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<br />flows de]i~'er only 0.021_0.44106 Mglyr
<br />of bouldcrs (BUIS>2.56 mm), these
<br />boulders have a crilical impact on the
<br />geomorphic fr.unt'work of the river,
<br />defining debris fans, rapids, and related
<br />sand bars, and are unlikely 10 be removed
<br />by regulated flows.
<br />
<br />Total Sediment Yield to the
<br />Colorado River
<br />
<br />Sediment-yield eSllmates for
<br />slreamflow and debris flow were
<br />combined to estimate total annual
<br />sedimcnt yield from the ungaged drainage
<br />areas (fig. 5A). The tOlal sediment yield is
<br />highest in Reach F (fig. 1), which has the
<br />highest streamflow sooiment yield. The
<br />percent contribution of debris-flow
<br />sediment yield is highest in Reaches D, C,
<br />and D because of the high frequency of
<br />debris flow in those reaches (Griffiths and
<br />others, 1996).
<br />A range of possible sand yields was
<br />calculated given the range of streamflow
<br />sand content and assumptions in the
<br />debris-flow sediment-yield model and
<br />low, average, and maximum sand delivery
<br />from the ungaged tributaries are reported
<br />(5b). The sand delivery rale from fully
<br />reworked debris fans, which reflects pre-
<br />dam conditions, a....erages 13 106 Mglyr
<br />for all ungaged tributaries in Grand
<br />Canyon. In Reach~ A. 8, and C (fig. I),
<br />!.he average tala] sand delivery is
<br />0.030'106,0.296'106, and 0.050"106 MgI
<br />yr, respeclively. limiled reworking of
<br />debris fans associated with the operation
<br />of Glen Canvon Dam reduces sand
<br />delivcry in Rea'ches B and C to 0.288106
<br />and 0.047"106 Mglyr, respectively.
<br />The combined average post-dam sand
<br />yield from ungaged tributaries in Reaches
<br />A and B is about 0.318' I 06 Mglyr, or 20
<br />percent of the approximately 1.5106 MgI
<br />yr of sand delivcred annually by lhe Paria
<br />Ri....er. The total sediment yield by
<br />streamflow and debris flow from the
<br />ungaged drainage areas is 2.8-3.0106 MgI
<br />yr. Of this lotal sediment yield, 0.4-2.0 Irf>
<br />Mglyr is sand, although a smaIl amount of
<br />this sand is storc-d in unre.....orked debris
<br />fans.
<br />Even .....ith slOragc- in debris fans. 0.1-
<br />0.5106 Mglyr of sand are added to the
<br />reaches betv>c-cn Glen Canyon Dam and
<br />!.he Linlc Colorado Rivcr annually. This
<br />amount is up to 33 percent of the &IDd
<br />deli\'ered by the Paria Ri\er, the only other
<br />
<br />
<br />- 1,0
<br />~
<br />~ A
<br />::. 0.8
<br />b
<br />-
<br />-
<br />'" 0.6
<br />"
<br />>=
<br />C 04
<br />~
<br />E
<br />'\j 02
<br /><f1
<br />~
<br />a 0.0
<br />...
<br /> 0.7
<br /> 06 B
<br />-
<br />~
<br />>
<br />'" 0.5
<br />::.
<br />b 04
<br />~
<br />-
<br />31 0,3
<br />~
<br />>=
<br />'" 0,2
<br />c
<br />~
<br /><f1 0.1
<br /> 0.0
<br /> A
<br />
<br />.
<br />.
<br />
<br />Slreamflow
<br />Debris flow
<br />
<br />. 15% Sand Content
<br />. 50~'o Sand Content
<br />. 75% Sand Content
<br />
<br />
<br />c
<br />
<br />D
<br />
<br />E
<br />
<br />F
<br />
<br />G
<br />
<br />B
<br />
<br />Sediment-Yield Reach
<br />
<br />Figure 5. Estimates 01 annual sediment yield from ungaged tributaries by reaCh. A.
<br />Total sediment yield delivered by debris flow and streamflow. B. Total sand delivered by
<br />streamflow and debris flow 10 the river under a regulated flow regime. Estimates are
<br />given for 15%, 50%, and 75% sand conlent.
<br />
<br />source of sand-sized particles in this
<br />critical section of Grand Canyon, and
<br />douh]e lhe 0.17'106 Mglyr estimated by
<br />the U.S. Department of the Interior
<br />(1995). Sand ddivered by dehris flows
<br />contributes up to 8 percent of the lotal
<br />sand yields. Particles larger than sand -
<br />particularly the boulders and connles
<br />deli\'cred by debris flow - arc ]argely
<br />unaffected by regulated nows from Glen
<br />Canyon Dam and continue to aggradc thc
<br />Colorado River in Grand Canyon.
<br />
<br />- Robar H. \.t.-~bb aM Peter G, Griffiths
<br />
<br />Selected References
<br />
<br />Gnffiths. P.G., Webb, R.H., and Melis. T.S.,
<br />1996, Inniation and frequency of !kbris
<br />ftO\'os In Grand Canyon. Arizona: U.s.
<br />Geo]ogical Sun-ey Open-File Repon 96-
<br />491. 35 p.
<br />Melis. TS.. Webb. R.H.. Griffiths. P.G, and
<br />Wise. TJ., ]994. \1agmtudt and frequency
<br />data for hiMoric dehns flows in Grand
<br />
<br />Canyon Natlonal Park and ....icinity.
<br />Arizona: U.S. Geological Sur...ey Water
<br />Resources Investigations Rtp:.n 94-4214,
<br />285p.
<br />Renard. K.G.. 1972. Sediment problems in th..'
<br />arid and semI-and southwest. in
<br />Proceedinp. 21th Annual Meeting. Soil
<br />Con5er\'alion Society of America:
<br />Ponland, Oregon, p. 225.232.
<br />U.S. Department of the Imerior, 1995,
<br />Operation of Glen Canyon Dam - final
<br />environmental impact Slatement: Coconino
<br />CounlY. A'L Colorado RI....er Storage
<br />Project, 237 p.
<br />Wehb, R.H" Griffiths, P.G.. Melis, T.S" and
<br />Hanle)'. D.R" 2CXXl, Sediment delivery hy
<br />ungaged tributaries of the Colorado Ri....er
<br />in Grand Canyon: U.S. Geological Sun-e)'
<br />Water Resources In~'estlgauons Rep:.n 00-
<br />4055,67 p.
<br />
<br />For funher infOn1lalion, contact:
<br />Roben H. Wehh
<br />U.s. Geological SUr\'C')
<br />1675 W. Ank]am Road
<br />Tueson. AZ 85745
<br />E.ffi3Jl: rflwthb@usgs.gov
<br />
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