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

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86. Not specifically addressed in detail as yat are disturbances to areas of ground water recharge for alluvial valley floors. Obviously, water providing subirrigation must be introduced into the alluvial and bedrock aquifers at some location. Little surface recharge to most alluvial aquifers takes place in the immediate alluvial val- ley floor area of any one mine. Therefore, one may envision that the aquifers shown in the preceding cross sections are connected to areas into which sufficient preci- pitation infiltrates to replenish the ground water. And the areas shown in the Figures are connected to surface watersheds which collect water during periods of high flow that can be spread over the alluvial valley floors. If these upstream areas of recharge are disturbed, the water supply in the alluvial valley floor may be significantly affected. Nor has the topography of the alluvial valley floor been adequately addressed. Surface water flow may be sufficient to supply valley floor vegetation. In all cases shown in the preceding figures, it will be necessary to recreate a low flood plain and a stable drainage system after mining. The flood plain may be necessary for periodic flooding and simple spreading of flood waters to restore soil moisture. The stable drainage system is necessary to control erosion and sedimentation. Sta- bility was the aspect of alluvial valley floors of greatest concern to members of the "Study Committee on the Potential for Rehabilitating Lands Surface Mined for Coal in the Western United States" (National Academy of Sciences, 1974, p. 45). Had- ley and Icing (1977) are developing a paper which identifies the problem of recreat- ing a quasi - equilibrium* in stream channels once the longitudinal profile is dis- turbed during surface mining. The most complete published data describing an alluvial valley floor situation known to the authors is that of Van Toast and Hedges (1975). Though Van Voast and Hedges' objective did not include an assessment of agriculturally- important subirri- gated areas, the hydrologic cross sections developed and analyses performed by them are pertinent to this analysis of alluvial valley floors. Figure 8 is a topographic map showing the alluvial valley floor identified by this reconnaissance study along the East Fork of Armells Creek and the locations of three cross sections developed by Van Voast and Hedges. The cross sections are reproduced, in generalized form, in Figure 9. Cross sections B -B' and C -C' intersect the alluvial valley floor of the East Fork of Armells Creek. The East Fork of Annelle Creek is described by Van Voast and Hedges as inter- mittent in the area west of Colstrip, Montana, and perennial downstream of the town. Immediately west of the north -south road the East Fork is somewhat incised, and the subirrigated alluvial valley floor is relatively narrow. During two field inspec- tions conducted in 1976, no hay cropping was observed in the area shown. Two surface - mineable coal seams exist in the Colstrip area, an upper Rosebud seam and a lower McKay seam, separated by "interburden" ranging in thickness from one to twenty meters. Van Voast and Hedges describe the interburden as including local aquifers of sandstone and clayey silt. Water has been obtained from aquifers below the McKay seam for use at Colstrip. Water levels in the alluvium and McKay coal seam near the East Fork of Armells Creek are estimated by Van Voast and Hedges to fluctuate about one meter between the spring and early summer recharge periods, and the fall. Van Voast and Hedges have concluded that the water in the coal -seam aquifers is at least partially confined and that the alluvium along the East Fork of Armells Creek receives discharge from these aquifers. This water then flows downstream in the alluvial aquifer. However, in cross section A -A' (Figure 9), the Rosebud Coal is reported to be recharged by the alluvium. This cross section is located west of * The state of balance or grade in a stream cross section whereby conditions of approximate equilibirium tend to be established in a reach of the stream as soon as a more or less smooth longitudinal profile has been established, even though down - cutting may continue (American Geological Institute, 1972), 87.