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

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70. high terraces adjacent to larger streams in the northeastern Montana study area. Along the Yellowstone and Missouri Rivers, somewhat higher terraces exhibit subirri- gated vegetative characteristics. Such characteristics are exhibited on nearly all small tributary streams in regions of wheat farming, producing a network of small alluvial valley floors in the headwaters. It appears then that subirrigation of desirable agricultural forage may occur in a slightly larger area in northeastern Montana than in the area of southeastern Montana studied by Malde and Boyles. Alluvial valley floors in the northeastern Montana study area are shown by Schmidt to exhibit the following additional characteristics relative to those in southeastern Montana: (1) a substantial percentage of valley floors in wheat grow- ing areas are in marsh areas which presently receive excess ground water flow from nearby fields; (2) a substantial percentage of valley floors exhibit slight to severe salinity problems; and (3) larger portions of valley floors are covered, as well as underlain, by thicker, poorly sorted alluvial valley gravels. This last factor can preclude production of sufficient grass for hay production and wildlife usage if the gravel deposit outcrops so as to limit the near - surface water retention capacity and provide poor plant growth medium characteristics. However, the gravels may remain highly productive aquifers and can be important as areas of recharge to downstream alluvial valley floors. In northeastern Montana, Schmidt found that wheat farmers viewed the drainage areas, including alluvial valley floors, as obstacles to development. This appears to be true also in North Dakota. Over the past ten years the amount of land utilized for wheat has grown while hayed lands have decreased (in northeastern Montana). How- ever, ranchers in Dawson County, Montana, interviewed by Schmidt indicated that the amount of land used for hay production will probably increase in response to higher hay prices and recent lower wheat prices. Thus the amount of alluvial valley floor land used for hay production may also increase in the near future. These types of changes show that assessments of land use in an area must be conducted over several years and supplemented by interviews with farmers and ranchers if the agricultural role of the valleys is to be determined. In any one year, an alluvial valley floor may be used only for unimproved rangeland, wildlife habitat and water drainage. Of course, regardless of land use, the physical or natural characteristics of subirri- gated alluvial valley floors probably remain. Methods Used to Identify Alluvial Valley Floors This reconnaissance review of alluvial valley floors in existing and proposed surface coal mine areas in the interior western United States was performed utiliz- ing aerial imagery, reconnaissance field work of other agencies, and limited field investigations by the authors. Data from a wide variety of sources were used to develop the conclusions and hypotheses that follow. A tabulation of the locations (i.e., township, range, and section) of lease tracts for existing and proposed surface coal mines in the interior western United States was provided by the U. S. Bureau of Mines, Intermountain Field Operations Center, Denver, Colorado. Since a field review of the ninety -two identified mine sites was not possible due to time and budgetary constraints, color and color in- frared aerial imagery were obtained for .the majority of mine sites. U. S. Geolo- gical Survey topographic maps, generally at a scale of 1 :24,000, were also obtained when available. The aerial imagery facilitated the assessment of relative heights of stream terraces, widths of lowland areas, and vigor and general types of vegeta- tion. The topographic maps served as base maps and permitted more accurate identi- fication of elevations and location of land unit boundaries. It should be noted that the mine sites considered in this analysis of alluvial valley floors include only those sites scheduled to be in production in the next 71. few years *. The site location information contains only general descriptions of the leasehold areas to the nearest full section and includes areas that do not contain strippable (i.e., surface mineable) coal. Alluvial valley floor determinations in this report therefore relate specifically to the description for each mine site as listed in Appendix 1. As a result, identified alluvial valley floors may encompass areas other than those underlain by strippable coal. This situation resulted from a lack of detailed information with respect to the locations of strippable coal within the proposed mining areas. The classification of lowland areas within the lease tract areas as alluvial valley floors was based principally on the following criteria: (1) underlain by alluvial deposits, where known; (2) on floodplains and low terraces assumed to be the surface features of alluvial deposits; (3) in areas exhibiting vigorous vegetative growth in comparison to adjacent areas and thus where subirrigation is likely; and (4) where valley width is greater than fifteen meters. Lowland drainage areas, floodplain and low terrace, were assumed to have rela- tively thick (on the order of a few maters) alluvial deposits. The higher inter - stream areas and minor alluvial washes are assumed to have thin veneers of alluvium rather than the thicker deposits of water -borne silt and sand materials which are more suitable for growth of forage. Or, they may be sufficiently high and above the water table such that subirrigation does not occur during the dryer months. As noted in the introductory section, in order to support vegetation typical of an alluvial valley floor, the character of this alluvial material must be such that sufficient water is retained for continued plant growth during the dry summer and fall months. Thus areas with a thin veneer of colluvium overlying unweathered strata and gravel - filled areas do not generally qualify as alluvial valley floors since too little soil is present. Also, thick, fine- grained silts or clays (such as lacustrine or playa deposits) may be too impermeable to hold sufficient water for subirrigation of the alluvial valley floor vegetation complex. Lowland areas more than 1.5 meters (on minor streams) and more than 2.5 meters (on principal streams) in elevation above the water level in the channel, or above the intermittent or ephemeral channel bottom, were generally considered to be out of the alluvial valley floor if infrared aerial photography did not show vigorous vege- tative growth during times of lower rainfall and relative to surrounding areas. Simi- larly, narrow streams incised more than 1.5 meters were generally assumed to indicate a water table too deep to sustain agricultural crops. However, it should be noted that crops grown in some situations could be subirrigated by a ground water system far outside the channel and low terraces of the valley floors. In these instances, the water table may be maintained by bedrock aquifers which recharge the colluvium. While this study did not identify any such cases of significance, the potential for such situations remains. The minimum fifteen -meter width for an alluvial valley floor designation repre- sents what is perhaps a lower limit of practical "farmability" and geologic stabili- ty. In fact, the fifteen -meter width represents a practicable limit to identifica- tion of elevations and boundaries on both topographic maps and aerial photographs. It is recognized that, particularly in the Northern Great Plains, some narrower val- ley floors are stable, subirrigated, and farmed. However, it was assumed for this analysis that these narrower valleys are not essential to agricultural operations. It is likely that the narrower drainages located upstream of the alluvial valley floors serve as wildlife habitats to some degree. Further, the water supplies for * An original listing of coal mine sites was developed in 1975 as representing western mines to be in production in 1979. That list was corrected by removing some sites that were scheduled to be underground operations and by adding a few existing mines. However, production estimates were not updated.