such as minimal spills can be used to improve conditions during critical periods (Daniil et al. 1991).
Dissolved nitrogen in reservoir tailwaters is problematic when concentrations exceed saturation
(i.e., supersaturation) and result in gas bubble disease in fish. Gas supersaturation occurs when air is
entrained a water jet that plunges into a significant depth of water. The entrained air is transported as
bubbles to the bottom of the stilling basin and the bubbles become dissolved in the water under
hydraulic pressure. The magnitude of gas supersaturation depends upon the type of hydrostatic
structures, magnitude of discharge, and depth of water in the stilling basin. Effects of nitrogen
saturation on fish have been documented in studies conducted in the Columbia and Snake River Basins
(Ebel et al. 1975; Dawley 1986). The severity of symptoms exhibited by fish exposed to high
concentrations of dissolved nitrogen depends on the saturation level (usually above 110 percent),
exposure time, water temperature, and the overall health of the
fish. External symptoms include exopthalmia or pop-eye, bubbles in the caudal fin or hemorrhage, and
emboli in gill blood vessels. Numerous studies conducted by the CE describing sampling techniques,
equipment deployment, data collection and analyses, and predictive capabilities and be found in U.S.
Army Corps of Engineers (1996).
Carbon dioxide is produced by respiration within tailwaters and concentrations may vary,
especially between riffles and pools (Neel 1951). Typically, in pools, dissolved oxygen decreases
while carbon dioxide concentrations increase as water moves through but equilibrium is restored as a
function of turbulence associated with riffles. Increased carbon dioxide concentrations raises the
carbonic acid content and lowers the pH (see discussion of the carbonate system in the overview of
reservoir limnology). Carbon dioxide is either lost to the atmosphere or removed by the interactions
with calcium carbonate. The equilibrium of carbon dioxide and calcium carbonate (especially in
limestone-rich areas) provides a good buffer for the system and helps stabilize pH. In highly productive
areas (areas with high photosynthetic rates), removal of carbon dioxide can result in deposition of
calcium carbonate, especially in hardwater (i.e. high calcium concentrations) systems.
1.3.6 CHEMICAL PROCESSES
Chemical processes in reservoir tailwaters are a function of the quantity and quality of chemical
constituents delivered from the upstream impoundment and reflect water quality conditions in the
reservoir. Chemical processes of major concern usually involve nutrients (nitrogen and phosphorus),
(PCBs), mercury, pesticides are usually considered on a site specific basis. Nutrient concentrations are
often elevated during summer stratification and with large runoff events. Monitoring is usually
conducted to evaluate eutrophication processes such as algal, periphyton, and macrophyte production
in the tailwater. Nutrient loading to downstream receiving water bodies is also a concern.
Concentrations of certain species such as ammonia or nitrogen are also used in describing water quality
for water supply. Processes involving metals are usually focused on oxidation of reduced iron and
manganese. These metals contribute to taste, odor, and staining problems for downstream water