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8 M. Grimm et al. / Geomorphology 00 (1995) ()()().-{)()() Table 3 Sensitivity analysis of hydraulic factors for the Lower Bear Creek site using step-backwater analysis Run Description.l Discharge (mJ s - I) Cross sections as surveyed n = 0.08. 0.07 nd = 0.3. 1.5 m contractionl expansion coefficients = O. 0.5 slope = 0.07. 85 water-surface = PSI 2 Run 1 withn=O.05.0.04 3 Run I with 11 = 0.03 critical depth calculation 4 Ftll removed from cross sections (avg. 0.6 m), n=0.05 slope=O.07 5 Run 3 with 11 = 0.06 106 110 113 104 In is Manning's roughness coefficient, for main channel and overbank; nd indicates roughness varied with hydraulic depth; PSI is paleostage indicator. ologic ages at these sites. Although repeated timber harvest throughout the basin has resulted in removal of older trees growing on flood deposits, the age of trees present provide a minimum estimate for the age of the flood deposit The radiocarbon ages indicate that the gaged or historically estimated discharges at the study sites have not been exceeded during a period several times the length of the historical record. 4.3. Coarse-sediment characteristics Particle-size distributions for the 19 sites in the Bear Creek basin indicate a substantial decrease in the mean diameter (D,o) from 26 em at the lower elevations to 2 cm in the upper basin, and a corresponding decrease in the size of the largest fluvially transported clast (Fig. 2). At tributary junctions below 2300 m, how- ever, particle sizes are locally controlled by the intro- duction of coarse material from small, flood-producing tributary basins, Comparison of particle-size distribu- tions immediately upstream and downstream from trib- utaries above and below 2300 m (Table 5) indicates substantial differences between paired sites below 2300 m, but very similar distributions above 2300 m. Parti- cle-size sorting also decreases below 2300 m. We were limited to six tributary junctions by the need to avoid sites potentially disturbed by human activities. 5, Discussion and conclusions The existence of an elevation limit on flash flooding in the Colorado Rocky Mountains is supported by dis. charge estimates from flood sediments in the Bear Creek basin. Peak unit discharge in the basin decreases systematically as elevation increases (Table 2), The geomorphi~ implications of this relation may be assessed from coars'e.sediment deposits. Flood boulder bars are absent along the channels at elevations above about 2100 m (Table I), The D,o and maximum clast size of coarse channel sediments decrease steadily up to this elevation (Fig, 2). The large differences in par- ticle.size distributions upstream and downstream from Cold Spring and Saw Mill Gulches (a t-test indicated that the D,o values were significantly different upstream and downstream of these tributaries) suggest that these tributaries are subject to flash floods capable of transporting coarse-grained sediments (Table 5). The particle size distributions upstream and down- stream from the four tributaries above 2100 m, in con- trast, are essentially identical, indicating that these higher elevation basins are not subject to rainfall- induced flash floods. In addition, the degree of sorting [(D84 - DI.) / D,ol increases at the high-elevation sites relative to the Saw Mill and Cold Spring Gulch sites. The channel bed sediment upstream from 2100 m is better sorted than the bed sediment downstream. The effect of the tribu- taries on particle-size distributions at the sites below 2100 m is to introduce coarse-grained material in a variety of particle sizes that substantially change the distributions downstream from the tributary junction. Upstream from 2100 m, the effect of the tributaries is to introduce material of about the same size distribution as that in the main channel. The discrepancy between Journal: GEOMOR Article: 368