Abstract
At the upstream end of the Grand Canyon, the Glen Canyon Dam has changed the Colorado River from a run-of-the-river flow to a deep, summer-stratified reservoir. This change in flow regime significantly alters the temperature regime of the Colorado River. Seasonal temperature variation, once ranging from near 0°C to almost 30°C, is now limited to 7 − 14°C. The lack of warm summer temperatures has prevented spawning of endangered humpback chub in the Colorado River. Implementation of a temperature control device, to allow for warmer summer releases to mitigate negative temperature effects on endangered fish, was considered by the federal government. Ultimately, this proposal was put on indefinite hold by the Bureau of Reclamation and U.S. Fish and Wildlife Service due to concerns of cost and unintended ecological consequences. The low-variability of the current dam-induced Colorado River temperature regime will continue into the foreseeable future. Agencies are reviewing humpback chub conservation efforts outside of temperature control.