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<br />002357 <br /> <br />Terraces associated Terrace height above river, <br /> <br />~0'i~-~ with Seg,::w debri~ -1 in m~~~~~~~=:am <br />~fcf.t[lj!~R~t~t;z~J~-_.~ Terra~ assoc~~d . ~.. ::3 <br />. ~ "X..", .,..._...,,,;......,.,....:.,.....,.,..,'. .,...:c.,..c.,,',.... .""......,..", With active debns fans <br />, /. .'. ~.,-J' :..:J:..;.........:!': .;,:,',.'.: ::";f!'.,:.,". ap,":...~:;...-t, {-'.; ','i.'f' .', "';'f.:-..'::J. <br />r-' ~7:':""'" '.' ........,..,...... "J; \-.,.::.. ..;......~,.,'.<......, umr"', .....~...,-.-.~ .-- 1983 flood sand <br />i-r0'..':.. '~*' ..; ..'~;:Yii~~)rj~1~~lt~~~~?-~~~~*r.~~~~4/Wj/~R~~~~ti; ~~ ~ ~ ~ ~ ~ ~ ~':~~. <br /> <br />t" '2'';' ,.....\..,.~...,..."....,.....,' ',. .... ',"., ,,', .,'... .1,. ...,..~ <br />%~" oF ~' , ~ $,.......;;:.~.-;~:~<(:.:!...~::.;:&~.::(:.!.:!:~:.;:::::"<--;..:::i::'~':!'f.:.I':~:'~;J':=::;':;: ~; ~:.: ~: ~ <br />~' ;~~" ~. " ..~%.......~.~y..,,:i.-:.::!!~...:.~...~f/I.:.~.:<;.>~.e;~...;...:....-:.~.Ij.-"....:.;~....~..~...;.:.l... .:-~ <br />T ,,",... / ~; , /? / ;.. , .....-:......;....;-....::..;..-~:r'-f..:i :..;...."..;.:-..:.I.t..:;.::.::/."':::::.:; :~:'. :':'1 <br />A-:j/" , " , ".. " ,"/' ,~"';~~.:i'.:;.:;:#.\';;':;"x:,;,:;;;;..::;;;;.tf-;.:-::.J\ <br />t-:. BedroCk~ndgravel' ~~/,<,., ,@'.../ "~"J%.',,"?7P~.:~"~" ':i:;_LH.ft~~- - . .. 140m3/s <br />~ . ,.:/, .:/ ,. /. '/ ,,/,. '/ ' /.. " .. " ,." ,',.' " .' .' ,,;(.. ;. '.. ~ ,'" , ~ --:: ./-;;'-,:.;:....~:.~:;-.~- . <br />~../../..Z..:!::L;:;'-..-.: ," k'L..:.~-..:.:',.. ..:.."::.LL,...~~.... ~L0;_...;'l~L2.~7Z77Z~~ <br /> <br />,- <br />'/( <br /> <br />.. . 2,800 m3/s <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-<lam 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 />M'"f' <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-fme 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 aboutA. 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 <br /> <br />vegetation is that of the upper riparian zone <br />community and includes mature western honey <br />mesquite (Prosopis glandulosa) trees 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, repqrted 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:!: 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 (Tamarix <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, <br /> <br />3.0 The Valley of the Colorado River 19 <br />