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· METHODOLOGY Onsite Paleoflood Investigations For the paleoflood investigation of the Elkhead Creek Basin, the strategy was to visit the most likely places where evidence of out-of-bank flooding might be preserved, had any flooding occurred, When flows (velocity and depth) are large enough, streambed and bank material are mobilized and transported. Such mobilization and transport are a function of channel gradient. As gradient increases, smaller velocities and depths are required to move sediment on the bed of a stream (Costa, 1983). When stream velocity, depth, and slope decrease, flowing water often is no longer competent to transport sediments, and they are deposited as flood bars and slackwater sediments in the channel or on the floodplain (figure 2). The sites where flow competence decreases and flood deposits commonly are found and studied include: (1) locations of rapid energy dissipation, where coarse sediments would be deposited, such as tributary junctions, sites of decreased channel gradient, abrupt valley expansions, or sites of increased flow depth; (2) locations downstream from moraines across valley floors where large floods would likely deposit sediments eroded from the moraines; (3) ponded areas upstream from channel contractions; and (4) locations along the sides of valleys in wide, expanding reaches where fine-grained sediments (SWD) would likely be deposited, In most basins (including the Elkhead Creek Basin), numerous culverts or . bridges act as channel constrictions providing ideal Sediment-depositional sites, either upstream or downslream from these structures and document if flooding has occurred. Flood-transported woody debris also provides a good indicator of flood height. In semi-arid streams, woody debris typically lasts about 60 to 80 years before completely decaying, Onsite paleoflood data included estimates of flood width and depth, channel gradient, maximum particle size in the channel and on flood bars, The paleodepth is established by using the elevation of the top of flood bars in the channel or on the floodplain. Some streams have no well defined flood bars, thus, the elevation of the top of the main-channel bank was used to define the depth and width, which is a conservative estimate. Using the paleodepth and channel geometry, the width and cross-sectional area below the PSI elevation was determined, Paleovelocity was determined from velocity-data relations for Colorado streams (Jarrett, 1984) and other flood data for Colorado streams (U.S. Geological Survey, unpublished flood data). Then, the continuity equation (Chow, 1959) and slope-conveyance method (Bames and Davidian, 1978) were used to estimate paleoflood discharge, Onsite data were collected for accessible sites on Elkhead Creek and its tributaries. Numerous sites also were visited in adjacent and nearby streams in the Yampa, White, Little Snake River Basins above about 6,000 ft in northwestem Colorado only to ascertain if any substantial paleoflood evidence exists. For these sites, the maximum paleoflood discharge was estimated from hydraulic measurements, . Another indicator of the occurrence of intense rainfall (U.S. Department of Agriculture, 1921; Costa and Jarrett, 1981; Jarrett, 1987; Jarrett, 1990b) is the development of rill (light) and or gully (deep) erosion on 15