Dams are pretty much any structure designed to stop the flow of water enough to result in a body of water forming upstream. They can be natural creations, as beavers are known to build them in order to have a habitat, shelter, and a food supply. Humans build them too, however.

Modern dams can actually be enormous structures. Some are used to form reservoirs for water supply, and others just create lakes used as recreational areas. Many dams are used to create a stable supply of water that power plants need for cooling, whereas other dams are actually hydroelectric power plants themselves.

Having access to a renewable resource and option for clean energy is favored by many worried about climate change. However, while dams don’t result in common forms of pollution, deadbeat dams can impact water in ways that hurt local ecosystems.

Some species of fish are very sensitive to temperatures in the water. This is especially true in rivers emptying out into the Pacific basin along the West Coast. Salmon and trout are just two cold-river species that can have their health drastically impacted by even the slightest variations in water temperature. Even if a dam doesn’t do anything to apply heat to water, it can still influence the temperature of the water, having ecological effects up to 31 miles downstream.

Three-quarters of the trout and salmon in California rivers are assumed to be at risk of extinction, and dams can be a potent source of trouble for these species. Not only are these fish crucial components of local biodiversity, they’re key to many other parts of the local ecosystems and also a considerable economic resource for the state.

A number of dam managers try to maintain natural water temperatures downstream by releasing cold water from reservoirs on a regular schedule, but if these releases don’t match the reproductive cycles and other timed-behaviors of the fish downstream, their lifestyles can be disrupted and even threatened.

Conservationists are naturally distraught about this. Using renewable resources can potentially save the entire world from CO2 emissions contributing to climate change, and hydroelectric dams can generate power nearly all the time. Wind turbines rely on atmospheric movement, while solar panels only work if the sun is up. Dams can run 24/7, but is saving the world enough if you destroy the local environment in the process?