Change In Water Characteristics:
Reservoirs increase the water surface area and decrease the flow of the water causing average surface temperatures of the water to increase. Additionally, the reservoir increases water depth, thus creating a stratified effect, in which water is significantly colder and oxygen depleted once below the hypolimnion mark, ultimately effecting plant and aquatic life alike (Host).
The increased surface area of the reservoir increases the rate of evaporation, therefore causing the salt levels in the reservoir to become more concentrated (Host).
Sediment, Organic, and Chemical Concentrations:
The flooding of the watershed raises water levels to unnatural points along the surrounding forest, pulling nutrient rich topsoil into the reservoir. As a result of fewer nutrients available, vegetation growth is appropriately hindered. This erosion also draws in herbicides, pesticides, and other harmful chemicals; therefore, severely contaminating the water supply. Besides contributing to the levels of methane emitted by the reservoir, this huge buildup of sediments, organic matter, and chemicals can become so concentrated in a reservoir, since the matter is unable to flow downstream because the dam blocks it, the water essentially becomes toxic to the extent that aquatic species can no longer survive.
Water pollution levels are also increased by the use of motorboats, jet skis, and other leisure activities that emit oil and other contaminants into the newly created recreation area. The emissions of natural gasses further inhibit the survival of native species and contribute to rising water temperatures (Host).
A dam that creates a watershed to be uninhabitable, also greatly decreases biodiversity throughout the area, which impacts the local ecosystem.
The decrease in water temperatures near the bottom of the watershed, and the increased concentrations of salt, sediments, chemicals, and organic matter, all decrease the availability of oxygen throughout the reservoir. The water will not become stagnant because, unless the incoming river dries up, because there will always be water circulation throughout the reservoir. Though, oxygen levels may become so depleted that the reservoir proves incapable of supporting flora or aquatic life.