Effects of Impoundment on River Water Quality

Water-quality changes downstream of dams may be slight or considerable, depending on water residence time in the impoundment and whether surface or deep water is released downstream.  Dams that release water from the bottom usually cause the most adverse effects. Water released into tailwaters is typically very clear due to sediment settling in the reservoir. Nutrient concentrations may be elevated, particularly below deep-release dams. Hydrogen sulfide is occasionally released from reservoir sediments, producing a "rotten egg" smell. The most serious change in water quality is due to the release of oxygen-depleted water from the bottom of deep reservoirs. However, turbulence in the river normally re-oxygenates the water within a short distance, and artificial aeration is a relatively simple solution (Allan 1995).

The temperature regime of a river is altered to varying degrees by impoundments, particularly by large reservoirs with deep release dams. The large volume of water in a reservoir has considerable thermal inertia; therefore, daily temperature fluctuations are reduced or eliminated. Deep release reservoirs may also reduce seasonal temperature variation (Allan 1995).

The habitat changes described above have a host of consequences for riverine organisms below dams. Effects range from greater aquatic plant abundance in clear, nutrient-rich waters to reduced species richness of benthic invertebrate communities, to establishment of tailwater trout fisheries in the cold-release water. The formerly rich unionid mussel fauna of the Tennessee River system, now largely impounded, has been reduced to about half the original 100 or so species (Isom 1969).

Management of water quality in reservoirs has its own unique set of problems.