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<br />
<br />" \'fete typical of the changes at the -18
<br />t'ement sites of the study area, but the
<br />'~de of adjustment was not always
<br />. Table 3 shows the nature of the
<br />e1, adjustments following the intro-
<br />'on of tamarisk: the data for the first
<br />at, each site, pertains to preinvasion
<br />'nditions. In every case but one, the bank-
<br />lo-bank distance declined. The width re-
<br />duction ranged from about 13% to 55%.
<br />1he exception, a cross section near Turk's
<br />pead at mile 20.5, may be misleading be-
<br />cause it is the location of an alluvial fan
<br />j,uilt by a tributary' to the main channel, so
<br />'~at the channel cross section responds to
<br />~anges resulting from flash-flood activity
<br />,~ the tributary.
<br />1
<br />l \,
<br />
<br />
<br />TAMARISK IN THE COLORADO PLATEAU REGION
<br />
<br />Near-channel Landforms and Flooding
<br />
<br />The expansion of stabilized deposits and
<br />concomitaQt reduction in mobile materials
<br />and channels is significant because of the
<br />decreased ability of the channel to adjust to
<br />substantial fluctuations in discharge. An
<br />annual range of instantaneous discharges at
<br />the town of Green River (the upstream edge
<br />of the study area) of 30,000 cfs is common.
<br />Before the arrival of tamarisk, the channel
<br />and near-channel landforms accommo-
<br />dated these fluctuations by the maintenance
<br />of large, unstable sand deposits that were
<br />set in motion during periods of high dis-
<br />charge, and that were temporarily depo-
<br />sited during periods of low discharge. Such
<br />
<br />1497
<br />
<br />an arrangement prevented overflow of the
<br />stable banks in all but the highest floods,
<br />During low-water periods in the 1920s,
<br />however, tamarisk colonized and stabilized
<br />many of the previously unstable sand de-
<br />posits. Now, after the tamarisk invasion,
<br />overbank flooding is a common charac-
<br />teristic of even modest discharges, because
<br />the channel and near-channel landforms are
<br />immobile and inflexible, Figure 9 shows an
<br />example of the restricted range of adjust-
<br />ments that are possible at one site, Flood
<br />debris and damaged vegetation are com-
<br />mon on the surfaces of the tamarisk-
<br />stabilized features.
<br />Overbank flooding enhances the de-
<br />velopment of the newly stabilized features
<br />
<br />· .!!. Ii
<br />
<br />1871 ,':.:. ., '. . :;.:.:.:....t.......:...:.:.::.;.:..:.::.::......::.....:.:..:::....
<br />'.' ...... .."'.. .. .." ........... ........... ............ ...... .....-.......................... ......... "" ........................O'...... ...... ".. .. ..'
<br />........... ......... ................ .....e................................... '.. ............................ ........ eo' ,
<br />
<br />
<br />..L . ,
<br />
<br />
<br />1909 ::::: :::::: :;:::::::: :::.'::':::':':'..:.','.:::::.::.'.::::.":'f ~
<br />...... -.: .~. :.:.... ....:.:~::: .-:.. .........:....,:...: ::: ::: :~_..::.::... ..~ ...::::::~ -III. -. .~ ~ ~ ~.......... ......... -"III." .. .. ~.. ...... .......:. .::.. :.:
<br /> ;. .. .. F. :... .~::
<br />
<br />
<br />-'-
<br />
<br />\S:'::':::i::f~:~~: ~ n ;H~==~';:~:,:,:,:::::::::.~:':: ~'::.::: ::.::'.::. .:: ',:.:: ::::.':~ :.:..:........:: :. ::.::.:....:.:::: -: ;.:::; :::::::'~':",'.:; :::..111
<br />
<br />J!.
<br />
<br />1914 :,:,':'=::':::~:::'.::::::.::.:~:'::::::::;::: :.:.:;:;:..': :.:,:.:.:.;.:.:.:: ::::~'. ":'" '.. ' ~
<br />..: .. .III.:~....... .. .. .. ~ .... .. ..: .. .. ...... ...... .. ~.. ~.......... .. .. .. .. -III. _ .... ._. .. _.. ...... .. .. .. _.. ..:~.._.. .. .. ...... ... ~.. .." .._.. ..
<br /> .. .. ,-III... ..... eo.... .. .. .... ~...III. eo ...... ...... ... .111.0 00.... .. ... .. .." .. .. .. ..:
<br />
<br />1911
<br />
<br />~.
<br />
<br />
<br />1951 '~:~.:~..:.,....:..!.:::.:...:. ......., .;.;..::.:....:.....:...;.. ,......:::;.... :M
<br />
<br />
<br />~ . .
<br />
<br />
<br />1976 ~:Y;;::::.......,...,.,........................,...,...,......"..".,......,... ......JS.
<br />
<br />
<br />~ STABLE SAND
<br />
<br />~;it.::;l UNSTABLE SAND . TALUS, BEDROCK . CHANNEL WATER
<br />
<br />200 Ft,
<br />50m.
<br />
<br />
<br />
<br />)figure 9. Cross section of Green River at Bowknot Bend, mile 69.2. Sources of data: 1871, Powell-Beaman photo; 1909, Stone photo;
<br />)911, Kolb photo; 1914, LaRue photo; 1951, U.S. Geological Survey aerial photo; 1976, Graf field survey.
<br />l~
<br />
<br />it~ I
<br />
<br />li~1
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