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<br />assumptions, such as equating decadal sediment <br />yield with the sum of sediment yield from one <br />ten-year, two five-year, and five two-year floods. <br />The technique was adjusted to fit the data <br />regression relation and is not strictly an <br />independent approach. <br /> <br />Nevertheless, close agreement with the other <br />two methods suggests that the technique has strong <br />potential as a new method for estimating <br />streamflow-sediment yield. The data regression <br />technique and the empirical approach of Renard <br />(1972), derived from different data sets and <br />methodologies, are independent methods that <br />provide similar sediment-yield estimates. Of the <br />three methods, we selected the regional regression <br />relation for calculating sediment-yield estimates <br />because it is derived specifically from Colorado <br />Plateau data. This technique indicates that the Glen <br />and Marble Canyon reaches (river miles -15 to <br />61.5) deliver 0.065'106 and 0.610'106 Mg/yr of <br />streamflow sediment, respectively, (0.68'106 total) <br />to the Colorado River. This amount is 20 percent of <br />the sediment yield of the Paria River, the only <br />gaged tributary in the reach. <br /> <br />1,500,000 <br /> <br />~ <br />~ <br />.J:: <br />o <br />~ 1,000,000 <br />II: <br />.!: <br />-0 <br />a; <br />>= <br />E <br />" <br />E <br />'6 500,000 <br />c?l <br />Iii <br />" <br />c:: <br />c:: <br /><{ <br /> <br /> <br /> <br />, . <br />...... '0 <br />....::..~ ~_..~#.....," ,.- <br />--..y-:-'..... .......1.:..,..,......... <br />o " ........... ': <br />1900 1920 1940 <br /> <br />Sand delivery by streamflow from the Glen and <br />Marble Canyon reaches is about 0.032'106 and <br />0.305'106 Mg/yr, respectively (0.340'106 total), <br />depending on the sand content of streamflow <br />sediment. Sand input to Glen Canyon is <br />significantly coarser (050 = 0.24 mm) than sand in <br />other reaches (050 0.15 mm). A relation is given <br />relating the possible variation of this sediment <br />delivery with climatic variability. <br /> <br />Debris flows transport poorly-sorted sediment <br />onto debris fans in the Colorado River. In the pre- <br />dam era, most debris fans were extensively <br />reworked during Colorado River floods, liberating <br />most fine-grained sediment to the river. In the post- <br />dam river, an average of only 25 percent of debris- <br />fan volume is reworked, leading to storage of sand <br />in the matrix of debris fans. We used a <br />logistic-regression model of debris-flow frequency <br />in Grand Canyon based on the interpretation of <br />1,297 historical photographs of the river corridor. <br />This analysis yielded information on the frequency <br />of debris flows in 168 of the 736 tributaries (23 <br />percent). Of the 168 tributaries, 96 (-60 percent) <br />had debris flows during the last one hundred years. <br /> <br />......... Reach A <br />_... - Annual Average, <br />Reach A <br />- Reach B <br />- - - Annual Average, <br />Reach B <br /> <br /> <br />. . , ,... :/":" ," <br />." 'r:f\:..,..f'T/1J,.. .V':::-"':..~...:::' <br /> <br />1960 <br /> <br />1980 <br /> <br />2000 <br /> <br />Year <br /> <br />Figure 15. Estimated time series of streamflow sediment yields in Reaches A and B <br />based on climatic variability (Fig. 4a) and using a Cv = 1.3. <br /> <br />DISCUSSION AND CONCLUSIONS 37 <br />