Water manipulation . One major problem caused by dams is alteration of stream flow. In extreme cases, enough water can be withdrawn to dry up streams, rivers and riparian habitat. Even when substantial water flow remains, it is often regulated so that periods of high and low flow are eliminated. Loss of historic flooding patterns can cause simplification of river channels, including loss of backwater channels required as habitat by native fish and other aquatic species.
Dams also interfere with the ability of fish to travel along streams.
For example, spawning and rearing areas in the upper Columbia River Basin were inaccessible to salmon after completion of the Grand Coulee Dam in 1941 and the Chief Joseph Dam in 1955. Fire Suppression . Over the past 80 years, people have prevented or put out fires in these forests to protect timber resources, human life and property. Fires have also been inadvertently suppressed by the creation of roads and other clearings that can prevent them from spreading. Forest Ecosystem Decline Cumulatively, the effects of the preceding activities have changed forests across the nation.