The quality of a river's water is primarily controlled by the climatic and geological characteristics of the drainage basin. Factors of climate such as precipitation (seasonal regime and storminess), and temperature (seasonal regime and diurnal variation) effect the rock, vegetation and soil character of the river. These factors are also directly connected with the river's runoff system, which is a compilation of flood hydrograph character, quality composition and fluctuations of the river. The runoff characteristics thereby effect the river discharge quality. Dammed water has different physical, chemical, and biological properties compared to the river's free-flowing water. This is because runoff now effects the reservoir morphology, which then effects the river's outflow, the outflow being factors of discharge volume and water quality. Factors of reservoir morphology are thermal stratification (which encompasses latitudinal variations as well as morphometric variations), chemical stratification (dissolved gasses, biogeochemistry and temporal patterns), and the density of the currents. The change in reservoir morphology of dammed water usually causes adverse effects on river species .
The Bonneville Dam: A Case Study
Water temperature is important because many physical, chemical, and biological processes are significantly influenced by the temperature of river-flows. In the case of impounded waters, the still water allows heat storage to take place, making the temperature of the dam water often seasonally higher than the normal temperature of the river basin. The increased temperature can cause many problems for river species. In the case of the Bonneville Dam, an increase in temperature has adversely effected salmon survival . State and Federal regulations require that the Bonneville Dam maintain temperatures of 68 F (20 C). During the summer of 1998, the temperature of the dam reached 74 F (23.33 C). Salmon need cooler waters for survival. Higher temperature rates often cause high mortality rates for juvenile salmon.
Industrial and agricultural pollution has adverse effects on a river's ecology. These effects can mostly be seen in the reduction of viable food supplies. The most toxic contaminants of the Bonneville Dam appear to be the pesticides DDE and DDT . PCB has also been found in the still waters of the Bonneville Dam. These toxins have likely lead to the deaths of many river species, as well as to the contamination of river food supplies.
Chemical changes within reservoirs are associated with their flow-dynamics and biological activity. Dissolved oxygen is not replenished in the hypolimnion layer, because sunlight can not reach this level. Oxygen is consumed in this layer and does not become replenished until autumn, when there is less aerobic activity. During low oxygen periods, carbon dioxide is released at high levels, and pH decreases. Conductivity, alkalinity, and orthophosphate levels all increase. When high bottom temperatures increase, anoxic conditions develop and deoxygentation occur at faster rates, and last for longer periods. In the upper layer (epiliminal) dissolved oxygen levels are maintained at near saturation. The rate of photosynthesis is limited to the supply of nutrients. In the Bonneville Dam there has been an increasing trend in the TDG (Total Dissolved Gases) between the years of 1995 and 1997. This trend leads to the conclusion that there are decreasing amounts of nutrients in the Bonneville Dam during peak summer months.