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<br />NAU Sand Bar Studies
<br />
<br />Final Report
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<br />and wide daily fluctuations in discharge were restricted (Randle and Pemberton, 1987; Randle et al.,
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<br />1993; Smillie et ai" 1993), However, transport rates are highest when the bed has a large proportion
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<br />of fines and sediment rating curves shift over time as a function of the sand grain sizes present on the
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<br />bed (Topping et ai" 2000a; 2000b). Therefore, previous studies of sand mass balance underestimate
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<br />sediment transport through the system immediately following tributary inputs and over predict
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<br />accumulation on the bed during the intervening periods, The rate of sediment transport in Marble
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<br />Canyon is a critical resource issue because it may not be possible to store large amounts of tributary
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<br />supplied fine sediment on the bed for periods longer than a few months (Topping et ai" 2000b) under
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<br />current ROD dam operating criteria (U,S, Department of Interior, 1995).
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<br />THE 1997 PARIA RIVER FLOODS
<br />The Paria River is an arid region stream that drains 3,600 km2 in southern Utah and northern
<br />Arizona, and is subject to infrequent floods of short duration, Historically, large floods on the Paria
<br />River are generated by runoff in the uppennost 14% of the drainage basin by eastern Pacific Ocean
<br />tropical stonns and intense but more isolated rainfall associated with the southwestern monsoon
<br />(Topping, 1997; Topping et al., 1998), Most of the sediment carried by the Paria River is derived
<br />from areas of lower basin elevation that are underlain by sedimentary rocks of Mesozoic and younger
<br />ages, Between 1923 and 1996, the mean annual sediment load delivered to the Colorado Ri ver by the
<br />Paria River was 9,1 x 105 m3 (2.4:t 1.2 million Mg) of sand, silt, and clay, of which about 50% was
<br />sand (Topping, 1997), However, annual inputs from the Paria River to the Colorado River ecosystem
<br />are not only variable but were mostly below average from 1980 through 1996, Variability in sand
<br />delivery from the Paria River has been attributed to long-tenn climate variations (Graf et ai" 1991;
<br />Hereford and Webb, 1992) and land-use changes (Topping, 1997),
<br />Heavy precipitation in the drainage basin produced four large floods in late summer 1997 (Fig,
<br />2a), The flood peaks were 115 m3ts (4,061 ft3ts) on August 10, 72 m3ts (2,542 ft3ts) on September 7, a
<br />double-peaked flood with peaks of 85 m3ts (3,001 ft3ts) and 110 m3ts (3,884 ft3ts) on September 15,
<br />and 95 m3ts (3,354 ft%) on September 26. Three of the floods exceeded the 90 m3ts bank-full
<br />discharge of the Paria River estimated by Topping (1997), These stonns were associated with a series
<br />of dissipating tropical stonns combined with a strong monsoon season that resulted in high antecedent
<br />moisture conditions. Approximately 7,7 x 105 m3 (2,O:t 0.4 million Mg) of sand and 9,2 x 105 m3
<br />(2.4:t 1,2 million Mg) of silt and clay were delivered to the Colorado River (Topping et ai" 2000b),
<br />nearly twice the Paria River mean-annual sediment input. The sand inputs from the Paria River in
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