Dams Disrupt River Flows

The volume, timing and temperature of a river’s flow impact everything from habitat quality to reproductive cues for fish to bankside vegetative growth. Many hydropower dams seasonally divert as much as 95 percent of a river’s water, reducing flows to a mere trickle in some reaches. In addition, generating power can pulse flows downstream and wreak havoc on aquatic communities. Larger dams with reservoirs often retain winter and spring flows that would naturally flush and renew river ecosystems.

Dams Block Fish and Wildlife Migration

Dams block the upstream and downstream migration of fish and other aquatic organisms. Loss of habitat has been a critical factor in the decline of the state’s migratory fish species. Some dams may be retrofitted with fish ladders, although these structures are costly and not completely effective. Dams also block the natural movement of nutrients and sediment that create habitat, nourish banks and replenish beaches.

Dams Degrade Temperature and Water Quality

When dams divert water out of the natural streambed, water temperatures generally rise — often to levels that cannot sustain healthy fisheries or dilute natural or man-made pollutants. Dams can also degrade water quality by reducing the oxygen content of downstream flows and trapping sediment and pollutants in reservoirs.

Dams Impact Recreational Values

Low river flows generally reduce recreational opportunities, harming local tourism-based economies and personal enjoyment of a public resource. Some hydropower dams block public access to rivers altogether and eliminate opportunities to boat or fish.

California Rivers are at Risk

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The Truth About Hydropower