1. Dams reduce river levels
By diverting water for power, dams remove water needed for healthy in-stream ecosystems. Stretches below dams are often completely de-watered.
2. Dams block rivers
Dams prevent the flow of plants and nutrients, impede the migration of fish and other wildlife, and block recreational use. Fish passage structures can enable a percentage of fish to pass around a dam, but multiple dams along a river make safe travel unlikely.
3. Dams slow rivers
Many fish species, such as salmon, depend on steady flows to flush them downriver early in their life and guide them upstream years later to spawn. Stagnant reservoir pools disorient migrating fish and significantly increase the duration of their migration.
4. Dams alter water temperatures
By slowing water flow, most dams increase water temperatures. Other dams decrease temperatures by releasing cooled water from the reservoir bottom. Fish and other species are sensitive to these temperature irregularities, which often destroy native populations.
5. Dams alter timing of flows
By withholding and then releasing water to generate power for peak demand periods, dams cause downstream stretches to alternate between no water and powerful surges that erode soil and vegetation, and flood or strand wildlife. These irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species.
6. Dams fluctuate reservoir levels
Peaking power operations can cause dramatic changes in reservoir water levels — often up to 40 feet — which degrade shorelines and disturb fisheries, waterfowl, and bottom-dwelling organisms.
7. Dams decrease oxygen levels in reservoir waters
When oxygen-deprived water is released from behind the dam, it kills fish downstream.
8. Dams hold back silt, debris, and nutrients
By slowing flows, dams allow silt to collect on river bottoms and bury fish spawning habitat. Silt trapped above dams accumulates heavy metals and other pollutants. Gravel, logs and other debris are also trapped by dams, eliminating their use downstream as food and habitat.
9. Dam turbines hurt fish
Following currents downstream, fish can be injured or killed by turbines. When fish are trucked or barged around the dams, they experience increased stress and disease and decreased homing instincts.
10. Dams increase predator risk
Warm, murky reservoirs often favor predators of naturally occurring species. In addition, passage through fish ladders or turbines injure or stun fish, making them easy prey for flying predators like gulls and herons.
“When I visit a dam, I often find a plaque honoring by name the engineer, government leader, contracting firm and the height, size, date, volume of water held or diverted, power generated, flood capacity measurements. And that’s fine.
But I don’t find a plaque with the names of any species hurt, the names of any people displaced, the cost to taxpayers, the price of maintenance or decommissioning, or why this option was chosen over, say, windmills, solar panels, natural gas, groundwater pumping, demand management or some decentralized tools.”
— From a World Commission on Dams member