<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 />
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