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During mining, the sediment control system will temporarily reduce surface flows and will remove sediment loads. Sediment ponds will not remove dissolved solids. Thus, the mine operation will increase salt loads. Reduction in flow will only occur during significant precipitation events and possibly during spring snowmelt. During these events, flows will be reduced for only a period of 24 hours. Even then, the amount of water stored in the sediment ponds is minor compared to the average flow of the Colorado River (3549 cfs). Salt loading effects on the sediment pond discharges are unquantifiable at present because none of the ponds have discharged. However, the comparatively minor quantity of discharge from the ponds would result in a negligible effect on water quality of the Colorado River following dilution. All effects from the mining operation will be short-termed. With reclamation and removal of the sediment control system, these effects would be mitigated. Based on the above discussion, the Division finds that the proposed operations will not materially damage the quantity and quality of water in the surface and underground water systems that supply the alluvial valley floor or portions of the alluvial valley floor. 3. The mining operations have already disturbed much of the alluvial valley floor under permit No. 77-442, with construction of the overland conveyor and loadout facility. No additional disturbance of this area is planned. The essential hydrologic functions of those areas which will remain undisturbed will be preserved. The applicant will be required to restore the essential hydrologic functions on the entire alluvial valley floor where disturbance has occurred. There are no "grandfathering" provisions for restoring the essential hydrologic functions on an alluvial valley floor. The applicant identified subirrigation and flood irrigation as the essential hydrologic functions of the AVF. In the past, the Highline Canal, which is immediately upslope of the AUF, has been the source of water for irrigating a portion of the AVF. No irrigation presently takes place in the site area. subirrigation is controlled by the water level in the adjacent Colorado River. Reclamation and restoration of the essential hydrologic functions will involve removing all facilities, regrading to the approximate original river terrace topography, replacing topsoil and revegetating. A flood irrigation system will be reestablished and the system will be connected to the highline Canal. This will provide for restoration of the essential hydrologic function of flood irrigation. -22-