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Last modified
1/25/2010 6:25:10 PM
Creation date
10/4/2006 11:04:31 PM
Metadata
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Template:
Floodplain Documents
County
Larimer
Basin
South Platte
Title
Flood Geomorphology of Arthurs Rock Gulch: Paleoflood History
Date
1/1/1994
Prepared For
Larimer County
Prepared By
Elsevier Science Publishers
Floodplain - Doc Type
Flood Documentation Report
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<br />~;:.t..~... :~:~.~~~;:{~;~:'":i't~~:;..;,~_: <br />_- ...... _ ....::.....,~....;...l..~-...--'..-. <br /> <br />" <br /> <br />_. - . .~,- . & ':\'1..... <br /> <br />.......~..,-_.'.- <br /> <br />C.F. Wayrhomas. R.D. Jan"efl / Geomorplwlogy I J (1994) /5-40 <br /> <br />19 <br /> <br /> <br />)ir <br />\ <br /> <br />2 <br /> <br />Fig. J. PhOtograph of pegmatite dike ("peiJfl3tite weir") that.cros.ses channel of ArJUIJ'S Rock Gulch. site S (outlined by dnshed line). Arrow <br />locates gr;modiorite flood boulders (FBw) on pegmatite weit. Dotted lines indicate paJeochanneL Active channel crosses the pegmatite weir (Q <br />the left (south) ofFBw tload boulders. <br /> <br />channel and scattered stands of Ponderosa pine are <br />present in a narrow zone along the channel where it <br />exits Arthurs Rock canyon. <br /> <br />4. Paleoflood evidence <br /> <br /> .. <br /> f <br />!Sits. <br />,cks. , <br />. <br />yof . <br />} <br />low ~ <br />:tive i <br /> j <br /> <br />Imbricate. well-sorted. and rounded bouldery depos- <br />its. longitudinal boulder bars and bepns. isolated <br />"ell'lltic" flood boulders. and localized concentrations <br />of coarse overbank (slaclcwater) sediment are the pri. <br />mary depositional evidence for paleolloods in Arthurs <br />Rock Gulch (Fig. 4). Flood deposits like these are <br />loosely termed slaclcwater deposits because they form <br />during the waning or slackening phase offloods in areas <br />of baclcwater or diminished flow (Baker et aI.. 1979, <br />1983; Kochel and Baker, 1982, 1988). The highest <br />limit of a slaclcwater deposit is a close approximation <br />of the minimum peak flood stage.and thUs can be used <br />as a paleostage indicator (PSI). In .this study we use <br />the boulder surface of the highest boulder per different <br />age deposit as a minimum estimate of the paleostage <br />of the flood flow that formed the deposit. In a compan- <br /> <br />ion paper (Jarrett and Way thomas, 1994) we use such <br />paleostage clata to estimate paleodischarge and flow <br />competence at four sites in the lower and middle parts <br />of the basin. <br />To facilitate geomorphic mapping and correlation of <br />1l00d deposits among sites, flood-boulder deposits were <br />sequentially designateClFBO through FB4. FBO depos- <br />its are oldest and are found at the highest elevations <br />above the active channe~ whereas.FB4 deposits are <br />youngest and are found closest to the active channel. <br />Data were collected ai four sites in the basin to assess <br />the age. weathering characteristics. sedimentology. and <br />general geomorphic relations among deposits. Samples <br />for radiO>;lrbon dating were collected (Table I) and a <br />variety of relative-age criteria (Table 2; Burke and <br />Birkeland. 1979; McFadden et a1~ 1989) were used to <br />infer age relations among flood deposits; Elevation dif- <br />,ferencesof PSI's at each site facilitate distinguishing <br />the different flood deposits. Fluvial geomorphic fea- <br />tures and flood deposits were mapped at about I: 1000 <br />scale, and 5 to 7 cross sections were surveyed at each <br />site. <br />
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