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<br />. hydrometeorologic assessment was made to provide additional supporting information on maximum-flood <br />potential in Elkhead Creek Basin, This study was done to complement a site-specific probable maximum <br />precipitation (PMP) and probable maximum flood (PMF) study and was conducted independently. <br />Paleoflood estimates were made for 93 sites above about 6,000 ft in northwestem Colorado, primarily in <br />the Yampa and White River Basin. <br /> <br />Paleoflood estimates, which have an approximate error of +/- 25 percent in mountain streams, were <br />made for 93 sites in northwestem Colorado, No evidence of substantial out-of.bank flooding in at least <br />10,000 years was found in any stream in Elkhead Creek Basin or vegetated basin in northwestem <br />Colorado above about 6,500 ft. The maximum paleoflood in Elkhead Creek varies from 1,700 to 3,300 ft3/s <br />for sites upstream from Elkhead Reservoir to 4,500 to 4,750 ft3/s for sites downstream from Elkhead <br />Reservoir. For comparison, the maximum paleoflood in the Yampa River is about 10,000 ft3/s at <br />Steamboat Springs and is about 31,000 ft3/S at Maybell in about the past 10,000 years. Evidence of <br />about a 1 ,300 ft3/S flood was found in the Fortification Creek Basin at Craig, This 1o-year flood resulted <br />from about 1 ,34-in, 24-hr rainstorm on May 30, 1995 measured at the Craig 4SW precipitation gage; larger <br />rainfall amounts may have occurred upstream from Craig. This rainstonn covered about 30 mj2 at an <br />elevation of about 6,200 ft to 6,800 ft. Craig residents indicated that the maximum flood known occurred in <br />May 1984 and provided estimates of flood stage. The estimated peak discharge was about 2,800 ft3/S, <br />which is about a 1 OO-year flood. A number of basins are steep and have sparce vegetation cover in <br />. northwestem Colorado such as Piceance Creek Basin west of Meeker. Thus, a few inches of rain in <br />several hours have larger peak flows than vegetated basins, but still substantially less runoff than basins <br />below about 6,500 ft in the Colorado River Basin or below about 7,500 ft in eastern Colorado, <br /> <br />A definitive relation of the height of paleostage indicators such as the top of flood bars to actual flood <br />height was developed. A comparison of flood bars (PSis) that fonned during record high flows during <br />1995, which included the 1995 flood in Fortification Creek, indicates that the elevation of the top of flood <br />bars generally is within about +/- 0,5 ft; most PSis are within +/-0,2 ft of HWM elevations. PSis for <br />streams having a gradient larger than about 0.04 ftIft can protruded ~ the maximum water surface. <br />Therefore, use of the top of flood bars as PSis for streams in the Elkhead Creek Basin paleoflood study <br />provides a reliable estimate of maximum paleostage and discharge. <br /> <br />An analysis of 3,512 station-years of record for 198 gages was made to define an envelope relation of <br />maximum unit-discharge and elevation for northwestem Colorado above 5,200 ft. The maximum unit <br />discharge in northwestem Colorado is 477 ft3/slmi2 for Cottonwood Gulch near Rio Blanco, which is a 1.1 <br />mi2 basin at an elevation of 6,335 ft. in Piceance Creek Basin. Envelope curves show that unit discharge <br />has not exceeded 100 ft3/slmi2 above an elevation of about 7,000 ft, except for small (<5-10 mi2) basins at <br />high elevations, Maximum unit discharge from snowmelt of 220 ft3/slmi2 is for a 0.7 mi2 basin located at an <br />elevation of 9,875 ft near Buffalo Pass, which is the snowiest part of Colorado. As drainage-basin size <br />. increases, maximum unit discharge (rain and snowmelt runoff) decreases to less than 50 ft3/sI mi2 for <br />basins larger than about 50 mj2. The maximum unit discharge for Elkhead Creek near Elkhead gage is 44 <br /> <br />32 <br />