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Alluvial Valley Floor Demonstration for Hayden Gulch Loadout <br />Introduction. An alluvial valley floor (AVF) study was performed by Keammerer <br />Ecological Consultants and Golder Associates, Inc. during summer and fall of 1990 <br />to evaluate drainage systems to the south and southeast of Hayden, Colorado. The <br />AVF study was performed for Seneca Coal Company's (SCC) Seneca II-W Mine. Seneca <br />Coal Company is a corporate affiliate of Hayden Gulch Terminal, Inc. The <br />information presented in this AVF demonstration for the Hayden Gulch Loadout <br />(Loadout) is adapted from the Seneca II-W Mine study (Seneca II-W Mine, Mining and <br />Reclamation Plan, Tab 16, Protection of the Hydrologic Balance). <br />The drainages that were evaluated for the Seneca II-W study include Dry Creek, <br />Sage Creek and Grassy Creek. The study area included the reach of Dry Creek from <br />its headwaters downstream to the Town of Hayden, which is downstream o£ the <br />Loadout. Sage Creek and Grassy Creek will not be specifically addressed in this <br />demonstration because there is no hydraulic communication between these stream <br />systems and the Loadout facility. The lateral extent of the study area was <br />defined by the extent of valley fill and alluvial deposits. Color aerial <br />photographs (1"=1,000' scale), false color infrared photographs (1"=39,000' scale) <br />and U.S. Geologic Survey (USGS) surficial geology maps were utilized in addition <br />to extensive field checking. <br />Geomorphology. A geologic deposits map (this map is included in the Seneca II-W <br />permit [Permit C-S2-057] as Exhibit 16-2) was constructed to analyze the <br />geomorphology of the area, Six types of geologic deposits were identified along <br />stream courses. These were anthropogenic fill, alluvium, valley fill deposits, <br />fan deposits, colluvium and landslide deposits, Anthropogenic fill (man-made) was <br />used as the map unit for some of the larger earthfill embankments. Alluvium was <br />mapped in areas where the floodplain was broad and it was clear that few of the <br />deposits in the valley bottom were related to the immediately adjacent valley <br />slopes (e.g., the landscape in Sections 22 and 23 upstream of the Loadout <br />facilities area). Typically, the alluvium in the broader floodplain areas <br />contained more sand and gravel than was present in the valley fill and other <br />deposits. <br />Valley fill deposits, as used on the geologic map, comprise alluvium, colluvium, <br />fan deposits and landslide deposits. In areas with narrow valley floors, and <br />where hillslopes adjoin floodplain terraces, the valley bottoms consist of more <br />TR-07 19-3.1 Revised 03/06 <br />