Conversely, dams can alter water temperatures on a daily basis. Water stored in reservoirs behind dams is often oxygen starved, which can kill fish and other organisms downstream when released from the dam. Dams kill Fish - Flows from dams can reduce dissolved oxygen in rivers that fish and other aquatic organisms need. Unscreened diversions associated with dams can capture fish or fish can be trapped against fish screens by water pressure. Fish caught in hydro diversions can be killed by spinning turbines or become disoriented and subject to predation.
Dams provide excellent habitat for predators that feed on migrating fish slowed by dams.
Fish that spill over the lip of a dam can be injured or killed, depending on the dam height. Low river flows generally reduce recreational opportunities, with adverse impacts on local tourism-based economies. Although reservoirs behind large dams can provide flat water recreation, seasonal reservoir fluctuations can degrade or eliminate this experience. Working to protect rivers for more than 25 years, it has played a key environmental role in the regulatory process governing hydroelectric operations in the state. In 1998, Friends of the River was one of the founding members of the California Hydropower Reform Coalition, which was created to enhance river restoration opportunities during the relicensing process for hydropower facilities around the state.