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Floods and Sandbars in the Grand Canyon



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INSIDE Vol 9 No 4 April 1999 GSA TODAY A Publication of the Geological Society of America Call for Papers Page D1 Electronic Abstract Submission Page D12 Registration Issue June GSA Today Floods and Sandbars in the Grand Canyon Ivo Lucchitta 6969 Snowbowl View Circle Flagstaff AZ 86001 Luna B Leopold 400 Vermont Avenue Berkeley CA 94707 ABSTRACT Erosion of sandbars and beaches in the Grand Canyon National Park downstream from Glen Canyon Dam has become a major problem that needs to be addressed Geomorphic and geologic mapping provide a link between sandbar elevations and discharge measurements This link allows an estimate of discharges that will deposit sand far enough above normal high water to prevent frequent depletions by erosion The sand is needed to protect habitats and archaeological sites and to maintain beaches used by recreationists It is proposed that when the Little Colorado is in flood discharge at Glen Canyon Dam be increased to bring the total discharge to the desired high value Analysis of the flow records show that such opportunities are presented on the average once in eight years suggesting that the proposal has a reasonable chance of success INTRODUCTION The Colorado River in the Grand Canyon section in Arizona once fluctuated greatly in its flow Year to year and seasonto season variability was large Peak discharges ranged from 300 000 cfs cubic feet per second to 19 200 cfs a difference of 16 times The amount of sediment transported as suspended load was very large Measurements carried out at the Grand Canyon for the period December 1940 to June 1941 show that at 50 000 cfs about 2 000 000 tons were moved per day during the rising stage of the flood 1 and 500 000 tons during the falling stage whereas almost 5 000 000 tons per day were moved during the peak of the flood Grand Canyon continued on p 2 1Sediment transported during the rising stage of a flood is much greater than that transported during the falling stage Leopold and Maddock 1953 Glen



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