2012-06-20_PERMIT FILE - C2010089 (88)

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72. alluvial valley floors are, most likely, accumulated in and pass through these narrower valleys. Thus their disturbance may be as important as the disturbance of the actual alluvial valley floor. But those upstream areas were not mapped here. Schmidt (1977) did map the upstream wildlife areas in the areas of northeastern Mon- tana he investigated. Aerial imagery was utilized to assess the relative vigor of vegetation in low- land areas. Relative vigor was shown in color infrared aerial imagery collected during the drier months since the more vigorously growing vegetation is emphasized at this time. When this sign (of growth) is noted, the photo interpreter is also identifying areas where shallow ground water is likely to be available or soil moisture is maintained at a level high enough to sustain the vegetation. There is no equally direct indication of the quality of ground water, though large differences in the quality of ground water would be reflected by the composition of vegetation. It was not possible to assess the species included in the vegetation of sub - irrigated lowland areas using aerial imagery. Therefore, the methodology incor- porated the use of available ground level photographs and other site - specific infor- mation gathered by investigators at a number of mine sites *. Therefore the vegeta- tive data presented are neither detailed nor complete since field surveys were not ,performed. Alluvial valley floor designations were verified where hay cropping in the val- ley floor was observed in the aerial imagery. However, occasional difficulties were encountered in identifying alluvial valley floors. Storage reservoirs located in the alluvial valley floors may restrict the extent of downstream flooding and usually increase the subirrigated area by raising the local water table in the vicinity of the impoundment. Such uncertainties in mapping, and others that exist where irrigated lands border the alluvial valley, were not completely resolved during the investigation. However, the effects of these impediments to accurate mapping were considered trivial for the sites evaluated. The methods used here to identify alluvial valley floors are therefore largely empirical. A definition of a complex interrelationship of geologic, hydrologic, biologic, and topographic conditions that have surface, subsurface, time - related and use- related components was attempted. Since this was done without benefit of ade- quate field data, it is likely that the definition will improve as field data are compiled. It is concluded that alluvial valley floors can be mapped in the manner used here with reasonable certainty. Geological, ground water, and vegetative studies would surely refine this analysis, but such investigations would require additional time and field efforts. The reconnaissance methodology utilized for this report may remain a suitable technique for initially identifying alluvial valley floors. Fur- ther calibration with ground truth and subsurface data to confirm its accuracy would be desirable. But it is believed that the procedures used adequately identify the alluvial valley floor areas that are subirrigated. What is not identified is the hydrologic system that supports the subirrigation. * Mines for which ground -truth data were available in some form and which had been visited include Black Mesa, Rayenta, Craig, Edna, Energy (1, 2 & 3), Seneca, Colo - wyo, Abealoka, Big Sky, Circle West, Rosebud (J iT), Decker, Savage, Spring Creek, Youngs Creek, Navajo, San Juan, Star Lake, Beulah, Center, Falkirk, Gascoyne, Glen - harold, Husky, Indian Head, Velva, Alton, Eagle Butte, Belle Ayr, Big Horn, Black Thunder, Coal Creek, Cordaro, Dave Johnston, East Gillette, Jacobs Ranch, Jim Brid- ger, Lake DeSmet, Medicine Bow, North Rawhide, PSO #1, Rosebud (WY), Seminoe, Soren- son, Welch, Whitney, and Wyodak (See Appendix 1 for locations). 73. Results of Assessment of Alluvial Valley Floors Land Surface of Alluvial Valley Floors Eighty -seven of ninety -two existing or proposed surface coal mine sites were studied. The mine sites are located in an eight -state area (Washington, Arizona, New Mexico, Colorado, Utah, Wyoming, Montana and North Dakota). All states except Washington are loosely grouped under the term "interior western United States ". Using the methodology discussed in the previous section, the locations of allu- vial valley floors were denoted on topographic maps or, if topographic maps were not available, on aerial photographs. These then provided the basis for planimeter measurements. Due to space constraints in preparing this report, neither the topographic maps nor the aerial photographs have been reproduced here, but are avail- able for reference at selected locations *. Table 3 provides the results of the planimeter analysis. The total leased area studied consisted of 369,836 hectares * *. Within this area, 11,002 hectares of allu- vial valley floors were identified. This amounts to 2.97 percent of the coal mine lease areas described in Appendix 1. Malde and Boyles (1976) determined that just under three percent of the strippable coal reserves of the three - county portion of the Powder River Basin they studied were overlain by alluvial valley floors. Of the total of eighty -seven mine sites, fifty -nine (68R) contained areas iden- tified as alluvial valley floors. The two mines on Black Mesa, Arizona, were com- bined and both contained alluvial valley floors. Twelve mine sites of seventeen analyzed in Colorado contained alluvial valley floors. Nine mina sites of the seven- teen analyzed in Montana contained areas mapped as alluvial valley floors. Of the seven mine sites studied in New Mexico, only one contained an area designated as alluvial valley floor. Of the eleven mine sites investigated in North Dakota, all but one appeared to involve some part of alluvial valley floors, neither of the Utah sites involved alluvial valley floors, while in Wyoming twenty -five of the thirty -one mine sites reviewed showed portions of areas designated as alluvial val- ley floors. The single site in Washington included an alluvial valley floor. The largest single alluvial valley floor identified was a 1,046 hectare lowland area within the Glenharold Mine "leasehold" (Mine Number 47 or ND -5). However, the area so identified consisted almost entirely of a segment of the alluvial valley flour of the Missouri River. Without the Missouri River alluvial valley floor area, about ten hectares of alluvial valley floors would be involved in the leasehold. It is probable that the coal now mined at the Glenharold Mine does not continue under the Missouri River. This mine site is an example of one where a large area of allu- vial valley floor within the leasehold is not indicative of an equally large impact on the amount of strippable coal. A similar example is at the Crain Nine_(Mine Number 5 or C -4), in northwestern Colorado. The coal in this area of the Yampa River alluvial valley floor is reported to be too deep for surface mining (Rural Electrification Administration, 1974). The alluvial valley floor identified for the Navajo Nine (Nine Number 36 or NM - 2) is in the San Juan River Valley, north of the northern-most limit of mining to date. The alluvial valley area identified for the Indian Head Mine (Nine Number 49 or ND - * Maps are on file in one copy at the Environmental Protection Agency, Office of Energy Activities, 1860 Lincoln St., Denver, CO. Aerial photography is similarly available. Single copies of the aerial photographs have been sent to the appro- priate states. ** 1 hectare = .00386 square miles, or 2.4704 acres.