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<br />Table 17. Annual aedlment yield and aand delivery by debrla flow In Grand Canyon.
<br />
<br /> VOLUME AND WEIGHTVIELD
<br /> M.udmum volume Maximum volulM Average volume Av....g. volume
<br /> model model model model
<br />Reach (m3lyr) (Mg/yr) (m3lyr) (Mg/yr)
<br />Reach A O. O. O. O.
<br />Reach B 40,100 80,300 19,000 38.000
<br />Reach C 14,800 29,600 7).20 14,400
<br />Reach D 24,900 49,900 12,000 24.000
<br />Reach E 2,640 S,290 1.200 20510
<br />Reach F 36,000 72 .000 17.100 34,100
<br />Reach G 29,200 S8.3OO 14.200 28,400
<br />All Reaches 148.000 .29S .000 70,700 141.000
<br /> SANDVIELD
<br /> FULLV REWORKED DEBRIS FANSt PARTIALLV REWORKED DEBRIS FANS'
<br /> Maximum volume AverIIge volume Maximum volume AV8IlIIle volume
<br /> model model model model
<br />Reech (Mg/yr) (Mg/yr) (Mg/yr) (MgIyr)
<br />Reach A O. O. O. O.
<br />Reach B 14,600 6.910 3,6S0 1,730
<br />Reach C S,380 2.630 I,3S0 6S7
<br />Reach D 9,070 4,370 2,270 1,090
<br />Reach E 960 460 240 110
<br />ReachF 13,100 6,210 3,280 I,SSO
<br />ReachG 10.600 S,I70 2,6S0 1).90
<br />All Reaches S3,7OO 2S,8OO 13 ,400 6,440
<br />
<br />. Tributaries in Reach A (Glen Canyon) do not produce debris flows.
<br />t Assumes that all debris flows average 18.2% sand content and that all sand entel'& the Colorado River (sceTable IS).
<br />t Assumea that every decade 25% of the volume of the average debris fan is reworked by a poat-dam ColOl"Bdo River flood lIJ1d only the
<br />reworked debris reaches the river (see text). We then distribute the reworked sand on an expected annual basis.
<br />
<br />and Grand Canyon upstream from Bright Angel
<br />Creek) the average total sand delivery is 0.030'106,
<br />0.296'106, and 0.050'106 Mg/yr, respectively.
<br />Limited reworking of debris fans associated with
<br />the operation of Glen Canyon Dam reduces sand
<br />delivery in Reaches B and C to 0.288 and 0.047"106
<br />Mglyr, respectively (table 18). The combined
<br />average post-dam sand yield from ungaged
<br />tributaries in Reaches A and B is about 0.318'106
<br />Mg/yr, or 20 percent of the approximately 1.5'106
<br />Mglyr of sand delivered annually by the Paria
<br />River. Sand contributed by tributaries in Glen
<br />Canyon is notably coarser (Dso = 0.24 mm) than
<br />sand from other reaches (Dso = 0.11 - 0.20 mm),
<br />including the Marble Canyon reach (Dso = 0.20
<br />mm; table 13; fig. 10).
<br />
<br />CLIMATIC VARIABILITY AND
<br />STREAMFLOW SEDIMENT YIELD
<br />
<br />Sediment yield in the Grand Canyon region
<br />occurs mostly in discrete events. The relation
<br />between the magnitude and frequency of discrete
<br />events and regional climate is complex for
<br />geomorphic processes such as debris flows, as
<br />discussed above. Streamflow, however, is strongly
<br />related to seasonal climate, and streamflow
<br />sediment yield should be related to climatic
<br />variability (Graf and others, 1991). In Bright Angel
<br />Creek, for example, most flood events (55 percent)
<br />occur in summer, summer floods have a higher
<br />mean discharge (13 m3/s) than floods in other
<br />seasons, and winter floods occur mostly in EI Nino
<br />years. We assumed that other small tributaries in
<br />
<br />34 Sediment Delivery by Ungeged Trlbutarle. 01 the Colorodo River In Grond Cenyon
<br />
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