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<br />10 <br /> <br />M. Grimm et ai. / Geomorphology 00 (1995) 000-00O <br /> <br />Tobie 5 <br />Particle sizes upstream and downstream from six major tributary junctions on Bear Creek <br />Sample site Particle sizea (em) Elevation (m) Sorting:::lll (Du - Dt6) I D5Q <br /> D" D", D.. <br />Saw Mill Gulch (u) 3.0 6.4 24.4 1981 3.3 <br />Saw Mill Gulch (d) 4,6 14,9 35.4 2.1 <br />Cold Spring Gulch (u) 3.7 7.3 23.5 2040 2.7 <br />Cold Spring Gulch (d) 5.5 11.6 21.3 1.4 <br />Yankee Creek (u) 3,7 6.7 11.3 2298 1.1 <br />Yankee Creek (d) 4.0 6.7 12.8 1.3 <br />Currnl Creek (u) 1.6 2.8 4.4 2300 1.0 <br />Corrnl Creek (d) 1.1 1.9 3.1 1.0 <br />Gross Creek (u) 1.6 3.4 6.0 2530 1.3 <br />Gl1lSS Creek (d) 2.6 4.4 7.0 1.0 <br />Unnamed Creek (u) 2.3 3.6 5.5 2705 0.9 <br />Unnamed Creek (d) 1.8 2.7 4.4 1.0 <br /> <br />:''016. D50. and DlW represent particle size.for which 16. 50. and 84% of the distribution. respectively, is finer. <br />b (u) means upstream. (d) downstream. <br /> <br />ral Creek, however, has the smallest D,o of the six <br />tributaries compared. We interpret this to indicate that <br />flood hydrometeorology (in this case, the predomi- <br />nance of snowmelt rather than rainfall floods in Corral <br />Creek) is a more important control than drainage area <br />on coarse-sediment characteristics. <br />One of the geomorphic implications of the elevation <br />limit on flash flooding is, therefore, a reversal of the <br />usual downstream-fining trend (Knighton, 1984) in <br />coarse channel sediments. Downstream changes in bed- <br />material characteristics have been attributed primarily <br />to abrasion (Schumm and Stevens, 1973) and soning <br />(Bradley et a!., 1972), although the imponance oflith- <br />ologic controls on weathering (Knighton, 1984), and <br />of sediment supply processes at tributary junctions <br />(Knighton, 1980; Troutman, 1980) have been noted. <br />To our knowledge, however, such changes have not <br />previously been explained in tenns of changes in unit <br />discharge associated with flood hydrometeorology. On <br />Bear Creek the D,o of channel sediments and the size <br />of the largest fluvially transponed clast increase down- <br />stream to the mountain front. and the sorting decreases. <br />probably because of an increased transport capacity <br />associaled with intense rainfalls, This relation is also <br /> <br />influenced by the junction of tributaries below 2100 m <br />that are subject to flash floods. <br />Although the data presented in this paper are limited <br />to a single basin, they illustrate the importance of flood <br />hydrometeorology in controlling both flood magnitude <br />and coarse-sediment distribution. Differences in peak <br />unit discharge and grain sizes above and below about <br />2100 m elevation imply differences in sediment trans- <br />port, channel morphology, and appropriate engineering <br />response. In regions where flood hydrometeorology <br />varies over short distances as a result of aspect or ele- <br />vation, the geomorphic effects of floods may also vary, <br />altering the usual downstream trends observed in basins <br />with consistent flood hydrometeorology. <br /> <br />Acknowledgements <br /> <br />This research fonned a parr of Michael Grimm's <br />M.S. thesis, which was supponed by the U.S. Geolog- <br />ical Survey, We thank Marsha Hilmes for field assis- <br />t,mce, Christopher Waythomas and John Elliott for <br />helpful discussions, and Jim Bennett, Gar Williams, <br />and two anonymous reviewers for thorough reviews. <br /> <br />Journal: GEOMOR Article: 368 <br />