,eds.,Proceedings of the 7th Australian Stream Management Conference.Townsville,pp.210-218.

Deitch,M.J.,Mathias Kondolf,G.M.,2012.Consequences of variations in magnitude and duration of an instream environmental flow threshold across a longitudinal gradient.J.Hydrol.420-421,17-24.http:// dx.doi.org/10.1016/j.jhydrol.2011.11.003.

Diao,W.,Cheng,Y.G.,Zhang,C.Z.,Wu,J.Y.,2015.Three-dimensional prediction of reservoir water temperature by the lattice Boltzmann method: Validation.J.Hydrodyn.Ser.B 27(2),248-256.http://dx.doi.org/10.1016/ S1001-6058(15)60479-6.

Erlewein,A.,2013.Disappearing rivers:The limits of environmental assessmentfor hydropower in India.Environ.Impact Assess.Rev.43,135-143. http://dx.doi.org/10.1016/j.eiar.2013.07.002.

Fan,S.F.,Feng,M.Q.,Liu,Z.,2009.Simulation of water temperature distribution in Fenhe Reservoir.Water Sci.Eng.2(2),32-42.http://dx.doi.org/ 10.3882/j.issn.1674-2370.2009.02.004.

Farnham,H.P.,1904.The Law of Waters and Water Rights:International, National,State,Municipal,and Individual,Including Irrigation,Drainage, and Municipal Water Supply.The Lawyers Co-operative Publishing Company,Rochester.

Foulds,W.L.,Lucas,M.C.,2013.Extreme inefficiency of two conventional, technical fishways used by European river lamprey(Lampetra fluviatilis). Ecol.Eng.58,423-433.http://dx.doi.org/10.1016/j.ecoleng.2013.06.038.

Gao,B.,Yang,D.W.,Yang,H.B.,2013.Impact of the three Gorges Dam on flow regime in the middle and lower Yangtze River.Quat.Int.304,43-50. http://dx.doi.org/10.1016/j.quaint.2012.11.023.

Gao,Y.X.,Vogel,R.M.,Kroll,C.N.,Poff,N.L.,Olden,J.D.,2009.Development of representative indicators of hydrologic alteration.J.Hydrol. 374(1-2),136-147.http://dx.doi.org/10.1016/j.jhydrol.2009.06.009.

Gippel,C.,2001.Hydrological analyses for environmental fl ow assessment. In:Ghassemi,F.,David,P.,Murugesu,S.,Robert,V.,eds.,Proceedings of the MODSIM 2001:International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand Inc., Canberra,pp.873-880.

Gippel,C.J.,Stewardson,M.J.,1998.Use of wetted perimeter in defining minimum environmentalfl ows.River Res.Appl.14(1),53-67.http://dx.doi.org/ 10.1002/(SICI)1099-1646(199801/02)14:1＜53::AID-RRR476＞3.0.CO;2-Z.

Graf,W.L.,2006.Downstream hydrologic and geomorphic effects of large dams on American rivers.Geomorphology 79(3-4),336-360.http:// dx.doi.org/10.1016/j.geomorph.2006.06.022.

Gustafsson,S.,¨Osterling,M.,Skurdal,J.,Schneider,L.D.,Calles,O.,2013. Macroinvertebrate colonization of a nature-like fishway:The effects of adding habitat heterogeneity.Ecol.Eng.61,345-353.http://dx.doi.org/ 10.1016/j.ecoleng.2013.09.023.

Han,B.P.,Armengol,J.,Carlos Garcia,J.,Comerma,M.,Roura,M.,Dolz,J., Straskraba,M.,2000.The thermal structure of Sau Reservoir(NE:Spain): A simulation approach.Ecol.Model.125(2-3),109-122.http:// dx.doi.org/10.1016/S0304-3800(99)00176-3.

Hong,L.X.,Zhou,N.,Fridley,D.,Raczkowski,C.,2013.Assessmentof China's renewable energy contribution during the 12th Five Year Plan.Energy Policy 62,1533-1543.http://dx.doi.org/10.1016/j.enpol.2013.07.110.

Humphries,P.,Walker,K.,2013.Ecology of Australian Freshwater Fishes. CSIRO Publishing,Collingwood.

Jin,Z.N.,2015.Environmental Impact Assessment Law in China's courts:A study of 107 judicial decisions.Environ.Impact Assess.Rev.55,35-44. http://dx.doi.org/10.1016/j.eiar.2015.06.008.

Katopodis,C.,Williams,J.G.,2012.The developmentoffi sh passage research in a historical context.Ecol.Eng.48,8-18.http://dx.doi.org/10.1016/ j.ecoleng.2011.07.004.

Keller,R.J.,Peterken,C.J.,Berghuis,A.P.,2012.Design and assessment of weirs for fi sh passage under drowned conditions.Ecol.Eng.48,61-69. http://dx.doi.org/10.1016/j.ecoleng.2011.06.037.

Lee,H.,Chung,S.,Ryu,I.,Choi,J.,2013.Three-dimensional modeling of thermal stratification of a deep and dendritic reservoir using ELCOM model.J.Hydro Environ.Res.7(2),124-133.http://dx.doi.org/10.1016/ j.jher.2012.10.002.

Li,F.Q.,Cai,Q.H.,Fu,X.C.,Liu,J.L.,2009.Construction ofhabitatsuitability models(HSMs)for benthic macroinvertebrate and their applications to instream environmental flows:A case study in Xiangxi River of Three Gorges Reservior region,China.Prog.Nat.Sci.19(3),359-367.http:// dx.doi.org/10.1016/j.pnsc.2008.07.011.

Li,J.K.,2012.Research on prospectand problem forhydropowerdevelopment of China.Procedia Eng.28,677-682.http://dx.doi.org/10.1016/ j.proeng.2012.01.790.

Li,J.P.,Dong,S.K.,Peng,M.C.,Yang,Z.F.,Liu,S.L.,Li,X.Y.,Zhao,C., 2013.Effects ofdamming on the biologicalintegrity of fish assemblages in the middle Lancang-Mekong River Basin.Ecol.Indic.34,94-102.http:// dx.doi.org/10.1016/j.ecolind.2013.04.016.

Li,R.N.,Chen,Q.W.,Tonina,D.,Cai,D.S.,2015a.Effects of upstream reservoir regulation on the hydrological regime and fish habitats of the Lijiang River,China.Ecol.Eng.76,75-83.http://dx.doi.org/10.1016/ j.ecoleng.2014.04.021.

Li,W.M.,Chen,Q.W.,Cai,D.S.,Li,R.N.,2015b.Determination of an appropriate ecological hydrograph for a rare fish species using an improved fish habitat suitability model introducing landscape ecology index. Ecol. Model. 311, 31-38. http://dx.doi.org/10.1016/ j.ecolmodel.2015.05.009.

Li,Y.,Li,Y.B.,Ji,P.F.,Yang,J.,2015c.The status quo analysis and policy suggestions on promoting China's hydropower development.Renew.Sustain. Energy Rev. 51, 1071-1079. http://dx.doi.org/10.1016/ j.rser.2015.07.044.

Liu,J.,Zuo,J.,Sun,Z.Y.,Zillante,G.,Chen,X.M.,2013.Sustainability in hydropower development:A case study.Renew.Sustain.Energy Rev.19, 230-237.http://dx.doi.org/10.1016/j.rser.2012.11.036.

Ma,J.,Liu,D.F.,Wells,S.A.,Tang,H.W.,Ji,D.B.,Yang,Z.J.,2015.Modeling density currents in a typical tributary of the Three Gorges Reservoir, China. Ecol. Model. 296, 113-125. http://dx.doi.org/10.1016/ j.ecolmodel.2014.10.030.

Mackie,J.K.,Chester,E.T.,Matthews,T.G.,Robson,B.J.,2013.Macroinvertebrate response to environmental fl ows in headwater streams in western Victoria,Australia.Ecol.Eng.53,100-105.http://dx.doi.org/ 10.1016/j.ecoleng.2012.12.018.

Maeda,S.,2013.Asimulation-optimization method forecohydraulic design of fish habitat in a canal.Ecol.Eng.61,182-189.http://dx.doi.org/10.1016/ j.ecoleng.2013.09.026.

Magilligan,F.J.,Nislow,K.H.,2005.Changes in hydrologic regime by dams.Geomorphology 71(1-2),61-78.http://dx.doi.org/10.1016/ j.geomorph.2004.08.017.

Mao,X.,Fu,J.J.,Tuo,Y.C.,An,R.D.,Li,J.,2012.Influence of structure on hydraulic characteristics of T shape fishway.J.Hydrodyn.Ser.B 24(5), 684-691.http://dx.doi.org/10.1016/S1001-6058(11)60292-8.

Marsili-Libelli,S.,Giusti,E.,Nocita,A.,2013.A new instream flow assessment method based on fuzzy habitat suitability and large scale river modelling.Environ.Model.Softw.41,27-38.http://dx.doi.org/10.1016/ j.envsoft.2012.10.005.

Milstein,A.,Feldlite,M.,2015.Relationships between thermalstratification in a secondarily treated wastewater reservoir that stores water for irrigation and filter clogging in the irrigation system.Agric.Water Manag.153, 63-70.http://dx.doi.org/10.1016/j.agwat.2015.02.007.

Morrison,R.R.,Hotchkiss,R.H.,Stone,M.,Thurman,D.,Horner-Devine,A.R.,2009.Turbulence characteristics of flow in a spiral corrugated culvert fitted with baffles and implications for fish passage.Ecol. Eng.35(3),381-392.http://dx.doi.org/10.1016/j.ecoleng.2008.10.012.

O'Hanley,J.R.,Wright,J.,Diebel,M.,Fedora,M.A.,Soucy,C.L.,2013. Restoring stream habitatconnectivity:A proposed method for prioritizing the removal of resident fish passage barriers.J.Environ.Manag.125, 19-27.http://dx.doi.org/10.1016/j.jenvman.2013.02.055.

Olsen,M.,Troldborg,L.,Henriksen,H.J.,Conallin,J.,Refsgaard,J.C., Boegh,E.,2013.Evaluation of a typical hydrological modelin relation to environmental flows.J.Hydrol.507,52-62.http://dx.doi.org/10.1016/ j.jhydrol.2013.10.022.

Orr,S.,Pittock,J.,Chapagain,A.,Dumaresq,D.,2012.Dams on the Mekong River:Lostfish protein and the implications for land and water resources. Glob.Environ.Change 22(4),925-932.http://dx.doi.org/10.1016/ j.gloenvcha.2012.06.002.

Pang,M.Y.,Zhang,L.X.,Ulgiati,S.,Wang,C.B.,2015.Ecologicalimpacts of smallhydropowerin China:Insightsfrom an emergy analysisofa case plant. Energy Policy 76,112-122.http://dx.doi.org/10.1016/j.enpol.2014.10.009. Piper,A.T.,Wright,R.M.,Kemp,P.S.,2012.The influence of attraction flow on upstream passage of European eel(Anguilla anguilla)at intertidal barriers. Ecol. Eng. 44, 329-336. http://dx.doi.org/10.1016/ j.ecoleng.2012.04.019.

Poff,N.L.,Richter,B.D.,Arthington,A.H.,Bunn,S.E.,Naiman,R.J.,Kendy,E., Acreman,M.,Apse,C.,Bledsoe,B.P.,Freeman,M.C.,et al.,2010.The ecologicallimits of hydrologic alteration(ELOHA):A new framework for developing regional environmental flow standards.Freshw.Biol.55(1), 147-170.http://dx.doi.org/10.1111/j.1365-2427.2009.02204.x.

Politano,M.,Haque,M.M.,Weber,L.J.,2008.A numerical study of the temperature dynamics at McNary Dam.Ecol.Model.212(3-4),408-421. http://dx.doi.org/10.1016/j.ecolmodel.2007.10.040.

Puertas,J.,Cea,L.,Bermu′dez,M.,Pena,L.,Rodríguez,ˊA.,Rabu~nal,J.R., Balairˊon,L.,Lara,ˊA.,Aramburu,E.,2012.Computer application for the analysis and design of vertical slot fishways in accordance with the requirements of the target species.Ecol.Eng.48,51-60.http://dx.doi.org/ 10.1016/j.ecoleng.2011.05.009.

Richmond,M.C.,Serkowski,J.A.,Ebner,L.L.,Sick,M.,Brown,R.S., Carlson,T.J.,2014.Quantifying barotrauma risk to juvenile fish during hydro-turbine passage.Fish.Res.154,152-164.http://dx.doi.org/10.1016/ j.fishres.2014.01.007.

Rodríguez,T.T.,Agudo,J.P.,Mosquera,L.P.,Gonzˊalez,E.P.,2006. Evaluating vertical-slot fishway designs in terms of fish swimming capabilities. Ecol. Eng. 27(1), 37-48. http://dx.doi.org/10.1016/ j.ecoleng.2005.09.015.

Roscoe,D.W.,Hinch,S.G.,2010.Effectiveness monitoring of fish passage facilities:Historicaltrends,geographic patterns and future directions.Fish Fish.11(1),12-33.http://dx.doi.org/10.1111/j.1467-2979.2009.00333.x.

Sajdlovˊa,Z.,Draˇstík,V.,J?za,T.,ˇRíha,M.,Frouzovˊa,J.,ˇCech,M.,Vaˇsek,M., Muˇska,M.,Blabolil,P.,Tuˇser,M.,etal.,2015.Fish behaviour in response to a midwater trawl footrope in temperate reservoirs.Fish Res.172, 105-113.http://dx.doi.org/10.1016/j.fishres.2015.06.025.

Santos,J.M.,Silva,A.,Katopodis,C.,Pinheiro,P.,Pinheiro,A., Bochechas,J.,Ferreira,M.T.,2012.Ecohydraulics of pool-type fi shways:Getting past the barriers.Ecol.Eng.48,38-50.http://dx.doi.org/ 10.1016/j.ecoleng.2011.03.006.

Schilt,C.R.,2007.Developing fish passage and protection at hydropower dams.Appl.Animal Behav.Sci.104(3-4),295-325.http://dx.doi.org/ 10.1016/j.applanim.2006.09.004.

Sui,X.,Wu,S.N.,Liao,W.G.,Jia,L.,Jin,T.T.,Zhang,X.,2013.Optimized operation of cascade reservoirs on Wujiang River during 2009-2010 drought in southwest China.Water Sci.Eng.6(3),308-316.http:// dx.doi.org/10.3882/j.issn.1674-2370.2013.03.007.

Tharme,R.E.,2003.A global perspective on environmental flow assessment: Emerging trends in the developmentand application of environmentalfl ow methodologies for rivers.River Res.Appl.19(5-6),397-441.http:// dx.doi.org/10.1002/rra.736.

Wang,X.,Guo,J.,2005.Brief review on research and construction of fishways at home and abroad.J.China Inst.Water Resour.Hydropower Res.3(3),222-228(in Chinese).

Wang,Y.H.,Jiang,Y.Z.,Liao,W.H.,Gao,P.,Huang,X.M.,Wang,H., Song,X.S.,Lei,X.H.,2014.3-D hydro-environmental simulation of Miyun Reservoir,Beijing.J.Hydro.Environ.Res.8(4),383-395.http:// dx.doi.org/10.1016/j.jher.2013.09.002.

Welcomme,R.L.,Winemiller,K.O.,Cowx,I.G.,2006.Fish environmental guilds as a toolfor assessmentof ecologicalcondition of rivers.River Res. Appl.22(3),377-396.http://dx.doi.org/10.1002/rra.914.

Yagci,O.,2010.Hydraulic aspects of pool-weir fishways as ecologically friendly water structure.Ecol.Eng.36(1),36-46.http://dx.doi.org/ 10.1016/j.ecoleng.2009.09.007.

Yang,M.,Li,L.,Li,J.,2012.Prediction of water temperature in stratified reservoir and effects on downstream irrigation area:A case study of Xiahushan Reservoir.Phys.Chem.Earth Parts A/B/C 53-54,38-42. http://dx.doi.org/10.1016/j.pce.2011.08.019.

Yi,Y.J.,Wang,Z.Y.,Yang,Z.F.,2010a.Impact of the Gezhouba and Three Gorges Dams on habitatsuitability of carps in the Yangtze River.J.Hydrol. 387(3-4),283-291.http://dx.doi.org/10.1016/j.jhydrol.2010.04.018.

Yi,Y.J.,Yang,Z.F.,Zhang,S.H.,2010b.Ecological influence of dam construction and river-lake connectivity on migration fish habitat in the Yangtze River Basin,China.Proc.Environ.Sci.2,1942-1954.http:// dx.doi.org/10.1016/j.proenv.2010.10.207.

Yi,Y.J.,Tang,C.H.,Yang,Z.F.,Chen,X.,2014.Influence of Manwan reservoir on fish habitat in the middle reach of the Lancang River.Ecol. Eng.69,106-117.http://dx.doi.org/10.1016/j.ecoleng.2014.03.026.

Zhang,Y.,Wu,Z.,Liu,M.,He,J.,Shi,K.,Zhou,Y.,Wang,M.,Liu,X.,2015a. Dissolved oxygen stratification and response to thermalstructure and longterm climate change in a large and deep subtropical reservoir(Lake Qiandaohu,China).Water Res.75,249-258.http://dx.doi.org/10.1016/ j.watres.2015.02.052.

Zhang,Y.,Yang,Z.F.,Wang,X.Q.,2006.Methodology to determine regional water demand for instream flow and its application in the Yellow River Basin.J.Environ.Sci.18(5),1031-1039.http://dx.doi.org/10.1016/ S1001-0742(06)60034-X.

Zhang,Y.Y.,Zhai,X.Y.,Shao,Q.X.,Yan,Z.Q.,2015b.Assessing temporaland spatialalterations offlow regimes in the regulated Huai River Basin,China. J.Hydrol.529,384-397.http://dx.doi.org/10.1016/j.jhydrol.2015.08.001.

Zhao,C.S.,Yang,S.T.,Liu,C.M.,Dou,T.W.,Yang,Z.L.,Yang,Z.Y., Liu,X.L.,Xiang,H.,Nie,S.Y.,Zhang,J.L.,et al.,2015.Linking hydrologic,physical and chemical habitat environments for the potential assessment of fish community rehabilitation in a developing city.J.Hydrol.523,384-397.http://dx.doi.org/10.1016/j.jhydrol. 2015.01.067.

Zhao,J.Y.,Dong,Z.R.,Sun,D.Y.,2008.State of the art in the field of river habitat assessment.Sci.Technol.Rev.26(17),82-88(in Chinese).

Received 28 December 2015;accepted 22 August 2016

Available online 7 January 2017

This work was supported by a General Financial Grant from the China Postdoctoral Science Foundation(Grant No.2016M592404)and the projects funded by the China Three Gorges Corporation(Grants No.0799556 and 0799564).

*Corresponding author.

E-mail address:angteenchen@gmail.com(Ang Chen).

Peer review under responsibility of Hohai University.

?2016 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).