Three Gorges Probe

Chapter 4 – Three Gorges Reservoir: Environmental Impacts

by David L. Wegner, M.Sc.


A reservoir is an impounded body of water created when a river or stream is dammed and water is allowed to store. This impoundment of water has an immediate impact on the physical and biological systems within the reservoir which needs to be understood before the full range of environmental impacts can be properly evaluated.

CYJV recognizes that the impoundment of the Yangtze River to create a reservoir hundreds of kilometres long would cause environmental impacts affecting physical, ecological and social change within the river basin and tributaries. It states that water velocity in the reservoir would be reduced to 20 percent of the river’s natural velocity at certain times of the year, and that the reservoir would disrupt aquatic and terrestrial (land-based) ecosystems. However, CYJV fails to assess the range of impacts that could occur, and instead depicts a reservoir that would always operate within very narrow, average conditions.

From River to Reservoir

Physical Changes

Of particular concern is CYJV’s failure to identify and evaluate the existing range of physical conditions in the reservoir. Based on the experience with reservoir impoundments elsewhere, it is reasonable to expect that the Three Gorges reservoir would initiate the following physical changes in the river system:

  • The free flowing river would be transformed into a slower moving, or still water system.


  • Migration of fish either upstream or downstream would be blocked by the dam.


  • As water velocity is reduced, fine particles of sand and clay, known as sediment, would settle to the bottom of the reservoir.


  • Initially, as the water level in the reservoir rose, land would be inundated and nutrients (and pollutants) would be drawn out from the flooded soil and decomposing vegetation.


  • The flooded vegetation would provide new habitat for young fish.


  • Evaporation of water would increase.


  • Temperatures in the upstream end, and in the upper portions, of the reservoir would increase.


  • Due to the combined influence of temperature, sediment and other chemical constituents, the water in the reservoir would become stratified. That is, the water body would not be evenly mixed, and this would have a wide range of results. For example, fish might be unable to use some areas because of lack of oxygen or temperature limits.


  • Water clarity in the reservoir might increase as the water slowed down and stratification occurred.


  • As the water seeped into the dry land, and as the reservoir level fluctuated, the reservoir shoreline would begin to erode.


  • Stratification could cause some chemicals attached to bottom sediments to be released into the reservoir, leading to water quality problems and potential impacts on the aquatic food base.


Biological Changes

A biologically productive reservoir depends primarily on the presence of a thriving population of various aquatic organisms such as plankton, zooplankton, and macro invertebrates, upon which fish populations depend.

As the reservoir fills, the flooding of land and vegetation would release nutrients and vegetative debris into the water. The increase in nutrients combined with the increased light penetration in the reservoir would cause the plankton to multiply rapidly. As the plankton food base responded to the new conditions, zooplankton and macro invertebrates would respond in much the same way, resulting in an overall short-term increase in the productivity of the reservoir ecosystem. But once the nutrients from the flooded soils were depleted, the plankton population might either decrease or increase, depending on the inflow of nutrients to the reservoir.

Fish presently in the Yangtze River have developed in a riverine system. If the river were to be transformed into a reservoir, the native fish species would, in all likelihood, be unable to maintain themselves. They would attempt to move upstream to more favourable waters or to seek other refuges. The new habitat created by the impoundment would tend to attract new fish species from upstream areas that are adapted to a reservoir-type environment. As well, exotic or commercially advantageous fish species would be artificially planted in the reservoir. The new fish would tend to dominate the reservoir and take over the ecological niches formerly used by the native species, causing a boom in numbers and biomass (number and total weight of fish) as they quickly exploited the available resources.

Once the new fish had exploited the reservoir resources there would be a decline in fish population. In the long term, whether or not fish populations could be sustained in their new environment would be determined primarily by the rate of flow through the reservoir and the amount of fluctuation in flows per year.

This phenomenon is widespread and has been well documented, particularly for reservoirs in the U.S. and the Soviet Union. For example, native fish species in the Colorado River (U.S.) are now either threatened or endangered due to the extensive development of dams and reservoirs along the river and its tributaries.

River Basin Impacts

The Yangtze River, like all rivers, is an ecological system: what happens upstream has an impact on what happens downstream; some impacts are immediate while others are more gradual. Many of the environmental problems associated with major U.S. rivers, such as the Mississippi, the Missouri, the Colorado, and the Columbia, can be largely attributed to a lack of systematic understanding of river basin impacts.

The overall basin relationships are what would ultimately define the inflowing water volume, water quality and biological responses. CYJV presents little information to provide an understanding of the river’s potential changes as a system; for example:

  • Historical or background information on how other reservoirs in China, or around the world, have developed as a result of reservoir impoundment.


  • Specific data on nutrients, productivity, and water quality, from other reservoir developments, such as those on the Yellow River.


  • Seasonal and annual water flows and volumes to be expected throughout the system. It is the ranges not the averages which define the aquatic system dynamics.

Cumulative Impacts

A study of cumulative impacts would attempt to relate the impacts of all of the mainstem developments to the ecological relationships along the river. An assessment of cumulative impacts is necessary to identify additive or synergistic effects of planned and existing developments downstream and upstream of the TGP.

CYJV failed to conduct a cumulative impact assessment of potential impacts both downstream and upstream, and the effects of upstream water development on the planned reservoir. On a river and project of this magnitude, potential environmental impacts and concerns cannot be thoroughly assessed by studying only one component of the river system.

Environmental Relationships in the Three Gorges Reservoir

CYJV’s assumptions in its ecological and environmental assessments are based on either no data or data that is outdated, non-verifiable, and representative of only a narrow band of conditions. With regard to the Chinese assessment of upstream environmental hydrology, CYJV simply notes:

Reservoir tributaries and reservoir characteristics were not addressed as they are not significant with regard to engineering studies for the Three Gorges Project.1

In general terms, CYJV discusses the environmental responses to be “expected” as a result of the dam and reservoir but fails to define adequately a range of new conditions that could occur.

Reservoir Productivity

CYJV’s conclusion that the change in aquatic environment would provide an increase in overall productivity of the aquatic environment is unsupported by world experience with large-scale reservoirs, and questionable for the following reasons:

  • Outdated data base

    Although CYJV recognizes that populations of aquatic organisms such as plankton and invertebrates are important parameters for relating predicted water quality changes to overall reservoir productivity, the assessment of existing populations in the river was conducted roughly fifty years ago by the Wuhan Aquatic Institute. In reference to this, CYJV states:

    The Chinese believe that, since river flow characteristics have not changed, the results obtained in the early 1950s are still representative of the present day situation.2

  • Inadequate analysis of river flow variability

    CYJV fails to provide adequate information on a range of variables which form the basis for conducting a thorough and comprehensive analysis of the effects of flow variability* on reservoir productivity. In particular, hydrological variability typically would affect retention time for water in the reservoir, and hence, reservoir productivity. Rather, CYJV bases its discussion of the reservoir on “average” proposed river operations without considering the wide variability and range of hydrological conditions in the Yangtze River system. The use of “average” flows to describe the system is misleading in many respects because “average” conditions occur only in a textbook. In reality, a reservoir ecosystem is dynamic and responds more to the variability of the system than to “average” conditions. Also, CYJV fails to consider “regulated” flow conditions due to future developments upstream which would ultimately affect flows into the reservoir.


  • Inadequate analysis of reservoir operation

    CYJV provides limited information on how the dam would actually be operated for flood control, hydropower generation, and navigation. For example, it states that the reservoir would be maintained at an elevation of 140 metres for as long as possible during the flood season but fails to explain specifically how the large annual flows would be routed through the reservoir, and what the flow management priorities would be for the entire river basin.

    CYJV does state that the reservoir would only be capable of storing nine days of the river’s average annual flow and that there would only be a short retention period for water in the reservoir before discharging downstream. As a flow-through system, the reservoir could have large impacts on the day-to-day reservoir dynamics and productivity, but CYJV fails to evaluate the significance of this on overall productivity.

    More specifically, CYJV’s discussion of reservoir productivity lacks adequate information on: the seasonal and annual ranges of water quality upstream of the dam site; the contribution of nutrients and pollutants from the Yangtze’s upstream tributaries; and the seasonal and annual variability in regional temperatures, water supply, and nutrient loading. CYJV also fails to analyze how the sediment would move through the reservoir, which would have a significant impact on how productivity in the reservoir would develop.

Clearly, there is little understanding of how the reservoir would react to the range of new conditions. But based on experience with other reservoirs, and CYJV’s description of the reservoir as a flow-through system with a short retention time, productivity in the reservoir would tend to be limited – contrary to CYJV’s prediction.


Currently, there are 172 species of fish in the reservoir region, 25 of which are caught commercially. These fish, like the common carp which comprises up to 18 percent of the total commercial catch in the region, are adapted to the rapidly flowing river, and spawn from April to June as water levels in the Yangtze rise.

CYJV reports that the reservoir would flood existing aquaculture facilities, which currently produce twenty times more than the annual natural fisheries. The irrigation ponds and rice fields used for raising fish would all be flooded.

To estimate aquacultural losses, two of the 14 counties that would be affected by the reservoir impoundment were surveyed. CYJV reports that the two main fry production facilities in those two counties, Wanxian and Fengjie, would be flooded. On the basis of this survey, CYJV then estimated that the reservoir “could eliminate over 3,888 tonnes of fish production at an estimated value of 15.5 x 106 yuan per year [$521,750].”3 Incredibly, CYJV predicts elsewhere in its report that aquacultural harvests in the reservoir area could increase from 54 to 109 percent.

For the effects of the reservoir on the natural fisheries, CYJV states without substantiation that the natural-catch fishery would increase by 59 percent. This statement is misleading for the following reasons:

  • The expected heavy build-up of sediment in the reservoir would likely have a negative impact on the spawning ability of the fish such that the reservoir would have to be stocked – CYJV acknowledges this.

  • The long-term decline in reservoir fisheries is well documented for many river systems around the world. If the Three Gorges reservoir were stocked, as is proposed by CYJV, native species would decline in numbers as the exotic species dominate. This would be followed by a slow reduction in the genetic quality of the natural or native fish populations, resulting in an eventual loss of fish populations, and of diversity and fish health.

  • CYJV fails to adequately define biological productivity and natural fish populations, potential modifications to habitat, and modifications to water quality due to upstream developments. All this is necessary to predict which fish species would increase or decrease, and what level of stocking would be required.

Overall, it appears highly unlikely that the natural fisheries would expand, and far more likely, based on the information presented by CYJV, that they would suffer a serious decline.

Other Reservoir Basin Impacts Not Considered

  • CYJV fails to assess how reservoir shoreline erosion would affect biological conditions (for example, fish spawning and terrestrial habitat) and human use of the reservoir, and conversely, how the physical features of the river banks and shoreline would be affected by reservoir operations.

  • CYJV assumes that sedimentation at the mouths of tributaries upstream of the dam would restrict the ability of fish to migrate back to their spawning grounds. No definitive discussion is provided on the upstream sources of sediment, the impact of sedimentation on fish reproduction, or measures that should be taken to reduce the river’s sediment load.

  • CYJV fails to recommend a program to maintain slope stability around the reservoir in order to minimize beach erosion and stabilize the reservoir basin. Substantial slope failure could reduce the reservoir area and have an adverse impact on productive aquatic areas.

  • CYJV fails to evaluate potential water quality problems, pollution, and heavy metal accumulation in the reservoir as a result of existing and future upstream land use, hydrological changes and industrial activity.

Impacts on Terrestrial Communities

A number of rare wildlife species (e.g., the clouded leopard, macaque, and tufted deer) are still sighted occasionally in the reservoir area, but most land has been cleared and intensely cultivated for many years. Except for the hilly areas along the Yangtze River and a few remaining tracts of forested lands, the only significant tracts of natural habitat are found along the tributary valleys, such as the Daning River Valley. CYJV acknowledges that the reservoir would change the ecological and social conditions in the tributary valleys but concludes:

Information on these tributaries is insufficient to assess the significance of flooding and resettlements on wildlife habitats.4

The limited discussion of the remaining natural habitat and wildlife in the reservoir area neglects the following:

  • effects on the various species in the reservoir area;

  • the number of acres of riparian vegetation which would be lost to the reservoir;

  • potential erosion of the riparian corridor along the 2500-kilometre reservoir shoreline; and

  • links between terrestrial communities and local people.

Since certain critical areas would have to be protected during and after construction, CYJV should have provided a map showing sensitive, critical, and developed areas.


Lacking key information on environmental hydrology, cumulative impacts, biological, physical, and chemical responses, and human use patterns, it is impossible to truly assess the impacts of the Three Gorges reservoir. There is enough doubt in CYJV’s data to warrant a much more extensive assessment of the potential impacts of the Three Gorges development.



Sources and Further Commentary


*As an example of the high flow variability in reservoirs, the Glen Canyon Dam’s reservoir, Lake Powell, has experienced inflows from 8 billion cubic metres to over 26 billion cubic metres over the past five years.

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