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<br />001585
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<br />Terraces associated Terrace height above river,
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<br />
<br />
<br />Ilil~~i~::;::::,~,,"~ Termres aSSOda:d m~~~aJ~~:~am
<br />
<br />"'/:,,.., ..'r.'..,......;.....,..,....,............. .....:................o!,....................::- with active debris fans j
<br />,/..' ~..... :;::.;;'.'.:l.:-;.:.!.'/.:./::.....:.;.j:.::::..i~: ap'::'.:..::;::~,'t'....;.;;:.;;.,:';:',,:"" '.~.:.:;.\.::::" --~-.- .-- 1983 fI d d ......,...
<br />[ ,~..........:... ......~...-.. .......~..... ....umt........f':. . , . I 00 san :
<br />(.. ','; ..~.~ 7/!!j}1~,;'~:~~t~*1rffllt.::~~}}ft;~:~}J#i;*%%1.~~:f~N: ~ j; ~; ~ i ~ :rJh ~\ ---~ 2,800 m3/s
<br />
<br />t:~;I.'~:/7>:..:.;.:;:,.: :"!-'~":":' ..;..:...:.:,..:v.....:]: . . : ',: : . :'
<br />
<br />(::;:" ,', ' ,)/~ ..);,(~~~~:~~~:~}~f;~*9f:~Jlt;ig~lUj ~~l ~ j: [l j ;'~{;~:,. I
<br />
<br />
<br />r .' Bedrock and gravel ~, , .. ' '@...,,'/0.... ';::7 .'/~Z~. ~.;: ~,~:~:":~":':\-' . -" . 1 ~ 140 m3/s
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<br />fn'~ ,,'j,l""/~0L'~' ",," ..//~; // ~~.;" .~./..... ~",,; .," ~'/~~L:/ // ..' Z..~ r/~~.....~_.~:....-:;!..,-.r;:-r~...~~.... ~
<br />l,l ",'/" J /._ ~ /,,,, ," ." /' ,"/ '" ;) ,,//,.~ "".','/' "", ," ,'" ;' .' /Z'77--r..-'7-7~7.~::a.
<br />.L..:... ._<..J_ .__.. ~~- ._._-~~_.- --.-. - --.--', .- ---~-.:./-,,;..~. ~ " .!
<br />
<br />.
<br />
<br />Figure 13. Diagram depicting generalized cross section showing geomorphic and geologic relations of late Holocene
<br />terrace-forming alluvium, eastern Grand Canyon. Sa is striped alluvium; ap is alluvium of Pueblo II age; umt is upper
<br />mesquite terrace. The unlabeled coarse stippled patterns depict the lower mesquite terrace and the pre..dam terrace
<br />at higher and lower positions, respectively. The densely stippled lowest surface are the deposits of the post-dam
<br />Colorado River. From Hereford et al. (1996, fig. 8).
<br />
<br />.
<br />
<br />levees on the surface of channel-margin depos-
<br />its are typically composed of a single set of
<br />foresets that record the onshore migration of
<br />the ridge (Schmidt and Rubin, 1995). Pre-dam
<br />flood deposits are primarily composed of
<br />poorly sorted silty very-fine sand, and post-
<br />dam deposits are primarily composed of very
<br />fine and fine sand with less silt. Deposits
<br />typically form distinct topographic surfaces,
<br />and most surfaces are underlain by discrete
<br />deposits separated by erosional unconform-
<br />ities.
<br />Hereford et aI. (1993, 1996) identified
<br />and determined ages of 5 late Holocene ter-
<br />races that pre-date completion of Glen Canyon
<br />Dam: the striped alluvium, alluvium of Pueblo
<br />II age, alluvium of the upper mesquite terrace,
<br />alluvium of the lower mesquite terrace, and
<br />pre-dam alluvium (Fig. 13). Deposition of the
<br />striped alluvium occurred from about 770 B.C.
<br />until around A. D. 300. The Pueblo II allu-
<br />vium accumulated between about A. D. 700
<br />and 1200. The three lower terraces formed
<br />during subsequent large floods and are much
<br />thinner, generally occupy less area, and are
<br />interpreted to represent the progressive decline
<br />in the magnitude of large floods (Hereford et
<br />aI., 1993). The upper mesquite terrace formed
<br />between aboutA. D. 1400 and 1880, and its
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<br />.
<br />
<br />vegetation is that of the upper riparian zone
<br />community and includes mature western honey
<br />mesquite (Prosopis glandulosa) tree:s and
<br />shrubs in lower Marble Canyon and upper
<br />Grand Canyon. The lower mesquite: terrace
<br />has similar vegetation. This surface had re-
<br />cently been overtopped in a January 1890
<br />photograph, reported by Hereford et aI. (1993);
<br />thus, the surface must have been inundated by
<br />the July 1884 flood of record whose: estimated
<br />peak discharge was 5935 m3/s j; 850 m3/s
<br />(Topping et aI., 2003).
<br />The pre-dam terrace level is a strath
<br />terrace established in older alluvium or is
<br />underlain by its own alluvium. The dominant
<br />vegetation on this terrace includes large,
<br />mature, and partially buried saltcedar (Tamari"C
<br />ramosissima). Hereford et aI. (1993) stated
<br />that this terrace formed during large: floods
<br />between about 1930 and 1960, based on types
<br />of driftwood and other debris. This terrace
<br />occurs near Lees Ferry (Hereford et aI.,
<br />2000b), in the Point Hansborough Reach
<br />(Schmidt and Leschin, 1995), near Nankoweap
<br />Rapids (Hereford et aI., 1998), and in the Big
<br />Bend Reach (Hereford et aI., 1993; Schmidt
<br />and Leschin, 1995).
<br />Post-dam flood deposits were created in
<br />1965, 1980, annually between 1983 and 1986,
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<br />3.0 The Valley of the Colorado Rivl~r 19
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