f DEPTH OF CINDER DEPOSITS AND WATER-STORAGE CAPACITY AT CINDER LAKE COCONINO COUNTY, AZ

The 2010 Schultz fire northeast of Flagstaff, Arizona burned over 14,000 acres on the east side of the San Francisco Mountain from June 20 to July 3. As a result, several drainages in the burn area are now more susceptible to increased frequency and volume of runoff and downstream areas are more susceptible to flooding. Resultant flooding in areas down-gradient of the burn has resulted in extensive damage to private lands and residences, municipal water lines, and roads. Coconino County, which encompasses Flagstaff, has responded by deepening and expanding a system of roadside channels. These channels move flood water away from communities and into an area of open Forest Service lands, known as Cinder Lake, where rapid infiltration can occur. Water that has been recently channeled into the Cinder Lake area has infiltrated into the volcanic cinders and could eventually migrate to the deep regional groundwater flow system that underlies the area. It is unknown as to how much water can potentially be diverted into Cinder Lake, and Coconino County is interested in determining how much storage is available. The U.S. Geological Survey conducted geophysical surveys and drilled four boreholes to determine the depth of the cinder beds and their capacity for potential water storage. Results from the geophysical surveys and boreholes indicate that the cinders are underlain by basalt at about 30 ft below land surface. Total porosity for the upper 30 ft of cinders was calculated at 43 percent for an area of 300 acres surrounding the boreholes, which yields a total potential subsurface storage for Cinder Lake of 3,900 acre-ft. Ongoing monitoring of storage change in the Cinder Lake area was initiated using a network of gravity stations.