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 predicted. Reaeration is the physical process by which turbulence at the air water interface provides
dissolved oxygen to the river through entrainment of air at the air/water interface. Since these
processes are related, they may be summarized using an equation (or a derivation) referred to as the
Streeter-Phelps equation (Streeter and Phelps 1925):
K1L O
&K1t
&K2t
&K2t
D'
(e
&e
)%DOe
K2&K1
where, D = the oxygen deficit, L0 is the ultimate BOD, K1 is the deoxygenation coefficient, K2 is the
reaeration coefficient, t is the time, and D0 is the oxygen deficit. The Streeter-Phelps equation has been
modified numerous times to include other sources and sinks of dissolved oxygen (i.e. sedimentation,
additional inputs, biological processes such as photosynthesis and respiration). Nitrogen also exerts a
demand on dissolved oxygen but at a much slower rate than carbon (about 20 days are required for
bacterial acclimation) and is a second order reaction in variations of the above equation. This type of
approach is useful in evaluating the recovery distance for dissolved oxygen concentrations at various
discharges.
The removal of reduced metals in tailwaters is also a function of physical, chemical, and
biological processes. While often modeled using chemical oxidation kinetics, Dortch et al. (1992)
recognize the significance of biological removal processes and association with streambed substrate.
Biological processes of metals removal in tailwaters have not been widely studied but inferences from
other freshwater and marine systems can be made. Good summaries of biological mechanisms for
metals removals are provided in Chapnick et al. (1982), Nealson et al. (1989), and Ghiorse (1984).
The effects of reservoir releases on invertebrates and fisheries and on tailwater ecology have
been summarized in literature reviews prepared by Walburg et al. (1980 and 1981) and discussed by
Petts (1984). Field studies conducted at seven reservoirs (Walburg et al. 1983) and fish recruitment
and movement in a flood control reservoir (Jacobs et al. 1985) suggest a variety of impacts on the
biotic community associated with reservoir operations. Detailed summaries of project impacts are
presented in Nestler et al. (1986) and comparisons to pre-project conditions are also included. In
general, invertebrates (particularly Ephemeroptera) responded to varied flow conditions with a
tendency to increase in number with distance from the dam. For surface release projects, the tailwater
benthos was dominated by filter-feeding organisms, such as net-building caddisflies. Deep-release or
bottom withdrawal projects, with anaerobic hypolimnia providing a clear, nutrient-rich release foster
periphyton development in the tailwater. Deep-release projects with aerobic hypolimnia provided a
source for phytoplankton, zooplankton, and fishes to the tailwater. The benthos in these tailwaters
were often dominated by grazers, such as some species of chironomids, oligochaetes, amphipods, and
isopods. The "surge" associated with hydropower production redistributes macrophytes, periphyton,
macroinvertebrates, and even fishes in a downstream direction. Fluctuation of flow (hence wetted
1.3-26
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