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4.8 Hydrology and Water Quality Impact 4.8.6: Mining out Hospital Creek and constructing a berm to form a large detention basin (Mining Plan Option 2) would substantially alter the natural hydrology of Hospital Creek.This option would permanently remove surface water flows from the reach of Hospital Creek downstream of the proposed mining operations leading to — depletion of flows to downstream receiving water bodies, reduction of surface water available for localized infiltration,or elimination of adequate water flow for aquatic and riparian habitats. This would be a significant impact. The proposed project design includes two options for mining near Hospital Creek. The first option would leave Hospital Creek in place and mine both sides of the creek. A 100-foot setback placed between Hospital Creek and the active mining area would serve to protect the creek and allow the creek to flow in its existing channel.The second option would construct a berm to separate mine Phases IH and V and Phases VI and VII to form a large surface water detention basin. Once constructed, the reach of Hospital Creek that traverses the proposed mining area would then be mined out,thus removing the existing alignment of the creek and the materials underlying it. The project applicant would construct an inlet flow dissipation structure at the — location that Hospital Creek currently enters the site and would construct a spillway at the location that the creek exits. Under the first mining option, Hospital Creek would remain an active stream channel with no alteration in existing flows downstream. However,because mining would take place on either side of the channel reach that traverses the active mine area, runoff from the project site would no — longer contribute to flows in the creek. The reduction in flow caused by eliminating the site contribution to Hospital Creek is not a significant loss due to the small volume of annual rainfall, the small contributing area of the project site, and the location of the project site in the lower — portion of the watershed. The second mining option would redirect all flows of Hospital Creek into the active mining area. Flows will enter the project site after crossing the conveyance structure over the Delta-Mendota Canal and there would be no flow in the creek downstream of the project site. As part of this option, a constructed retention pond would be used to manage flood flows. No off-site discharge is planned in this proposed design unless, in the very rare case,the detention pond overflows to discharge water downstream. However, considering the size of the detention basin,it is unlikely that Hospital Creek, in combination with a large storm event,could generate a volume of runoff — capable of exceeding the detention basin volume. In most cases, the detained flows would infiltrate through the bottom of the pond. — According to the significance criteria, the second mining option results in significant impacts because it depletes flows to downstream receiving water bodies and reduces surface water — available for localized infiltration. Discussion of these potential impacts is presented below. Hospital Creek,Lone Tree Creek, and Ingram Creek combine downstream of the project site and enter the San Joaquin River near Vemalis. The flow contribution provided by Hospital Creek represents approximately 50 percent of the total flows provided to the San Joaquin River by these three creeks. Hospital Creek is the largest nearby stream that conveys drainage from the central — Coast Range. Surface water generated from the 32-square-mile watershed drained by Hospital RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-31 ESA/203015 Draft Environmental Impact Report May 2006