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~ , Interdisciplinary Paleoflood Investigation of the Muddy Creek Basin for Rltachard Dam near Kremmllng, Colorado Robert D. Jarret!z Hydrologist U.S. Geological :;urvey, WRD National Research Program P.O. Box 25046, MS 412 Denver, CO 80225 In cooperation with Colorado River Water Conservation District Glenwood Springs, Colorado EXECUTIVE SUMMARY Since about 1950, the design crlterfa for dams has included an estimate of the probable maximum flood IPMF), which Is derfved from tile probable maximum precipitation (PMP). Accurately esUmatlng maximum bounds of extreme precipitation and flooding requires a delicate balance between cost and safely. The National Research Council (1994) recognized that the greatest dellclency of the PMP/PMF Is for extreme storms In mountain basins less than about 400 ml2 and the need for regional analy&ls of extreme preCipitation and flooding. A paleoflood study was conducted for RIlschard Dam In Muddy Creek near KremmDng, Colorado. Rltschard Dam Is located at an elevation of about 7.300 ft. and the drainage area upstream from the dam Is 269 mi2. The objecllve of this study was to determIne the magnitude of maximum rainfaU and flooding that has occurred In and near Muddy Creek Basin. An Interdisciplinary paleoflood methodol~ was used In the Upper Colorado River Basin above the Eagle River, whIch Includes Muddy Creek BaSin! and Is a homogeneous hydrometeorologlc region. The methodology Included onsile pafeoRood investigations to determine maximum paleoflood magnitude and ages and analysis of extreme-precipitation ancfflood data, The primary paleoflood evidence utilized In this study was bOuldery flood-bar deposits used as Indicators of past flood levels, which Is long lasting. Analysis of extreme rainfall data for Colorado Indlcates an elevation limit between 6,500 and 7,000 ft In Upper Colorado River Basin. Maximum intense, rainfall above the elevaUon Omit Is about 3 In. in 6 hours and there ~re no known flood-producing rainstorm having an areal extent of more than 10 to 20 m/2. Below . the elevation limit in westem Colorado, the maximum 6-hr amount Is less than about 7 In. and cover I maximum of about 20 to 30 mi2. No evidence of substantial out-of-bank flooding in at least 10,000 years was found In any Investigated stream In the Muddy Creek Basin or In the Upper Colorado River Basin. The maximum paleoflood In Muddy Creek varies from 2,000 to 3,000 ft3/s at the upper USGS gage (near Kremmling) to abOut 4,000 to 5,000 ft3/S at the two downstream USGS gages fabove Antelope Creek and at Kremm~) and at Rl!schard Dam. These are the laruest flows since glaciation ended about 1 0,000 years ago. FI evidence was found related to an Intense 3.2-ln. ralnstonn In about 1 hr that occurred on June 20, 1994. about 1 0 miles north of Kremmllng. The 1994 rainstorm or!marily occurred fNer the lOWer part of Carter Creek Basin below about 7,800 ft and covered about 10 mia. the flood was about 400 ft3/S (unit dlscharue of1 00 ft3/slmi2) In Carter Creek. . A comparison of Rood bars (PSis) that formed durfng record hlah flows durfng 1995 indicates Ihallhe elevation of the top of flood bars generally are about +/-0,5 ft of HWM elevallons for 40 Colorado streams having a broad range of hydrauOc characterfsUc:s. Therefore, use of the top of flood bars as PSis for streams in the Muddy Creek and upper Colorado River Basins provides a hydraulically reliable and accurate esl/mate of the maximum paleoflood depth and discharge. GO'd tEOS9EG 'ON l{I;I.:i O.:ia ES a18 HH8 SE:Sl IH.:i 9B-E -A~ ~.....~ .--h.,_.........