Three Gorges Reservoir: Environmental Impacts

by David L. Wegner, M.Sc.

Background

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.

Fisheries

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.

Conclusion

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.

Continue to Chapter 5

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Chapter 3