Dave Heramimtschuk-USGS/Freshwater Illustrated

Several species of aquatic insects are mysteriously missing from the Colorado River in the Grand Canyon. Scientists now know that’s because dam managers rapidly change the river’s level to meet electricity demand.

Over the past hundred years, people have introduced dozens of non-native fish species into the Colorado River and its tributaries. During that time, populations of native fish species have dropped, in some cases dramatically. It’s easy to guess at the causes of native species decline, like predation and competition for food. But it’s far more difficult to prove.