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<br />. smaller and are indicative of insufficient flow to mobilize readily-available streambed material. Data on <br />maximum partides available in the channels (dBed) for transport and those transported by streamflows <br />that are located on in-channel bars (dBar) are presented in table 1. The coarse material on the streambed <br />and under the fine-grained, flood-plain sediments were deposited by glacial outwash about 10,000 years <br />ago. If substantial flooding had occurred since glaciation the streambed material would have been <br />transported and deposited as identifiable flood bars. The knowledge of glaciation, soils development, and <br />a lack of flood-bar development provides a minimum time of at least 10,000 years of no substantial <br />flooding, In all but the very fined-grained channels, the largest partides in flood bars are substantially <br />smaller than available partides even moderate out-of-bank floods would be competent to mobilize and <br />transport (table 1). The streambanks and low-relief floodplains contain few sediments larger than silt and <br />sand-sized particles (figures 5a, 5c, and 5d). Had substantial out-of-bank flooding occurred, the coarse <br />streambed material would be transported onto the floodplain. Soils on the floodplain have developed <br />since glacial meltwash (about 10,000 years ago) (Birkland, 1984). Scour of the fine-grained soils on the <br />floodplain or deposition of coarse-grained material from flooding would be easily recognizable, A lack of <br />flood evidence or non-inundation of valley surfaces provides an age that can be used in flood-frequency <br />analysis. <br /> <br />The lack of flood evidence probably is explained by the climatology of the region. Northwestem <br />Colorado, including Elkhead Creek Basin, has had no documented historical severe flooding. Small <br />. streams such as Fortification Creek in Craig, and Spring, Soda, and Butcherknife Creeks in Steamboat <br />Springs have had localized minor flooding (Federal Emergency Management Agency, 1976, 19n, 1982, <br />1984). However, that flooding was due to undersized culverts or bridges, which usually become clogged <br />with debris or ice, and homes built too close to the streams. Flood damage in northwestem Colorado <br />generally is limited to basement flooding, bank erosion, and inundation of lowlands along streams. The lack <br />of summer rainfall flooding probably is a result of high mountains fOllTling the basin divide, which <br />substantially reduces the flow of wallTl moist air fOIlTl the Pacific Ocean or Gulf of Mexico. <br /> <br />Precipitation in Elkhead Creek Basin (and most of northwestem Colorado) also can be inferred from <br />botanic infollTlation (Hupp, 1988). Sagebrush, which generally grows where average annual precipitation <br />is less than about 15 to 20 in. in Colorado (detellTlined by comparing locations of plant species and <br />average annual precipitation in Colorado), is the predominant vegetation up to within a few miles of the <br />headwaters of Elkhead Creek Basin. The wetter headwater areas of the basin support mature stands of <br />aspen, spruce, pine, and other coniferous species. These tree species generally only grow naturally <br />where average annual precipitation is greater than about 25 to 30 in. The majority of the basin (and <br />northwestem Colorado) has moderately well-drained soils (Soil Conservation Service, unpublished data, <br />1995). Thus, small precipitation amounts (rain or snow) that falls on the relatively flatter, lower-elevation <br />areas in the basin, generally results in little runoff. This is particularly true for the majority of the basins in <br />. northweslern Colorado that have sparse vegetation, If these basins were subject to substantial rainfall, <br />then flood evidence would be preserved. <br /> <br />19 <br />