A dam and reservoir can alter temperature characteristics of a river

Dams and reservoirs can affect the ecology of the river they are placed on.

Over the ages, the fish in the  McKenzie River in Oregon were adapted for the local conditions. For eons fish were adapted to live and reproduce in a river having certain environmental characteristics that would not change quickly. But, after the construction of Cougar Dam, one thing that did change for the fish was the water-temperature patterns below the dam at certain times of year. The McKenzie River supports the largest remaining wild population of Chinook salmon in the upper Willamette River basin, and the South Fork McKenzie River provides good spawning habitat. It was found that the altered temperature pattern downstream of Cougar Dam created problems with regard to the timing of migration, spawning, and egg hatching for the fish.