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',...... "t~;.~""- '~1~~- ~;~~3-~';""..~~'~:S~~'^: -. ...., ..-':.,-., -.-,." ,- ~~~~~~..1'~ ::'~~'"1"~~' -~ <iiI"~iOC: --<'" "'""~".:'~ '. ~''''~~<oi{~..: ..~.".:.../. - ELSEVIER ""~"""-'-~""""-"'._-"-"--", -.--. GfDMDHPHOlDGY Geomorphology II (1994) 15-40 Flood geomorphology of Arthurs Rock Gulch, Colorado: paleoflood history Christopher F. \Vaythomas', Robert D. Jarrettb au.s. G<ological Survey. 4230 Unh"usity Dr.. Suite 20/. Anchorag~. Alaska. USA "U.S. Geological Survey. MS 4/8. P.O. Box 25046. Denver Federal Cemer, D~nver. CO. 80225. USA (Received February 3. 1993: revised January (9. (994; accepted January J~. 1994) Abstract Episodic late Quaternary flooding is recorded by bouldery deposits and slackwater sediments along Arthurs Rock Gulch. an ephemeral stream west of Fort Collins. Colorado. Flood deposits consist of individual granodiorite and pegmatite bouJders. boulder bars. and coarse overbank sediment that rest on 'erosional terrace segments along the channel. We identified evidence for at Ie:tSl five floods in the lower two thirds of the 1.84 km2 dr:linage basin. Flood deposits are differentiated by their position 3bove the active channel. weathering characteristics. degree of boulder burial by colluvium. amount of lichen cover. 30d position with respect to lerraco and colluvial deposits. Age estimates for the "ood deposits are based on radiocarbon dating. tree.ring analyses. and relative-age criteria from four sites in the basin. At least two "oods occurred in the last 300 years; a tbird "cod is at least SOOO years old. but likely younger than 10.000 yr BP; and lhe two oldest "oads occurred at least 40.000 years BP, '?- " :; 1. Introduction Recent large-magnitude flash floods in the Rocky Mounlain Front Range foothills of Cololado. such as the 1916 Big Thompson flood (Costa, 1918; Shrobaet al.. 1919), have drawn attention to the geomorphic significance offlash floods in the foothills region. Flash floods like the Big Thompson flood are relatively rare events that transport large amounts of sediment, have subst:!Otiallong-lasting effects on channel geometry. and pose significant hazards to humans (Jarrett and Cost:!, 1988). However, the timing of flash floods in the Colorado Front Range foothills is precisely known only for the period ofinstrumental :!Od historic records. or roughly the last 50-15 years (Jarrett. 1981). With few exceptions (Costa, 1918; Waythomas and Jarrett, 1991; Jarrett and Waythomas. 1994) the paleoflood record of the region is unknown. VirtuaII y every drain- 0169.5SSX/94/S07.00 C 1994 EI...ier Science B.V. All rights ....rved SSD/O 169.SS5X (94 )00004.B !it :'J! :Ol '" age basin in the region contains stratigraphic and sed- imentologic evidence of fonner flash floods (Bradley and Mears. 1980; Costa, 1983; Waythomas and Jarrett, unpubl. neld data); however. a temporal framework for these floods is lacking. In this paper we (I) prese~t evidence for multiple late Quaternary paleofloods in Arthurs Rock Gulch. a small ~phemeral drainage basin in the Colorado Front Range foothills near Fon Collins. Colorado (Fig. I); and (2) describe our approach to paleoflood retrodiction in this geomorphic setting. Arthurs Rock Gulch is a relatively small. ungaged catchment (drainage area = L&4 km') and' contains abundant bouldery deposits that formed mainly by flash flooding. Similar basins are common in the Colorado Front Range foothills {Jarrett. 1981}. Thus, the results of this study provide an example of the frequency and geomorphic effects 'of 'extreme flooding in a typical small Front Range foothills drainage'basin.