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 1.3 OVERVIEW OF LIMNOLOGICAL PROCESSES IN TAILWATERS
1.3.1 INTRODUCTION
Reservoir tailwaters may be considered a unique type of lotic (riverine) aquatic system since the
quantity and quality of the source is a function of an upstream control structure (e.g., dam). The
impoundment of the river results in water quality changes and regulation of flow, both of which impact
water quality processes in the downstream area or tailwater. Seasonal and operational influences are
observed in the tailwater and are often reflected as spatial gradients as well. Thus a tailwater may be
defined as that region downstream from a dam that reflects water quality and flow conditions directly
influenced by the impoundment and regulation of the river. Often, the "tailwater" is limited to a reach
defined by a particular study or features such as downstream dams, secondary tributaries, or point
sources that greatly influence the water quality or hydrology.
The regulation of flow in a reservoir tailwater is a function of the operation of the dam which
was built for purposes such as flood control, navigation, hydropower production, water supply, and
recreation. Operation for specific project purposes results in different release regimes or hydrographs.
For example, the high flow of a runoff event may be "attenuated" by the operation of the dam resulting
in a decrease in the peak discharge occurring for a longer duration, "absorbed", resulting in a constant
outflow, or manipulated as a staged discharge (Figure 1.3.1). Each of these hydrographs establishes
wetted areas, depths, currents, resuspension and transport processes, and mixing zones that define the
physical conditions for biological and chemical processes. The extent of impact is a result of discharge
level and duration. During the high flow discharge, wetted areas, depths, and currents are at a
maximum throughout the tailwater area. Maximum chemical concentrations occur on the leading edge
of the hydrograph (as observed for inflow hydrographs), and coincident with resuspension and
transport conditions (Ashby et al., 1995; Barillier, et al., 1993). Typically, water quality in the tailwater
reflects near steady-state conditions, spatial and/or temporal changes (dependent upon the constituent),
or conditions in the upstream reservoir (Nix et al., 1991). Channel morphometry, flow dynamics,
secondary inflows, and point/nonpoint sources are also major contributors to the water quality of the
tailwater. The length of the tailwater required for any constituent to return to values close to those
measured at the inflow to an impoundment, or to achieve a new equilibrium may be considered as the
"recovery distance" (Palmer and O'Keefe 1990). Typically, recovery distance is directly proportional
to river size.
General features of reservoir tailwaters are summarized in Figure 1.3.2. In this figure, the
velocity of the release from the dam is high and processes such as mixing, aeration, sediment transport,
and degassing occur downstream at a riffle zone. Velocity and currents are varied further downstream
in the vicinity of an island and additional mixing occurs at the confluence of a secondary tributary.
Sediment resuspension and transport also occur at areas with high
1.3-1
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