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<br />M. Grimm etal. /Geomorphology 00 (1995) ()(}()...(){)() <br /> <br />5 <br /> <br />Table I <br />B~ Creek basin study.site characteristics <br /> <br />Sitel Elevation (m) Drainage area (km2) Channel gradient Nearest USGS gage Paleoflood evidence <br />Turkey C....k" 1768 130 0.025 1.6 kIn downstteam USGS 2 ftood boulder bars <br /> CSG' 06711000 <br />Lower Bear Cr.b 1795 425 0,070 0.8 Ian upstream from USGS 2 flood boulder bars <br /> gage 06710500 <br />Mt. Vernon Cr. 1865 25 0.033 USGS CSG 06710600 upstream 1 flood boulder bar <br /> from mouth <br />Saw Mill Gulchl 1981 6 0.090 I ftood boulder bar <br />Cold Spring Gulchb 2040 14 0.040 absent <br />Parmalee Gulch 2054 15 0,033 USGS CSG 06710900 upstream absent <br /> from mouth <br />Lower Evergreenb 2121 200 0.021 just downstteam from USGS absent <br /> CSG 06710350 <br />Upper Evergreenb 2170 170 0.019 2.4 km upstream from USGS absent <br /> C5G 06710350 <br />Above Evergreenb 2195 110 0.007 5.6 kIn upstream from USGS absent <br /> CSG 06710350 <br />Cub Creekb 2255 130 0.040 2.5 km upstream from USGS fine.grained overbank deposits <br /> CSG 06710400 <br />Upper Bear Cr.'" 2268 249 0.007 I flood gravel bar and fine- <br /> grained overbank deposits <br />Yankee Creek!' 2298 235 0.028 absent <br />Corral Creekll 2300 80 Om8 absent <br />Grass Creekll 2530 6 0.024 absent <br />Unnamed Cree~ 2705 2 0.148 absen( <br /> <br />Iflood-evaluation sites are set boldface. <br />bOast sample site. <br />'USGS CSG: U.S. Geological Survey crest.stage gage. <br /> <br />open.channel flow is commonly unsteady and <br />nonuniform, hydraulic step-backwater routines assume <br />steady flow conditions between cross sections. To pre- <br />dict water-surface profiles associated with gradually <br />varied flows, a necessary assumption is that the head <br />loss at a section is the same for a uniform flow having <br />the velocity and hydraulic radius of the section <br />(O'Connor and Webb, 1988), The assumption of <br />steadiness probably is justified because at peak flows <br />the change in stage is minimal over short distances; <br />thus, flow approximates steady conditions (Davidian. <br />1984; Jarrett, 1986). <br />Step-backwater programs are limited to modeling <br />nondeformable boundaries, such as bedrock channels <br />or channels with minimal erosion. Selection of an <br />appropriate reach may be the most important part of <br />the hydraulic analysis. Good high-water marks, uni- <br />form channel geometry, a uniform or slightly contract- <br />ing reach, fully effeclive cross-sectional area, and <br /> <br />sufficient length are the basic requirements for reach <br />selection (Benson and Dalrymple, 1967; Davidian, <br />1984; Williams and Costa, 1988). Cross-section selec- <br />ticn includes a minimum of three cross sections that <br />represent the geometry of the reach, <br />Channel characteristics are obtained from cross-sec- <br />tion surveys. The standard-step method is used to bal- <br />ance the Bernoulli (energy) equation between adjacent <br />cross-sections (Chow, 1959). Water-surface eleva- <br />tions are determined for each cross-section at a speci- <br />fied discharge. A range of discharges is used to develop <br />stage-<lischarge relations at cross-sections where <br />paleostage indicators (PSI), such as the boulder and <br />gravel bars of Bear Creek basin, have been identified. <br />Paleodischarge estimates are then determined from the <br />elevations of specific PSI, which are assumed to rep- <br />resent minimum flood stage. <br />A number of uncertainties affect paleodischarge esti- <br />mates; therefore, an essential component of a paleo- <br /> <br />Iournal: GEOMOR Article: 368 <br />