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 concentrations are increasing in the morning until generation establishes a relatively constant temperature
near 26 OC and decreased dissolved oxygen concentrations near 5 mg L-1. Once generation stops
(around 9:00 to 10:00 p.m. after the peak in demand) temperature and dissolved oxygen concentrations
return to pre-generation patterns until the onset of the next generation cycle. Of interest in Figure
1.3.10, is the increase in dissolved oxygen concentration around 11:00 p.m. which may be a function of
reaeration associated with exposed riffle zones as the river stage drops coincident with the decrease in
discharge. Temperatures and dissolved oxygen concentrations may be quite different depending on the
conditions in the withdrawal zone in the upstream impoundment, especially during summer stratification.
Alternatively, winter releases may result in warmer temperatures with adequate dissolved oxygen
concentrations in reservoirs located in northern latitudes.
Chemical changes are often reflected as decreased concentrations near the dam (attributed to
dilution of hypolimnetic water with mid-depth or surface water) during generation with gradual increases
or decreases in total concentrations occurring in the tailwater as a result of resuspension or settling,
respectively, associated with discharge rates. These changes will be discussed in more detail later in
this section.
Regardless of project purposes, the discharge in a reservoir tailwater is a function of water
availability and quite often only a minimum flow is maintained. Recently, augmentation of minimum
flows has been viewed as an opportunity to improve tailwater habitats and water quality. Augmentation
of minimum flow is usually accomplished with changing reservoir operating guidelines for storage and
release. Quite often this is accomplished via a more frequent review of water allocation and results in
minimal impacts to project purposes. The net result of flow augmentation is a more stable aquatic
habitat, improved water quality via dilution of downstream inputs, increased navigation capabilities, and
increased supply for downstream users.
1.3.3 MORPHOLOGY
Rivers and tailwaters follow various courses as they progress seaward (from high altitude to
low altitude) as determined by the geologic setting. Consequently, they display a variety of shapes and
channel configurations that effect flow patterns and material transport as described above. The channel
morphometry downstream of a reservoir may have been modified, particularly for navigation and flow,
and is almost always different from conditions existing before the construction of the dam. Major
morphometric features important to water quality processes in tailwaters include such things as bank
erosion potential, riffles and pools, backwater areas, stream bed composition, and hydraulic features
that would affect currents and water movement. Bank erosion would increase sediment transport and
alter habitats. Riffles and pools, backwater areas, stream bed composition and hydraulic features
would affect habitats of aquatic organisms and may contribute to spatial gradients in water quality. Sear
(1995) suggests that changes in the sediment transport regime in the tailwater of a hydropower project
are controlled by the hydraulics of the channel morphometry (riffles, pools, etc.) and contribute to the
degradation of habitat for fish spawning and changes in riparian areas. Due to changes in energy
associated with velocity, riffles would tend to act as areas of transport and erosion of particulate matter
whereas pools would serve as areas of deposition unless velocities were sufficient (e.g., high discharge)
1.3-12
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