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4-3 <br />4.1.3.3 Geomorphic Criteria <br />4.1.3.3.1 Alluvial Sediments of East Salt Creek <br />The alluvial sediments of East Salt Creek comprise a well- <br />defined, wide strip of sediments which can be clearly dis- <br />tinguished from other geomorphic units. <br />The channel of the creek frequently has eroded into these <br />deposits and, as a result, the creek flows at an elevation <br />between ten to twenty feet below the surface of the river <br />terrace. Because of this deep erosion, the active floodplain <br />is absent in most cases. It has developed locally but is <br />limited in width. <br />Above the present creek bed, the alluvium (forms) a distinct <br />terrace that constitutes a former floodplain. <br />There is no historical data available on the thickness of <br />• the alluvial deposits of the East Salt Creek. During the <br />installation of groundwater monitoring wells, the thickness <br />was determined to be in excess of eighty feet (see Figure <br />9.1-25). Exploratory drilling performed north of the site <br />showed thicknesses in excess of one hundred feet. <br />The width of the alluvial sediment of East Salt Creek in- <br />creases generally toward the south. The valley is wider and <br />the alluvial sediments extend into wide and mostly short side <br />valleys that have developed with the ephemeral tributaries <br />of East Salt Creek. The boundaries between East Salt Creek <br />alluvium and the alluvium of its tributaries are difficult <br />to distinguish. <br />4.1.3.3.2 Other Alluvial Sediments <br />As indicated on Figure 4.1-15, there are numerous side valleys <br />within the area. On the east side of the East Salt Creek, <br />• they are represented by McClane Canyon in the northern part <br />