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f Yampa River/Big Bottom Alluvial Valley Floor The approximate boundary of the Yampa River/Big Bottom alluvial valley floor is shown on Map IV-8 of the permit application. Hay fields and pasture located in the Big Bottom area are flood irrigated. The main irrigation ditches are shown on Map IV-2. Crop production may be augmented by subirrigation, but the regional practice of flood irrigation is dominant. Thus, the essential hydrologic function of this alluvial valley floor is the capacity for flood irrigation. The Yampa River/Big Bottom alluvial valley floor will not be affected by the Eagle No. 5 Mine. Those portions of the Yampa River alluvial valley floor that will be undermined have already been undermined by past mining activities at the Eagle No. 5 Mine. No additional mining is proposed beneath the Yampa River by the Eagle No. 5 Mine. No surface facilities are proposed to be located on the alluvial valley floor. The quantity of ground water in the alluvial aquifer of Big Bottom would not be significantly impacted by the Eagle No. 5 Mine, given the limited extent of mining under the alluvial aquifer, and given the location of the mine workings in relation to ground water flow. The areas of the Yampa River alluvium which have been undermined by the Ea91e No. 5 Mine are located at the downstream end of the Big Bottom alluvial valley floor. Thus, these areas are downstream of the diversion points which supply Yampa River water to the Big Bottom area. Therefore the Eagle No.S Mine will not impact the quantity of water supplied to the Big Bottom alluvial valley floor. The Eagle No. 9 Mine, however, will undermine the Yampa River/ Big Bottom alluvial valley floor. This undermining could result in subsidence of irrigation ditches and the land surface of the Big Bottom alluvial valley floor. Subsidence of the Big Bottom alluvial valley floor would result in an undulating topography of troughs and ridges. If this type of topography were allowed to develop, there could be a disruption in the supply of flood irrigation waters through grade changes on irrigation ditches, and there could be ponding of water on the irrigated land surface. Both of these subsidence effects would result in the inability of the Big Bottom to support flood irrigation, the essential hydrologic function of the alluvial valley floor. Therefore, the potential for impacts to this alluvial valley floor are greater at the Eagle No. 9 Mine than at the Eagle No. 5 Mine. Empire Energy Corporation has made several pro3ections of the extent of subsidence due to mining under the Yampa River and the Big Bottom alluvial valley floor. For complete extraction, the company has predicted 2.6 feet of subsidence. For partial (50X) extraction, the company has predicted 0.0 feet of subsidence as a maximum (no collapse of support pillars), and they predict 1.1 feet of subsidence as a worst case (e.g., collapse of all support pillars). The company has designed the support pillar size and spacing to support the overburden strata and to prevent subsidence of the land surface. The designs, however, are only predictive.