Figure 8.16 Typical hydroelectric damThere are approximately 75,000 dams in the United States that block
600,000 miles (about 1 million km) of what had once been free-flowing rivers.
Salmon are migratory fish that hatch in streams and rivers and then swim
downstream to the ocean to live most of their lives. They return to the rivers and
streams from which they hatched to spawn (anadromous). Dams now block
almost every major river system in the West. Many of those dams have
destroyed important spawning and rearing habitats for salmon. As a result of
habitat destruction, at least 100 major U.S. West Coast salmon runs are extinct.
Dams also change the character of rivers, creating slow-moving, warm-water
pools that are ideal for predators of salmon. Low water velocities in large
reservoirs can also delay salmon migration and expose fish to higher water
temperatures and disease.
To reduce the impacts of dams on fish, fish passage facilities and fish ladders
have been built to help juvenile and adult fish migrate over or around many
dams. Spilling water at dams over the spillway can help pass juvenile fish
downstream because it avoids sending the fish through turbines. Water releases
from upstream storage reservoirs have been used to increase water velocities and
to reduce water temperatures to improve migration conditions through
reservoirs. Juvenile fish are also collected and transported downstream in barges