Evaluation of Decreased Flows in the Grand Canyon to Benefit Native Fish Species

The current flow regime of the Colorado River through the Grand Canyon, as altered by dam construction and operations, has led to declines in native fishes such as humpback chub (Gila cypha) and razorback sucker (Xyrauchen texanus) in the river. The native fish are heavily impacted by changes in flow regime, lack of seasonal warm temperatures, and introduction of invasive species. In an effort to model some possible improvements for fish, especially in their larval stages, we worked with CRSS and a model designed to predict water temperature by river mile to test the impacts of redistributed flow regimes on the native fish. Three scenarios were run in the model to determine the hydraulic and thermal effect of shifting flows and maintaining Pearce Ferry Rapid. First the model was run with no changes and acts as the base scenario for comparison. The second scenario was aimed to shift springtime releases from Glen Canyon Dam (GCD) to earlier in the year. Finally, the third was aimed at maintaining the pool elevation of Lake Mead below 1,135 feet and includes the shifted delivery schedule. Scenarios 2 and 3 were found to produce the lowest mean discharge for the month of May. Additionally, scenario 2 proved most reliable at reducing the mean discharge in May below 10,000 ft3/s. In the end, though minimal variation in river temperatures were achieved, all three scenarios provided acceptable thermal regimes in areas of the Grand Canyon.

Please note this model incorporates results from a temperature model developed by Dibble, K. L., C. B. Yackulic, J. C. Schmidt, T. A. Kennedy, and K. R. Bestgen. This model is still in review. For more information regarding how the temperature model was developed, please contact Kimberly Dibble with the USGS at [e-mail].