<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
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