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 to provide resuspension of particulates. Water quality, which is related to particulate matter, would
also be affected by changes in sediment transport regimes.
1.3.4 THERMAL PATTERNS
Temperatures in tailwaters are a function of daily and seasonal cycles in solar heating and
release operations at the dam (Figure 1.3.11). In the northern hemisphere, maximum temperatures
occur in July through September, minimum temperatures occur in January and February, and are often
very similar over a period of years (Figure 1.3.11a). For hydropower operations, daily ranges in
temperature can be quite extreme (e.g. 27 to 12 OC, Figure 1.3.11b, upper panel). Variability is often
reflective of latitude, altitude, and project depth and operation as well as extremes in annual climate.
Release temperatures can be manipulated by changing intake levels in the outlet works or employing
alternative release strategies such as bottom releases or spills (release through flood control gates)
(Figure 1.3.12). Annual or seasonal temperature management strategies may be developed, primarily
for fisheries requirements, using selective withdrawal for maintaining downstream temperatures during
critical periods such as spring spawning and fall migration seasons. However, changes in operations to
develop desired conditions in the tailwater may result in an adverse impact, such as a decrease in
available habitat, in the upstream impoundment (Hudgins 1995).
1.3.5 DISSOLVED GASES
The primary dissolved gases of concern are dissolved oxygen, nitrogen, and carbon
dioxide. In well aerated releases and releases from aerobic areas of the reservoir, favorable
concentrations are maintained. Dissolved oxygen dynamics are most pronounced in releases from
hypolimnia with low dissolved oxygen concentrations and projects with varied operations (i.e.
hydropower as depicted in Figure 1.3.13). In this figure and in Figures 1.3.10 and 1.3.11b, minimum
dissolved oxygen concentrations occur coincident with hydropower generation. Seasonally, lower
values occur during maximum thermal stratification in the upstream impoundment (Figure 1.3.11a). As
these release waters move downstream, turbulence, reaeration, and primary productivity increase the
dissolved oxygen concentration (Figure 1.3.14). However, oxygen demand may be increased due to
metabolism of carbon (from increased hypolimnetic concentrations). Generally, a recovery to near
equilibrium concentrations occurs at some point downstream. Critical periods for dissolved oxygen
occur primarily during late summer stratification and the establishment of anoxic conditions in the
withdrawal zone when concentrations in the releases are often low (see Figure 1.3.11a). There is often
a diurnal cycle to oxygen production and consumption in productive systems (Figure 1.3.15) that may
be disrupted by reservoir operations (see Figures 1.3.10 and 1.3.13). For example, in Figure 1.3.10,
1.3-13
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