Laserfiche WebLink
<br />coordinated to stable features on the aerial photographs for reference points. Coordinates <br />for the reference points were obtained from digital orthophotographs. <br />Each deposit was mapped according to depositional setting or facies, elevation <br />category (formative discharge), surface texture, and extent of vegetation cover. The map <br />units are similar to those used by Schmidt and Leschin (1995), Schmidt et al. (1999), and <br />Schmidt et al. (2002). The distinction between pre- and post-dam deposits is also <br />consistent with that of Hereford et al. (2000). <br />Depositional facies were determined by interpretation of the photographs in stereo <br />and by field inspection. Colorado River alluvial deposits were mapped either as fine- <br />grained eddy sand bars, fine-grained channel-margin deposits, or gravel bars. Eddy sand <br />bars were further subdivided into separation, reattachment, and undifferentiated eddy bars <br />according to the classification scheme proposed by Schmidt and Rubin (1995). Channel- <br />margin deposits are typically linear river-parallel bar and bank deposits, but include all <br />Colorado River alluvium not deposited in eddies (Grams and Schmidt, 1999, their Figure <br />4.7). Mapping was checked in the field in September 2000. A complete description of <br />the map units is included as Appendix D. <br />The post-Glen Canyon Dam flow regime and the timing of aerial photographs <br />makes it possible to classify post-dam deposits along the Colorado River into elevation <br />categories according to formative discharge, which in many cases corresponds directly <br />with deposit age. The October 1984 aerial photographs were taken following the 1983 <br />post-dam flood of2755 m3/s and the 1984 flood of 1648 m3/s. Because flows had just <br />dropped from about 708 m3/s prior to the photographs, that water's edge was visible as a <br />line on the photographs. Deposits below the 708 m3/s stage were mapped as fluctuating- <br />flow deposits (Figure 3). Above this abandoned high-water line were the bare sand <br />deposits from the high flows of 1984, mapped as high-jIow deposits. The deposits from <br />the 1983 flood were also distinct, and are higher in elevation than the 1984-flood deposits <br />and were mapped as flood-sand deposits. Alluvial deposits above the 1983 and 1984 <br />deposits and lacking evidence of deposition in the previous two years were mapped as <br />pre-dam deposits. Pre-dam terrace deposits include the levels mapped as high tamarisk <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />18 <br />