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

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90. the mapped alluvial valley floor. In cross section B -B', the gravels are recharged by discharges from the underlying McKay coal, indirectly by discharges from the Rosebud coal, and by surface flow along the channel. Farther downstream and out of the surface - mineable coal area (cross section C -C'), the creek receives discharge from the gravels and adjacent consolidated materials, Thus the stream alluvium loses water in the upstream reaches and gains water from the bedrock materials through most of the alluvial valley floor areas. The clay layers shown in cross sections B -B' and C -C' have not been thoroughly described but appear to have potential to hold some water in the overlying silt so as to be available for subirrigation. Mining of the coal seams shown outside the alluvial valley floor in Figure 9 will interrupt the flow of water from the coal seam aquifers to the alluvium during mining. Intercepted water of acceptable quality could be discharged back into the stream (downstream) during mining. Assuming that spoiled overburden removed outside the alluvial valley floor is placed by draglines and /or scrapers (without additional compaction) to approximate the pre - mining configuration of the surface and that water quality is maintained, we concur with Van Voast's and Hedges' preliminary findings that the hvdraogic system cold be reestablis a the marsins-af_the_Easr Eork of Arm Creak. However, the geologic system in the creek itself should probably not be disturbed since the reestablishment of the semi- confining clay layers would be more difficult. Final opinions in this matter would be dependent on more detailed field investigations. Alluvial valley floors overlying a confined coal aquifer appear to create a most difficult situation to mine and reclaim if it is necessary to recreate the confining bed or aquiclude. Thus these situations pose problems for the extraction of under- lying and nearby shallow coals. Substantial lowering of the land surface (elevation) as would occur when mining thick coals under thin overburden could also create situa- tions (shallow water tables and steep surface channels) creating impediments to the reestablishment of subirrigated alluvial valley floors. The greater the area of alluvial valley floors on a leasehold, the greater the likelihood that it will not be possible to mine shallow coal by surface methods without adverse impacts on the alluvial valley floors. While it is obviously necessary to assess the impacts of alluvial valley floor designations on the coal reserves in a site - specific manner, it is suggested that the average amount of coal affected would be about ten percent on a lease tract con- taining coal under an alluvial valley floor. This ten percent was developed earlier in this section by assuming the need to maintain highwall stability along the allu- vial valley floor and the concurrent lose (for economic reasons) of coal under thick overburden. Further refinement of this estimate necessitates additional geohydro- logic and specific mining plan information. Such refinement, presently necessary on a site - specific basis, could decrease or increase this estimate. Obviously there can be situations in which little coal could be mined in a single leaseable coal unit since water quality deterioration will be assured. It is believed that at least a few places exist where essential functions of subirrigated alluvial valley floors can be recreated after mining. A subsequent section of this report suggests infor- mation requirements necessary to make pre - mining evaluations of such impacts. 91. Illustrations of Alluvial Valley Floors Photographs are provided on the following pages to assist the reader in identifying alluvial valley floor areas. The photographs are representative of allu- vial valley floors in Montana, Wyoming, and Colorado. Plates 1 and 2 were taken in spring and fall of 1976 along the East Fork of Armells Creek. Plate 1 is a view to- ward the southwest across the East Fork near County Highway 315. The channel is marked by the bushes in the center background of the photograph. The foreground is covered principally with grasses and big sage. The alluvial valley floor area is about 200 meters wide at this point and is mapped as including the brushy area within the lower terrace which is barely visible in the center of the photograph (see arrow). Plate 2 was taken about five kilometers west of Colstrip from the north side of the East Fork valley. The stream channel area is barely visible where subirrigated brush shows in the left center of the photograph (see arrow). A short distance upstream of this point the alluvial valley floor ends. The stream channel is deeply incised at this location, and the valley floor is a narrow flood plain not more than 100 meters wide. The upland valley is bordered by clinker outcrops associated with the Rosebud coal seam. Plate 3 is a view of the Yellowstone River near Fallon, Montana. Drainages of this magnitude are characterized by wide flood plains, and the alluvial valley floor may extend up into the higher terraces. Though not examined in the field, it is likely that the lower terrace in the distant center of the photograph (see arrow) is an alluvial valley floor. The boundary of the alluvial valley floor along the near side of the valley appears to coincide with the dirt road in the foreground. Plate 4 shows the Tongue River in the vicinity of Birney Day School, Montana. The arrows show the general extent of the alluvial valley floor. Here the alluvial valley floor is relatively broad and appears to be within the flood prone area (areas which have a one in 100 chance on the average of being inundated during any year). The foreground in Plate 4 is near the coal outcrop and, at this point, the surface mineable coal does not underlie the Tongue River (Heide and Boyles, 1976). In the vicinity of the area shown in Plates 5 and 6, the coal does extend under the alluvial valley floor of the Tongue River. This area is upstream of that shown in Plate 4. Plate 5 is a view of the Tongue River looking south - southwest. The southern edge of the alluvial valley floor borders the low hills to the south (see arrow). Plate 5 shows this broad, irrigated field in November 1976. Plate 6 was taken earlier in May 1976, looking southwest of the upstream portion of the Tongue River and shows the western boundaries of the alluvial valley floor (see arrow). The northern limit of the alluvial valley floor is evident along the dirt road in the foreground of Plate 6 (see arrow). This alluvial valley floor is cropped for hay and has a ditch irrigation system. Plate 7 shows the Tongue River near Decker, Montana. The Tongue River Reservoir is visible on the left side of the photograph, while the channel of Spring Creek is located at the foot of the low hill extending into the flat area on the right. The loading silos of the Decker Mine are along the horizon and right of center. The low- land area here produces hay, and the vegetation shows healthy growth during the drier months. Prior to establishment of the reservoir, it is possible the subirrigated area was somewhat smaller. The principal alluvial valley floor in the area remains the relatively narrow valley of Spring Creek (see arrow). The remaining flat area requires additional investigation before it could be judged an alluvial valley floor. Plates 8 and 9 are photographs taken in the fall of 1976 along the Spring Creek drainage basin, about eight kilometers west of the Tongue River. Plate 8 is a view of a wet area along the South Fork of Spring Creek where ground water appears to be discharged. No area of alluvial valley floor has been identified between this area and the lower portion of Spring Creek shown in Plate 7. This wet area occurs along about 200 meters of the stream channel. Plate 9 shows a small ephemeral tributary to the South Fork of Spring Creek. This is not mapped as an alluvial valley floor because of its narrowness, but denser vegetation shows evidence of some water