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<br />4 <br /> <br />M. Grimm etal. /Geomorphology 00 (1995) {)(){)....()()( <br /> <br />potential, and high susceptibility to erosion (Diebold, <br />1939; Shroba et al., 1979). Sheetflooding, rill erosion, <br />and gullying provide a continuous supply of colluvium <br />to the channels. Open montane forest of ponderosa <br />pine, Douglas fir, and sparse undergrowth (Marr and <br />Boyd, 1979) is supported by these soils. <br />Mean annual precipitation in the Bear Creek basin <br />ranges from 760 mm'at the upper elevations to 400 mm <br />on the plains. Above 2300 m, the basin is dominated <br />by snowmelt runoff, with rainfall-runoff dominating <br />below 2300 m. From historical records, Bear Creek <br />appears to be subject to more frequent cloudbursts than <br />most South Platte River tributaries (Follansbee and <br />Sawyer, 1948), although this may result from its prox- <br />imity to Denver and increased attention to flooding. <br />Streamflow, fed by mountain snowpack, is perennial, <br />with periodic rises from accelerated snowmelt and <br />intense rainstonns. <br />Discharge in Bear Creek has been gaged at Morrison <br />since 1888, and crest-stage gages have been operated <br />throughout the basin since 1978 (Fig, I). Mean annual <br />flow (for 77 years through 1992) at Morrison is 1.5 m3 <br />s -1, with a recorded peak of 244 m3 s - 1 in 1896 <br />(Grimm, 1993). <br />Flash floods are most common on Bear Creek <br />between Evergreen (2135 m elevation) and Morrison <br />(1770 m elevation) (Fig. 1). They are especially fre- <br />quent on Mt. Vemon Creek and Cold Spring Gulch <br />(Diebold, 1939). Twenty.five flash floods have <br />occurred in the Bear Creek basin since 1876, causing <br />45 deaths and extensive property damage and leaving <br />geomorphic evidence in the fonn of flood boulder bars <br />and slackwater and overbank deposits (Grimm, 1993). <br />Flood deposits along Bear Creek are preserved (i) <br />at sites of rapid energy dissipation, such as tributary <br />junctions, abrupt decreases in channel gradient, and <br />abrupt valley expansions, and (ii) downstream from <br />cross. valley glacial moraines that serve as sediment <br />sources. The coarse-grained flood sediments typically <br />fonn longitudinal boulder bars that may be differenti- <br />ated from debris-flow deposits on the basis of clast size <br />and orientation. weathering characteristics, and bar <br />morphology (Costa and Jarrett, 1981; Costa, 1984, <br />1988; Jarrett and Waylhomas, 1995). Deposits from <br />recent floods indicate that, in high-gradient channels, <br />these bars closely approximate the water-surface ele- <br />vation (Jarrett and Way thomas, 1995; Jarrett and <br />Grimm, 1993, unpub!. data). <br /> <br />." <br />dJA 3- <br /> <br />8 6/0 ;{'/ S <br />I <br /> <br />d:O/:<O ;V1 <br /> <br />'" l~-l-t <br /> <br />3, Methodology <br /> <br />We estimated the magnitude, frequency, and geo- <br />morphic effects of flooding at four sites in the Bear <br />Creek basin (Fig. I). In addition, we measured particle <br />size distribution at 15 sites along Bear Creek. Criteria <br />for selection of the four flood-evaluation sites were (i) <br />relatively straight and unifonn reaches that could be <br />most accurately modeled with a step-backwater pro- <br />gram, (ii) existence and degree of preservation of <br />recent flood and paleoflood evidence, and (iii) location <br />in the basin with respect to elevation and hypothesized <br />distribution of flood-producing rainfall. Characteristics <br />of all of the study sites are summarized in Table 1. At <br />each of the four flood-evaluation sites, field methods <br />included estimation of peak discharge, geochronologic <br />examination, and measurements of coarse-sediments, <br /> <br />3.1. Discharge estimation <br /> <br />A laser theodolite was used to survey four to eight <br />channel cross-sections at each of the four flood-evalu. <br />ation sites (Table I), Cross-sections were located to <br />adequately characterize channel geometry for model- <br />ing step-backwater flow (Chow, 1959; Davidian, <br />1984). <br />For conditions of unifonn flow, discharge is usually <br />computed from the Manning equation that involves <br />channel characteristics. water-surface elevations, and a <br />roughness coefficient (Chow, 1959). Downstream <br />changes in the water.surface profile in a unifonn reach <br />are accounted for as losses of energy caused by rough- <br />ness elements in the channel bed. The Manning equa- <br />tion was developed for conditions of unifonn flow in <br />which the water-surface profile and energy gradient are <br />parallel to the streambed and the area, hydraulic radius, <br />and depth remain constant throughout the reach. In <br />natural channels, however, unifonn conditions rarely <br />exist (Jarrett, 1984). We assume that the equation also <br />is valid for nonunifonn reaches if the energy gradient <br />is modified to reflect only the losses that result from <br />boundary friction (Dalrymple and Benson, 1968). <br />Step-backwater analysis was done using WSPRO, a <br />program developed by the V.S, Geological Survey for <br />estimating flow characteristics in rivers (Sheannan. <br />1991). The step-backwater method evaluates energy <br />losses between any two cross sections caused by non- <br />uniform flow conditions (Davidian, 1984), Although <br /> <br />Journal: GEOMQR Article: 368 <br />