Consequences of an Altered Hydrologic Regime for Colorado River Fishes in Grand Canyon

Large hydroelectric dams directly affect fish in four primary ways: they change the timing and magnitude of flows, reduce turbidity, impede fish movement, and alter water temperature.  All of these effects are evident below Glen Canyon Dam on the Colorado River in Grand Canyon. However, these effects have recently changed as a result of ongoing drought conditions.  High, stable flows in 2011 designed to equalize water volumes in Lake Powell and Lake Mead created conditions that increased survival of Rainbow Trout with unintended negative consequences for the recreational trout fishery.  Since 2005, decreases in water level within Lake Powell has allowed warmer surface waters to become entrained downstream, resulting in increases in populations of rare native fishes in the Colorado River including Humpback Chub Gila cypha. These warmer water temperatures, while positive for populations of native fish in the short-term, could ultimately prove harmful if new warm-water nonnative species like smallmouth bass Micropterus dolomieu become established.  The Colorado River below Glen Canyon Dam provides a good example of how even small alterations to hydrology can have dramatic consequences for resident fish populations.