The dam prevents the movement of fish upstream, and frequently downstream as well. Sediment carried by the river is deposited in the still water impounded behind the dam, gradually filling in the impoundment. Because the energy of the moving water must be in balance with the amount of sediment it carries, this often results in erosion below large reservoirs as the river picks up additional material to compensate for that deposited in the reservoir. 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.