Human activities affecting water temperature can include the discharge of cooling water or heated industrial effluents, agriculture and forest harvesting (due to reduced shading), urban development that alters the characteristics and path of stormwater runoff, as well as other human activities that produce climate change.
Dams can alter the naturally occurring temperature regimes.
Source: Kellner 2010
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Another means of influencing instream temperatures is to change the depth of the water through damming or water abstraction. This leads to a thermal stratification, which refers to a change in the temperature at different depths. Depending on the stratification patterns in the reservoir, temperature, dissolved oxygen, turbidity and nutrient concentrations may change drastically when compared to the previous flowing river environment, and can have negative impacts on the river downstream and on aquatic life. For some fish species, water temperature acts as a stimulus to start spawning. Releasing warm water from the upper layer or cold water from the bottom into the river below can lead to the disruption of life cycles of fish species, elimination of species, or even extinction. It is therefore important to maintain the natural temperature regime in the area downstream of a reservoir. One measure to reach this is due to a selective withdrawal of water from the reservoir. This means a careful mixing of oxygen rich, warmer water released from the upper layer and oxygen poor, colder and nutrient rich water from the lower layer (NAMANG 1998).
