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, and human activities that produce climate change.
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 has negative impacts on the river downstream and on what lives there. For some fish species water temperature acts as a stimulus to start spawning. Releases of warmed 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 the 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).
Case Studies
The Blyde River System
The Blyde is a robust system even though it is disturbed by the present release strategies. One of the reasons is that a good vegetation zone is combating many possible problems associated with the releases. However, the system could be on the brink of a disaster as a lot of ‘red flags’ have been raised. Some of the fish species are not breeding downstream of the Blydepoort Dam because of temperature differences. Vegetation roots on the marginal edge are exposed due to pulsed operation.
Dams can alter the naturally occurring temperature regimes.
Source: ARA-SUL 2008
( click to enlarge )
Mzingwane River System
The water temperature varies with depth. Water temperature affects fish movement, e.g., some fish prefer to move between noon and mid afternoon when the temperatures are high, meanwhile, others move in the late afternoon and evening. Egg and larvae survival is temperature specific. Water temperatures are usually warmer at the surface than at the bottom. For temperature-specific species, adults may be able to withstand the changes, but be affected in their reproductive state. The environment at Site A, B and C in Mzingwane River does not provide any choice of habitat based on temperature. The shallow pools at times get too warm for fish survival.
