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<br />. INTRODUCTION <br /> <br />An interdisciplinary paleoflood study of northwestem Colorado was conducted to help assess the <br />flood hydrology for Elkhead Reservoir in Elkhead Creek Basin near Craig, Colorado, for the Colorado River <br />Water Conservation District (CRWCD). The objective of the paleoflood investigation was to determine <br />estimates of maximum flooding in the basin from evidence preserved in the floodplain. The paleoflood <br />investigations were made from May through July 1995, The paleoflood study was conducted to <br />complement a site-specific probable maximum precipitation (PMP) by North American Weather <br />Consultants (NAWC) in Salt Lake City, Utah, and probable maximum flood (PMF) study by Ayres <br />Associates in Fort Collins, Colorado, The site-specific PMP/PMF study and paleoflOOd investigations <br />were conducted independenUy, <br /> <br />During 1995, substantial peak streamflows occurred from near to record snowpack in most of the <br />Colorado mountains and moderate rainfall along the Front Range in Colorado. On many streams, the <br />recurrence interval of 1995 peak flows exceeded 75 years. Substantial mobilization, transport, and <br />deposition of channel sediments occurred in many Colorado streams, including in Fortification Creek at <br />Craig, These sediments were deposited as fine-grained, slack-water deposits (SWD) at the edge of the <br />stream and coarse-grained flood bars (FB) in the channel and on the floodplain. These deposits (SWDs <br />and FBs) are the most commonly used paleostage indicators (PSis) used to estimate the height of <br />. paleofloods, The 1995 runoff season provided a unique opportunity to conduct a systematic evaluation of <br />the relalion between paleoslage indicators (height of SWDs and FBs) and high-water elevations (HWMs) <br />of flood depths. These results directly affect the reliability and utility of maximum paleoflood estimates for <br />the paleoflood study of streams in northwestem Colorado. Therefore, the methods results of the HWM- <br />PSI comparison are summarized in this report, <br /> <br />BACKGROUND <br /> <br />Worldwide, floods are one of the most destructive events related to meteorological processes. In the <br />United States, an average of 95 people are killed and about $2.4 billion in damages occur annually from <br />floods (U.S. Anny Corps of Engineers, 1989). Poor understanding about floods contributes to <br />unnecessary loss of life and increasing flood damages or costly overdesign and construction of hydraulic <br />structures located in the floodplain (Jarrett, 1991,1993; Baker, 1994). Estimating the magnitude of extreme <br />floods in many river basins is difficult because of the relatively short systematic streamflow-gaging station <br />records, The average period of record is slightly more than 20 years per station in Colorado and generally <br />the longest gaged records are slightly more than 100 years (Jarrett, 1991, 1993). For many streams, <br />gaged records do not contain large-magnitude, low frequency floods. Estimates of large floods are needed <br />to provide accurate magnitude-frequency relations and for design of structures in floodplains (such as <br />dams and highway facilities). <br /> <br />. <br /> <br />For many years, the design criteria for construction of high-hazard structures such as dams have <br /> <br />8 <br />