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<br />Table 6. Estimated annual streamflow sediment yield from ungaged tributaries of the Colorado River in Grand <br />Canyon. <br /> <br />RENARD (1972) EQUATION <br /> <br />DATA REGRESSION EQUATION <br /> <br />Trlbulary Extra-area Tolal <br />sediment" sediment" sediment" <br />yield yield yield <br />(Mg/yr) (Mglyr) (Mg/yr) <br /> <br />58,300 6,490 64.800 <br /> <br />585,000 25,300 610,000 <br /> <br />90,700 7,010 97,700 <br /> <br />311,000 20,300 332,000 <br /> <br />47,300 9.650 57,000 <br /> <br />792,000 29,300 821,000 <br /> <br />661,000 8,400 669,000 <br /> <br />2,550,000 106.000 2,650,000 <br /> <br />Sediment <br />yield reach <br /> <br />A <br /> <br />B <br /> <br />C <br /> <br />D <br /> <br />E <br /> <br />F <br /> <br />G <br /> <br />River miles <br />,15.5 toO.9 <br />0.9 to 61.5 <br />61.5 to 87.8 <br />87.8 to 143.5 <br />143.5 to 156.8 <br />156.8 to 225.8 <br />225.8 to 276.0 <br />TOTAL <br /> <br />Tributary <br />sedlmentt <br />yield <br />(Mg/yr) <br /> <br />67,100 <br /> <br />556,000 <br /> <br />115,000 <br /> <br />343,000 <br /> <br />52.200 <br /> <br />737,000 <br /> <br />614,000 <br /> <br />2,480,000 <br /> <br />Extra-areB <br />sedlmentt <br />yield <br />(Mglyr) <br />9.330 <br />36.800 <br />12,000 <br />32.000 <br />11,500 <br />41.900 <br />18,900 <br />162,000 <br /> <br />Total <br />sedlmentt <br />yield <br />(Mg/yr) <br /> <br />76,400 <br /> <br />593,000 <br /> <br />127,000 <br /> <br />375,000 <br /> <br />63.700 <br /> <br />779,000 <br /> <br />633,000 <br /> <br />2,650,000 <br /> <br />. Sediment yield is calculated using a regression equBlion developed during this study. <br />t Sediment yield is calculated using [he Renard (1972) equation converted to metric units and using a sediment density of 1.2 Mglm3, <br /> <br />outlined by the PSIAC (pacific Southwest inter- <br />Agency Committee, 1968). The USLE was <br />developed strictly for low-slope agricultural land <br />and is not appropriate for the steep terrain of Grand <br />Canyon. Likewise, the CREAMs and WEPP <br />models were developed for relatively low-slope <br />agricultural and rangeland and require considerable <br />watershed data for proper application. The PSIAC <br />method involves rating a watershed on the basis of <br />nine factors related to erosion (surface geology, <br />soil, climate, runoff, topography, land use, upland <br />erosion, and channel erosion/sediment transport) to <br />produce an estimate of sediment yield. This method <br />can be applied to large areas using pre-calculated <br />PSIAC sediment-yield ratings mapped by the Soil <br />Conservation Service (SCS, 1975; Hedlund and <br />Curtis, 1984). It produces a high estimate of 4, II 0 <br />Mg krn-2yr-1 (table 5) and was rejected as a viable <br />method for estimating sediment yield in Grand <br />Canyon. <br /> <br />The Flood-Frequency, Rating-Curve <br />Technique <br /> <br />We developed a flood-frequency, rating-curve <br />technique to estimate streamflow sediment yield <br />based loosely on the work of Strand (1975) and <br />Strand and Pemberton (1982). This technique <br />requires numerous assumptions, one of the most <br /> <br />important of which is that the decadal streamflow <br />sediment yield in a tributary can be described by <br />several floods of recurrence intervals described by <br />regional flood-frequency relations (table 7; Roeske, <br />1978; Thomas and others, 1997). Considering the <br />intermittent-flow regime of these tributaries, which <br />probably have flow less than one percent of the <br />time, this is likely not to be an egregious <br />assumption for most of the ungaged tributaries. <br />Once flood-frequency has been established for a <br />tributary, we use a relation between peak discharge <br />and total-event sediment yield developed from <br />hydrographs of floods on Bright Angel Creek (fig. <br />I) and sediment-rating curves from Black Mesa <br />gaging stations (table 8). <br /> <br />Regional flood frequency <br /> <br />The regional-regression equations reported by <br />Thomas and others (1997) for the southwestern <br />United States (table 7) are not without significant <br />problems when applied to the Grand Canyon <br />region. Few small drainages from Grand Canyon <br />have gaging records, and therefore these tributaries <br />are not well represented in the regional flood- <br />frequency relations. Webb and others (1999b) <br />found that the equations for region II in central <br />Grand Canyon overestimated flood frequency for <br />the Prospect Valley drainage basin in western <br />Grand Canyon. As an alternative, we evaluated <br /> <br />STREAMFLOW SEDIMENT YIELD 15 <br />