The water in the impoundment is heated by the sun, often raising the temperature beyond the threshold where cold-water fish species, like trout, can survive. And water that settles into the deeper portions of an impoundment can have so little dissolved oxygen that it won't support fish or other life. Dams also impose societal costs. While some dams are built for flood control, the presence of dams can also increase the likelihood or severity of floods by constricting the flow of the river or causing ice jams. Finally, the financial and legal liability associated with dams is a significant issue.
Dams are expensive to build and maintain.
The Agency of Natural Resources owns and maintains nearly 90 dams, at a significant cost to Vermont's taxpayers. Dams that are abandoned by their owners after they are no longer useful can fall into disrepair, resulting in hazardous, and perhaps dangerous, conditions downstream in the event of dam failure. Most existing dams in Vermont were built well before passage of the Federal Clean Water Act and institution of other environmental laws and regulations. When the Department of Environmental Conservation reviews conditions at these older dams or receives an application for a new dam, the Water Quality Division carefully weighs the benefits against the social and environmental costs. While the benefits of flow regulation hydroelectric power, flood control, water for numerous purposes are obvious, the impacts often are not.