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2. As previously discussed, a portion of the non-exempt alluvia] valley floor was historically flood <br />irrigated. In addition, a portion is subirrigated. The remaining area is potentially flood imgable. <br />Surface waters which could be and were used to irrigate the non-exempted alluvial valley floor <br />would be diverted from the Highline Canal. Water in the Highline Canal is diverted from the <br />Colorado River approximately one mile upstream of the disturbance. The Highline Canal will <br />not be affected by subsidence or by mine discharges. Therefore, the quantity and quality of <br />surface water which supplies the AVF would not be affected by the proposed operations <br />(4.24.3(3)). <br />Dischazge from the Roadside and Cameo Mines will have a minor salt loading effect on the <br />Colorado River. The affect on the water quality from this discharge will be to increase the <br />specific conductance and total dissolved solids about 0.2 percent. This, in tum, would have a <br />minor effect on the water which supplies the subirrigated portion of the AVF, since rechazge <br />water from the adjacent Colorado River is the source of alluvial groundwater which subinigates <br />the AVF. <br />During mining, the sediment control system will temporarily reduce surface flows and will <br />remove sediment loads. Sediment ponds will not remove dissolved solids. Thus, the mine <br />operation will increase salt loads. Reduction in flow will only occur during significant <br />precipitation events and possibly during spring snowmelt. During these events, flows will be <br />reduced for only a period of 24 hours. Even then, the amount of water stored in the sediment <br />ponds is minor compared to the average flow of the Colorado River (3549 cfs). <br />All effects from the mining operation will beshort-termed. With reclamation and removal of the <br />sediment control system, these effects would be mitigated. Based on the above discussion, the <br />Division finds that the proposed operations will not materially damage the quantity and quality <br />of water in the surface and underground water systems that supply the alluvial valley floor or <br />portions of the alluvial valley floor (4.24.3(3)). <br />3. The mining operations have already disturbed much of the alluvial valley floor within the permit <br />azea prior to August 3, 1977, with constmcGon of the overland conveyor and loadout facility. <br />No additional disturbance of this azea is planned. The essential hydrologic functions of those <br />azeas which will remain undisturbed will be preserved. <br />The permittee will be required to restore the essential hydrologic functions on the entire alluvial <br />valley floor where disturbance has occurred. There aze no "grandfathering" provisions for <br />restoring the essential hydrologic functions on an alluvial valley floor. <br />The permittee identified subirrigation and flood irrigation as the essential hydrologic functions of <br />the AVF. In the past, the Highline Canal, which is immediately upslope of the AVF, has been <br />the source of water for irrigating a portion of the AVF. No irrigation presently takes place in the <br />site azea. Subirrigation is controlled by the water level in the adjacent Colorado River. <br />Reclamation and restoration of the essential hydrologic functions will involve removing all <br />facilities, regrading to the approximate original river terrace topography, replacing topsoil, and <br />revegetating. A flood irrigation system had apparently been abandoned at some point prior to <br />loadout site development in 1977, and azeas designated as historically flood irrigated in and <br />adjacent to the permit azea, have not been flood imgated since that time. The approved <br />50 <br />