Repeat photographs of a sandbar along the left shore of the Colorado River near river mile 30 shortly before (top) and shortly after (bottom) the November 2004 high-flow experiment. Scientists have learned that substantial increases in sandbar volume and area are only possible when high flows are conducted following large tributary inputs of sediment. (Photographs courtesy of Joseph E. Hazel, Jr., Northern Arizona University.)
Today, the Colorado River usually runs clear below Glen Canyon Dam because the dam all but eliminates the mainstem supply of sand to Grand Canyon. Dam-induced changes in both sand supply and flow have disrupted the sedimentary processes that create and maintain Grand Canyon sandbars and associated habitat. The effects of Glen Canyon operations on sediment resources, particularly the erosion and restoration of sandbars, are of interest because sandbars are a fundamental element of the landscape of Grand Canyon and its ecosystem processes. Additionally, throughout Grand Canyon, sandbars create habitat for native plants and animals, supply camping beaches for river runners and hikers, and provide sediment needed to protect archaeological resources from weathering and erosion.
The U.S. Geological Survey’s Grand Canyon Monitoring and Research Center has responsibility for scientific monitoring and research efforts related to sediment and sandbars in Grand Canyon. Extensive research and monitoring during the past decade have resulted in the identification of possible alternatives for operating Glen Canyon Dam that hold new potential for the conservation of sand resources. For example, scientists have learned that conducting experimental high-flow releases when sediment supplies are depleted will not successfully sustain sandbar area and volume. With this understanding, scientists and managers focused on the need to strategically time high-flow releases to take advantage of episodic tributary floods that supply new sand to the Colorado River downstream from the dam. As a result, high-flow experiments in 2004 and 2008 were conducted after tributaries had enriched the sediment supply in the main channel.
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