Effects of the 2008 high-flow experiment on water quality in Lake Powell and Glen Canyon Dam releases, Utah-Arizona

Under the direction of the Secretary of the Interior, the U.S. Geological Survey‘s Grand Canyon Monitoring and Research Center (GCMRC) conducted a high-flow experiment (HFE) at Glen Canyon Dam (GCD) from March 4 through March 9, 2008. This experiment was conducted under enriched sediment conditions in the Colorado River within Grand Canyon and was designed to rebuild sandbars, aid endangered humpback chub (Gila cypha), and benefit various downstream resources, including rainbow trout (Oncorhynchus mykiss), the aquatic food base, riparian vegetation, and archaeological sites. During the experiment, GCD discharge increased to a maximum of 1,160 m³/s and remained at that rate for 2.5 days by near-capacity operation of the hydroelectric powerplant at 736 m³/s, augmented by discharge from the river outlet works (ROW) at 424 m³/s. The ROW releases water from Lake Powell approximately 30 m below the powerplant penstock elevation and bypasses the powerplant turbines. During the HFE, the surface elevation of Lake Powell was reduced by 0.8 m.

This report describes studies that were conducted before and after the experiment to determine the effects of the HFE on (1) the stratification in Lake Powell in the forebay immediately upstream of GCD and (2) the water quality of combined GCD releases and changes that occurred through the tailwater below the dam. The effects of the HFE to the water quality and stratigraphy in the water column of the GCD forebay and upstream locations in Lake Powell were minimal, compared to those during the beach/habitat-building flow experiment conducted in 1996, in which high releases of 1,273 m³/s were sustained for a 9-day period. However, during the 2008 HFE, there was evidence of increased advective transport of reservoir water at the penstock withdrawal depth and subsequent mixing of this withdrawal current with water above and below this depth. Reservoir hydrodynamics during the HFE period were largely being controlled by a winter inflow density current, which was moving through the deepest portion of the reservoir and approaching GCD near the end of the experiment. Compared to the beach/habitat-building flow experiment of 1996, the 2008 HFE had less affect on the reservoir because of the decreased volume of discharge from the dam and the different behavior of the winter inflow density current.

The operation of the ROW increased the dissolved oxygen (DO) concentration of GCD releases and resulted in DO supersaturation at higher release volumes. The jets of water discharged from the ROW caused these increases. Elevated DO concentrations persisted through the tailwater of the dam to Lees Ferry. At maximum ROW operation, downstream DO concentrations increased to approximately 120 percent of saturation.