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<br />M. Grimm etal. /Geomorphology 00 (1995) {)(){)....()()(
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<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).
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<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
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<br />Journal: GEOMQR Article: 368
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