Those Dammed Lakes - Hydro-Environmental Analysis: Freshwater Environments - page 294

Endangered plants

We tlands protect

We tlands planning

Endangered species

Stream augmentation

Estuarine protection

Water quality

Fish and wildlife conservation

Emergency water supply

Irrigation

Water supply

Recreation

Flood control

Hydroelectric power

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1824 ... 1910 1920 1930 1940 1950 1960 1970 1980 1990

Ye ar

FIGURE 10.7 Chronology of acceptable reservoir purposes and programs as authorized by Congress. (From
USACE, Engineering and design—Hydrologic engineering requirements for reservoirs, U.S. Army Corps of
Engineers, Davis, CA, 1997.)

signiicant portion of the total water storage in the West. The report indicated that as a result of
climate change, water managers may be forced to make changes in reservoir operations and rule
curves.
A second problem is that these multiple purposes are often in conlict with each other. For exam-
ple, the manner in which a reservoir would be operated for lood control may be different from the
manner in which it would be operated for isheries management. For in-pool wildlife and isheries
management in luctuating warmwater and cool-water reservoirs, the water level is usually increased
during the spring to enhance spawning and the survival of young predators and it is then lowered
during the summer to permit regrowth of vegetation in the luctuation zone. In addition, the pool
levels are often increased during autumn for waterfowl management (USACE 1997). Conversely,
spring and fall drawdowns are common for lood control management, allowing increased storage
during wet seasons. Also, the manner in which a reservoir may be operated for isheries may differ
from the management of in-reservoir isheries to those in the reservoir tailwaters. As an example,
in many hydropower reservoirs in the southern United States, the bottom, cold releases allow the
supporting of cold-water (trout) isheries downstream. The manner in which the reservoir operates
clearly impacts the sustainability of that tailwater ishery. So, the management of multipurpose
reservoirs is often a problem in optimization.
In addition, needs or conditions change so that it may be necessary to update the reservoir opera-
tions to relect new or changing demands. Examples include a desired shift in operations in response
to increased water supply needs, or to a need to increase releases to improve water quality as part of
efluent trading, or to respond to the potential impacts of climate change. In addition, there may be a
desire to change the reservoir operation to relect some purpose that previously could not be used to
justify the reservoir. For example, it is only since the 1960s that ish and wildlife and environmental
issues have been an accepted reservoir design purpose (Figure 10.7). Therefore, federal reservoirs
constructed prior to that time and authorized by Congress were precluded from including ish and
wildlife and environmental considerations in their operations guide or rule curves.