FLAGSTAFF, Ariz. – High-volume water releases from Glen Canyon Dam can increase sandbar area and volume, but may also result in large increases in non-native rainbow trout downstream of the dam, according to a new U.S. Geological Survey report released today.
The report, Effects of Three-High Flow Experiments on the Colorado River Ecosystem Downstream from Glen Canyon Dam, Arizona, documents the effects of high-flow experiments (HFEs) on resources in Glen Canyon National Recreation Area and the Grand Canyon National Park. The U.S. Department of the Interior conducted HFEs at Glen Canyon Dam in March 1996, November 2004 and March 2008.
“This important scientific research has paved the way for better management of Glen Canyon Dam to enhance protection of downstream resources,” said Anne Castle, Assistant Secretary for Water and Science in the Department of the Interior. “The report pulls together the results from three different high-flow releases so that such events in the future can be targeted for optimal resource benefits and minimizing adverse effects.”
Grand Canyon sandbars provide habitat for wildlife, serve as camping beaches for recreationists, and supply sand that may preserve vegetation and help protect archaeological sites. High flows also create areas of low-velocity flow, or backwaters, used by young native fishes, including endangered humpback chub.
“Research and long-term monitoring of the effects of three high-flow experiments have allowed scientists to unravel some of the many uncertainties about how these Glen Canyon Dam releases affect downstream river resources,” said Dr. Ted Melis, deputy chief of the USGS Grand Canyon Monitoring and Research Center and the report's editor. “We believe that the scientific findings presented in the report will allow managers to better plan future flow operations to meet desired resource goals.”
HFEs, also known as artificial or controlled floods, are effective at increasing both sandbar area and volume when they are conducted soon after “new” sand has been supplied to the system by flooding from tributaries downstream of the dam. In the absence of new sand supplies, as was the case in 1996, HFEs may still build sandbars, but only by eroding the lower portions of existing sandbars. In other words, sandbars became higher and not wider in 1996.
According to the report, the best possibility for rebuilding and maintaining sandbars is to time HFEs to follow the seasonal flooding of tributaries downstream of the dam. During years of below-average upper Colorado River Basin precipitation, allowing multiple new sand inputs to accumulate before conducting an HFE would result in the greatest sandbar building, the report said. However, during years of wetter upper basin hydrology, HFEs might be more effective immediately following or even during tributary flooding.
Sandbars are built relatively quickly (hours to a few days) when new sand is available from tributaries, as occurred in 2004 and 2008, but they also tend to erode within days to several months under normal dam operations following an HFE. Despite this ongoing erosion, long-term monitoring indicates that about 75 percent of sandbars measured in Marble and Grand Canyons were larger in October 2008 than in February 1996, before the first HFE was conducted.
The 2008 HFE was followed by large increases in non-native rainbow trout in the Lees Ferry reach, the sport fishery immediately downstream of the dam. These fish moved downstream and into areas that support native fishes, including the area with the largest population of endangered humpback chub.
The eight-fold increase of rainbow trout that occurred in 2008 is of particular note, the report said, because rainbow trout are known predators of young humpback chub and may also compete with native fish for limited food resources. Overall, the studies found that HFEs have had no measurable positive effects on juvenile or adult humpback chub populations.
The report is a product of the Glen Canyon Dam Adaptive Management Program, a federally authorized initiative to ensure the mandate of the Grand Canyon Protection Act of 1992 is met through advances in information and resource management. The USGS Grand Canyon Monitoring and Research Center has responsibility for scientific monitoring and research efforts for the program. The Bureau of Reclamation provides financial support for the program that is derived from hydropower revenues from Glen Canyon Dam operations.
The Bureau of Reclamation recently released for public comment two draft Environmental Assessments (EAs) related to topics addressed by this report (1) Development and Implementation of a protocol for High-Flow Experimental Releases from Glen Canyon Dam, Ariz., 2011 through 2020 and (2) Nonnative Fish Control Downstream from Glen Canyon Dam.