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<br /> <br />.~__. n. . ....._.~ _.._. <br /> <br />C.F. Way/horn"". R.D. Ja"'tt/G<omorpholo~ Il (1994) 15-10 <br /> <br />the <br />! lnd <br />rest <br />pen <br />red <br />)an <br />the <br />~ine <br />.t~d <br /> <br />gneissic and schistose lithologies, are the dominant <br />bedrock in the foothills zone (Tweto. 1979). Paleozoic <br />sedimentary rocks lap onto the Precambrian core of the <br />Front Range. forming prominent north-south trending <br />hogbacks and strike valleys in the eastern foothills. <br />Much of the foothills zone is below the limits of PIe is- <br />tocene glaciation (Tweto, 1979). <br />Exposed bedrock exhibits varying degrees of weath- <br />ering. The resistant Precambrian rocks form rounded <br />hills and level upland surfaces of moderate relief ( 10- <br />1000 m; Madole et aI., 1987).ln these areas, bedrock <br />is exposed or is mantled by thin colluvium composed <br />of grus. Colluvial deposits composed of weathered rock <br />fragments are abundant on most hillslopes and typically <br />form colluvial aprons and wedges that thicken down- <br />slope. Bouldery debris is transported to bottomland <br />areas mainly by rockfall and debris-flow processes. <br />These deposits are an important source of sediment for <br />transport by flash floods. <br />All areas below about 2300 m in the Colorado Front <br />Range are susceptible to intense rainfall (Jarre[[, 1987; <br />Jarre[[ and Costa, 1988). The foothills zone between <br />1525 and 2135 m is panicularly prone to f1ash:f1ood <br />producing thunderstorms (Henz, 1974; Jarrett, 1987). <br />The Fort Collins area averages about 60 thunderstorms <br />per year, which is greater than most other instrumented <br />foothills locations (Wirshborn, 1978). The southerly <br />flow of moisture-laden air from the GulfofMexico, or <br />from the subtropical Pacific Ocean (the summer mon- <br />soon), frequently initiates widespread heavy rainfall <br />and intense local thunderstorm activity in the Colorado <br />Front Range foothills and piedmont (Hansen et aI., <br />1978). In,summer (June through mid-SJptember) the <br />orographic effect of the Front Range blocks the south- <br />erly flow of humid air. This affects the passage offron- <br />tal boundaries causing unstable meteorologic <br />conditions and upslope winds that result in intense local <br />convective thunderstorms or cloudbursts (Follensbee <br />and Sawyer. 1948. p. 22; Wirshborn. 1978; Hansen et <br />aI.,1978). <br />Streamflow and precipitation (lata, and the spatial <br />distribution -of paleoflood deposits in the Colorado <br />Front Range. indicate that flood discharge decreases <br />dramatically at higher elevation ( > 2300 m) (McCain <br />and Jarre[[, 1976; Jarrett, 1987, 1990). Above about <br />2300 m unit discharge typically is Jess than 1.1 m' s ~1 <br />Ian ", whereas at lower elevations in the foothills, unit <br /> <br />, <br />,=< <br /> <br />--; <br /> <br />:~ <br /> <br />'" <br /> <br />j;-): <br />.,-.- <br /> <br />1 <br />-1 '. <br /> <br />- <br /> <br />. <br />1 <br />i <br />~ <br />1 <br /> <br />17 <br /> <br />discharge commonly exceeds 44 m' s -, Ian -, (Jarretl. <br />1987; Jarrett and Cost:l, 1988). <br /> <br />3. Study area <br /> <br />The Arthurs Rock Gulch drainage basin (Fig. 2) is <br />subdivided into three sub-basins (upper, middle and <br />lower), based on general geomorphic characteristics. <br />The upper pan of the basin ranges in elevation from <br />1980 to 2100 m and is underlain by Boulder Creek <br />Granodiorite of Precambrian age (Fig. 2; Braddock et <br />aI., 1989). In the upper pan of the basin the active <br />channel is str:light and not more than 1 m wide. The <br />active channel gradient is about 0.1\ m/ m. Fluvial <br />deposits in the active channel are limited to a few lon- <br />gitudinal bars 1-2 m in length that are composed of <br />sand- and granule-sized sediment. Some bouldery <br />rockfall deposits,.are found on hillslopes but are not <br />common in or near the active channel. Vegetation in <br />this pan of the basin consists of open Ponderosa pine <br />and Douglas fir forest with an herb and grass unders- <br />tory. Scattered shrubs are present along the active chan- <br />neL <br />The middle pan of the basin (Fig. 2) ranges in ele- <br />vation from 1680 to 1980 m. The channel is incised in <br />bedrock and colluvial deposits, and the gradient is steep <br />(0.17 m/m). Bedrock consists of Boulder Creek Gran- <br />odiorite and discordant pegmatite dikes that locally <br />crop out along the channel. One of tJlese pegmatite <br />dikes -(hereafter called the "pegmatite weir") extends <br />across the channel of Arthurs Rock Gulch forming a <br />prominenl constriction (Fig. 3). Catastrophic paleo- <br />floods breached the pegmatite weir, deposited grano- <br />diorite boulders onits surface, and cuI a narrow, deep <br />notch in the bedrock. Downstream from the pegmatite <br />weir, the channel is confined to a narrow bedrock can- <br />yon,( "Arthurs Rock canyon", FIg. 2), where bouldery <br />flood and rockfall deposits are common. Vegetation in <br />the middle part of the basin consists of open forest to <br />scattered stands of Ponderosa pine with an understory <br />of grasses and low shrubs. A few cottonwood trees are <br />present along the active channel but are restricted to <br />isolated groups. <br />The lower pan of thebasin is downstream from the <br />foothills zone where the channel debouches from <br />Arthurs Rock canyon:"'11.e";;hannel is incised in collu- . <br />vial deposits of Qualernary age and weathered sedi- <br />