“Virtually all dams block the migration of fish,” said Grant, a research hydrologist with the U.S. department of agriculture’s forest service. There are several strategies that can be employed to help fish continue on their seasonal journeys, but all are very expensive.
Dams also block the flow of sediments that sometimes contain harmful chemicals.
“You may have 40, 50, 60 years of agricultural chemicals that may have come down and accumulated in the sediments,” Grant said.
