Cumulative Effects on Stream Temperatures

Warmer Temperatures

• Sunlight produces heat, which warms water. The vegetation from overhanging tree branches reduces the amount of sunlight reaching the stream water. It follows that shade from trees lowers stream temperatures. If those trees are clear-cut, stream temperatures should rise. A paper published by John M. Bartholow and reprinted by the U.S. Geological Survey verified the cumulative effects of clear-cutting trees along streams. Water temperatures rise, at least during the day. The opposite effect occurs at night.

Stream Width

• The report examined other cumulative effects from clear-cutting. One such effect was stream widening. Without trees and vegetation close along the banks, streams tend to widen. This spreading effect creates more water surface, which absorbs more sunlight, which produces a temperature increase.

Hydroelectric Dams

• An article in Lewis and Clark Law School's "Environmental Law Online" discusses the cumulative effects on stream temperatures of the operation of hydroelectric dams; specifically, the effects on salmon. Operation of the dams in the Columbia River Basin results in elevated temperatures owing to slower water flow and delayed seasonal cooling. These warmer waters lead to problems with migration and disease for salmon.

Climate Change

• In addition to increased water temperatures in streams caused by power plants, climate change is also warming streams. The gradual warming of the Earth over a period of time, when coupled with the long-term effects of the dams, leads the Lewis and Clark Law School article to suggest either removal of the dams or a substantial change in their operation.