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•° N The only anticipated effect to surface water in the Alkali Creek drainage is a slight increase in suspended solids. The light use road construction techniques are in conformance with all applicable design standards; therefore, the slight increase in suspended solids should have an insignificant effect on the quality of water in Alkali Creek. There should be no effect on the quantity of water in the Alkali Creek drainage basin due to the light use road. The roads will be constructed with adequate cross-drains capable of passing the peak flow from a 10-year, 24-hour precipitation event. In addition, there will be no detention structures associated with this road. Based on the above discussion, the Division finds that the proposed operation 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 proposed mining operation will not physically disturb the alluvial valley floor. As discussed above, there will be no effects on the quantity and quality of water supplied to the AVF. Therefore, the Division finds that the proposed mining operations will be conducted to preserve, throughout the mining and reclamation process, the essential hydrologic functions of the alluvial valley floor. Colorado River The applicant identified unconsolidated streamlaid deposits within and adjacent to the proposed permit area which meet the minimum size requirement of 50 feet in width by 10 acres in extent as set by the OSM alluvial valley floor guidelines. These unconsolidated streamlaid deposits border the Colorado River. Three distinct types of unconsolidated deposits were mapped which meet the geomorphic criteria for alluvial valley floors. These deposits are quaternerary terrace deposits, quaternerary transition zone between terrace and colluvial outwash, and quaternerary colluvial outwash. The unconsolidated sediments cover approximately 500 acres in the area of the surface facilities between the Colorado River and the Grand Hogback. Of the 500 acres, 216 acres are colluvial outwash; 136 acres are terrace deposits. Most of the area mapped as terrace and transition zone deposits is presently used for irrigated agriculture. Only a small portion of the area mapped as colluvial outwash is presently used for irrigated agriculture. The source of water for irrigation is the Vulcan ditch which diverts water from Canyon Creek. Canyon Creek is located north of the Colorado River. In addition to the area presently used for irrigated agriculture, two areas exhibited characteristics of subirrigation (Figure 5.2-2). The largest area is adjacent to the Colorado River. The source of the water for subirrigation of this area is most likely the Vulcan ditch, since the area is approximately 20 feet above the level of the Colorado River. The only other area exhibiting subirrigation is in a small ephemeral drainage. This area is split in half by the Vulcan ditch. _P7_