There is not a simple answer regarding dams and their effect on trout populations.It all depends on the type and operation of the dam itself.
Large dams that discharge water from the bottom of their impoundments (the water held back by the dam) often release clean, cold water that can be beneficial for trout populations downstream. Many large dams have "created" trout habitat in places where there previously was not trout habitat. Another important consideration for large dams is that they have adequate fish passage facilities because healthy fish populations need to move to different areas of the river for different aspects of their life cycles. Salmon and other anadromous species migrate from the ocean and dams significantly impact their ability to reach their spawning grounds, ultimately affecting the overall species survival rates.
The majority of dams in the United States are not these large bottom-release dams. Thousands of dams, most of them small dams, block fish passage and significantly impair water quality. As water sits behind these dams, it warms in the sun often to a point where downstream reaches can no longer sustainably support trout populations (trout need clean, cold water to survive). Also, decaying material and excessive vegetation growth behind the dams can cause the oxygen levels in the water to drop significantly, further degrading habitat quality. The sheer number of these types of dams in rivers and streams around the country have had a serious impact on trout habitat. The removal of small dams is becoming more common and is recognized as one of the most effective ways to restore trout habitat in streams that naturally support trout populations.
If you're interested in more information on small dam removal visit our Conservation Opportunities.
How do dams affect trout populations?