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<br />001576 <br /> <br />SYSTEM-WIDE CHANGES IN THE DISTRIBUTION <br />OF FINE SEDIMENT IN THE COLORADO RIVER <br />CORRIDOR BETWEEN GLEN CANYON DAM AND <br />BRIGHT ANGEL CREEK, ARIZONA <br /> <br />. <br /> <br />By John C. Schmidt, David J. Topping, Paul E. Grams, and Joseph E. Hazel <br /> <br />ABSTRACT <br /> <br />. <br /> <br />The riverine ecosystem of the Colorado <br />River between Glen Canyon Dam and Bright <br />Angel Creek had less fine sediment on its bed, <br />in eddies, and as channel-margin deposits in <br />2001 than it did prior to completion of the <br />dam. Changes in dam operations in the 1990s <br />did not arrest this trend. <br />The decrease in fine sediment storage is <br />documented by comparison of historical <br />oblique photographs, analysis of historical <br />aerial photographs, and field surveys since <br />1990. The magnitude of the decrease is uncer- <br />tain. The loss of sand is probably about 25% of <br />the area typically exposed at base flow in pre- <br />dam photographs, but estimates range between <br />o and -55%, depending on study reach and <br />method of analysis. There is no indication that <br />the magnitude of decrease is less in the down- <br />stream part ofthe study area. The cumulative <br />loss of eddy sand is about 1 m in thickness but <br />also varies greatly. <br />Eddies are now the primary storage site of <br />fine sediment. Eddies have always been a very <br />large storage site for fine sediment, but the bed <br />once played a more important role than it does <br />today. The bed has been significantly lowered <br />in Glen Canyon, but the bed has only degraded <br />in pools and ponded backwaters in Marble and <br />Upper Grand Canyons. There is no evidence <br />that fiIle sediment aggrades on the main chan- <br />nel bed or in the deep parts of eddies for longer <br />than a few weeks to a few months, and these <br />parts of the river respond quickly to changes in <br />flow and sediment transport. These areas <br /> <br />. <br /> <br />evacuate fine sediment during flows typical of <br />the 1990s. Post-dam flood deposits have a <br />longer response time and adjust over a period of <br />years to decades to changes in dam operations. <br />These deposits are only constructed by dam <br />releases that exceed power plant capacity. They <br />are subject to large erosion rates during the first <br />months following flood recession, but erosion <br />rates thereafter decrease. The area of these <br />deposits caused by the 1996 Controlled Flood <br />lasted about 5 years, although some individual <br />deposits remain large today. <br /> <br />1.0 INTRODUCTION <br /> <br />Today's Colorado River in Glen Canyon <br />National Recreation Area and Grand Canyon <br />National Park is heavily used as a recreation <br />corridor. Between 15,000 and 20,000 persons <br />annually float through Marble and Grand Can- <br />yons (U. S. Department of the Interior, 1995), <br />and these river trips often are remembered by <br />the participants for the rest of their lives. More <br />than 50,000 people visit Glen Canyon each year, <br />where they primarily fish for rainbow trout <br />(Oncorhynchus mykiss) or take scenic boat trips <br />from Glen Canyon Dam to Lees Ferry (U. S. <br />Department of the Interior, 1995). Thousands of <br />hikers and backpackers scramble or walk into <br />Marble and Grand Canyons to enjoy an after- <br />noon visit or an evening camp. <br />The Colorado River corridor is also a <br />unique riverine ecosystem. Parts of the Colo- <br />rado River in Glen, Marble, and Grand Canyons <br />are critical habitat for the endemic endangered <br />humpback chub (Gila cypha) and razorback <br /> <br />Abstract 1 <br />