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<br />cro~s ~ections that had been mea~ured on
<br />Augu~t :!3. Parts of cross section pot. near
<br />the center of the channel. filled with
<br />nearly 20 feet of new sand (fig. 2). A
<br />nood of this size or larger occur~ on {he
<br />
<br />o
<br />
<br />el 1I~9
<br />10""'" ..' .~z
<br />"'~.Oep05'led:' '.~~'9;
<br />:. '.\:.':.::.~lJq. :.:
<br />t:i 20 '. .~. _~+.". .
<br />w " " ..%"'". ""."
<br />lI. "!l\0'., . .
<br />z 30
<br />o
<br />
<br />1992 and 1993. relca~es from G]en depo~ition and erosion will result from
<br />Canyon Dam have been high enough to dam relea~es, To date (1995), we have
<br />redistribute large amounts of ~and only been able to study llows of less than
<br />transported to the Colorado River channel 20,(X)(} cubic feet per second, but the
<br />by tributary tloods. Just downstream from cro~s.st:ction net....ork will still be in place
<br />
<br />ErO(led.
<br />
<br /> 0
<br /> "
<br /> 20
<br />" 30
<br />w "
<br />w
<br />"
<br />. 50
<br />z 0
<br />"
<br />. "
<br />w
<br />0 20
<br /> 30
<br /> "
<br /> 50
<br /> 0
<br />
<br />" ".
<br />
<br />"
<br />"
<br />.
<br />w
<br />o
<br />
<br />Channel bOnom All ,24.1992 ... '/_--y
<br />10.~ Eroded ,I': Dep~~: . ~f~~?
<br />, . ,~o~ ;;7..'" '" ..
<br />20. ..'~<{p-? ~i.", .' ..'.
<br />....:~..:.: ' '.'
<br />
<br />""~",,;~"""""..""".":
<br />': . " 992 :--, '.,
<br />l\~ ..... ,'I, . ,. . .
<br />""#.: \l)Of1omj"''j.'':~...'
<br />~ .
<br />'....:.,.' ...'..
<br />.-:....:.----: .
<br />
<br />, "
<br />Er<Xled --:.' 9Q}'/. .
<br />~I'\'
<br />h3.""e\tP. ,.Q' 'P .
<br />C"af\~e~bOltO~. .
<br />.' .'
<br />
<br />30
<br />o
<br />
<br />500
<br />
<br />\00 200 300 400
<br />DISTANCE FROM LEFT BANK. IN FEET
<br />
<br />Ffgure 2, A 1I0od on the Paria River on August 23, 1992,
<br />deposited about 2,400 square feel 01 sand in the Colorado
<br />River at cross section p4 (fig, 1) just downstream from the
<br />confluence. Five months fater, about 2.880 square feet of
<br />sand had been eroded trom the main channef at the cross
<br />section and 317 square feet had been deposited in the eddy.
<br />
<br />Paria River about once every 2 years. The
<br />amount of sediment transported can vary
<br />greatly.
<br />In January] 993, a llood on the Lillie
<br />Colorado River "'ashed about 10.100,000
<br />tons of sediment into the Colorado Ri\'er,
<br />about -L(,OO.OOO tons of which was sand.
<br />enough to fill a channel an average of 400
<br />feet wide \,vith 10 fee! of sand for about
<br />5 miles. However. the sand was no!
<br />evenly deposited, Some cross sections
<br />below the mouth of the Linle Colorado
<br />Riv'er had very little sand accumulation
<br />and some gained almos! 30 feel (fig. Jl.
<br />This flood lasted about 3 weeks and had J
<br />peak llow of Jbout 16.-+00 cubic feet per
<br />second. A flood with such a fong dUJ.ition
<br />and high peak 110w occurs on the lillie
<br />Colorado Ri\'er about once ever~ 50
<br />years,
<br />
<br />Sand is Carried Downstream
<br />by Dam Releases
<br />
<br />Cross-section measurement., shew. thaI
<br />even during the restricted dam relea~es in
<br />
<br />o,la~ 1995
<br />
<br />~POS'led' .
<br />
<br />"" 200
<br />DISTANCE FROM LEFT BANK. IN FEET
<br />
<br />JOO
<br />
<br />Figure 3. A l!ood on the Lillie Coforado River in January
<br />1993 deposited about 5,300 square feet of sand in the
<br />Coforado River at cross section a6 (fig. 1), downstream
<br />!rom the conffuence. Three months tater, onfy about 220
<br />square feet of sand had been eroded.
<br />
<br />the mouth of the Paria River, TnO'.t of the
<br />sand deposited in the main pan of the
<br />river channel during the llood of August
<br />]992 had bccn scoured out by January :!3,
<br />1993 (fig_ :!l. Sand did accumulate in the
<br />large t:ddy al this location during lhis
<br />time. The sand in the Colorado River
<br />deposited downstream from the mouth of
<br />the Little Colorado River in January] 993
<br />also began 10 move downstream lfig. 3),
<br />The river bed changed more slowly in the
<br />study reach downstream from the mouth
<br />of the Liule Colorado River than in the
<br />sludy reach downstream from the moulh
<br />of the Paria River, probably because a
<br />larger amount of sand had been deposited
<br />there and because the Little Colorado
<br />River and olher tributaries continued to
<br />supply some sand after the ll('ll.xl.
<br />
<br />The cross-,ection monitoring provides
<br />infonnatlOn on change~ in "and ~torage at
<br />spe.:ific locations. Computer modeh
<br />under de\'dopmem will makt.' il possit.le
<br />w compute changes in storagt: in ri\ er
<br />rt.'ache~ in Grand Canyon ;\"ational Park
<br />and to predi.:t ~ here and ho~ much ~and
<br />
<br />when higher flow~ occur in the future.
<br />Data collected at that lime will provide
<br />detailed infonnation about hoW' l100ds
<br />scour and redeposit sand.
<br />
<br />Selected References
<br />
<br />Bureau of Redamation, 1989, G]en
<br />Canyon Environmental Studies-Final
<br />Repon. January. 1988: Bureau of
<br />Reclamation Repon. 84 p.
<br />
<br />Bureau of Reclamation. ]994, Operation
<br />of Glen Canyon Dam-Draft Environ-
<br />ental Impact Statement: Bureau of
<br />Reclamation Repon. 32-l- p.
<br />
<br />-5,/ln/lt'/ .H.D. Jon$I'1I. Julia B. Gra/.
<br />Jonathan E. .\farlow. Gild Gregary G.
<br />Fisk
<br />
<br />Fur mort' information. please contact:
<br />U.S. Geo]ogical Suney
<br />375 S_ Euclid A\enue
<br />Tucson, AZ 85719
<br />(60:!1670-M7\
<br />
<br />F5-120-95
<br />
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