Dam Tailwaters - Hydro-Environmental Analysis: Freshwater Environments - page 493

Epilimnion (warm; oxygenated)

Metalimnion

Hypolimnion (cool; low oxygen)

FIGURE 18.3 Illustration of hydropower release. (From Peterson, M.J., Cada, G.F., Sale, M.J., and
Eddlemon, G.K., Regulatory approaches for addressing dissolved oxygen concerns at hydropower facilities,
U.S. Department of Energy, Idaho Falls, ID, 20 03.)

high in nutrients (e.g., ammonia and dissolved phosphorus), and high in reduced materials (reduced
iron and manganese, sulides, etc.; Figure 18.3).
There are a variety of other issues related to tailwater systems, including:

• Release magnitude and designated uses (minimum lows/maximum lows, pulsed lows)

• Erosion, scour, and bank sloughing

• Gas bubble disease

• Entrainment and impingement (e.g., from pumpback operations)

• Release temperature and water quality

• Habitat loss for existing or desirable species

• Changes from cold-water to warmwater species (or inversely, changes from warmwater to

cold-water species)

• Blockage of ish passage, or loss of spawning or necessary habitats

Many of the same characteristics of tailwaters that may be judged from some perspectives as
undesirable often make them popular and productive from other perspectives. For example, the dam
may provide a barrier to migrating ish, and may also provide conditions unfavorable to the predam
river biota. On the other hand, dam tailwater often provides very productive managed isheries.
For example, the high nutrient and organic concentrations of reservoir releases often result in high
productivity in the reservoir tailwaters. Also, due to the presence and operation of dams, tailwaters
often have less seasonably variable conditions than natural streams, so there is often more food
available for longer periods of the year than there is in natural systems. As a result, tailwaters are
often some of the most popular recreational isheries available. Since tailwaters are not considered
natural systems, often the isheries may include introduced rather than, or in addition to, native
species. Examples include popular hybrid striped bass tailwater isheries. In some areas, bottom
releases of cold hypolimnetic waters change the tailwater areas from a warmwater system (such as
prior to the construction of the dam) to a cold-water environment (such as with bottom releases),
allowing for highly popular and productive trout isheries below many dams in the south, where
such a ishery would not be possible without the dam. The presence of these isheries has resulted in
a highly successful and productive recreational industry below many dams.

18.2 DAM RELEASES AND IMPACTS

18.2.1 r eLeaSe M aGnItude and d eSIGnated u SeS

One of the major inluences on the physical, chemical, and biological characteristics of tailwaters
is the magnitude and the timing of releases from a dam. These releases may be highly variable, as
illustrated by Figure 18.2.