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.. Hydrologic Analysis Willow Creek Introduction This report discusses the hydrologic analysis of the Willow Creek watershed near South Fork, Colorado, located completely within Rio Grande County. The report also provides information about the flood risks and hazards along Willow Creek and compiles new and existing information about the watershed. The Million fire burned a portion of the Willow Creek watershed. Because fire can change runoff characteristics, a new Flood Insurance Study was undertaken by the U.S. Geological Survey (USGS) at the request of Federal Emergency Management Agency (FEMA) to predict the long-term effects of the wildfire on the hydrology of Willow Creek. v'" Purpose The purpose of this hydrology report is to document the analysis and hydrologic methods used to determine the flood hydrology for Willow Creek. The report will explain how the peak flows for long-term recovery after wildfire were determined, present the peak flow information for the 10-, 50-, 100-, and 500-year recurrence intervals for each subbasin, and show the variation in peak flow along the floodplain of Willow Creek. The information in this report will be used in the hydraulic analysis of Willow Creek. Acknowledgments This study was authorized by FEMA under Interagency Agreement XXX-200X-IA-_HSFE08-04-X-0004 and was conducted in cooperation John Liou, FEMA project officer, who provided guidance and advice used to complete the study. Description The Million wildfire (fig. 1) started on June 19,2002, near South Fork, Colorado, and burned approximately 9,346 acres and several homes. Elevations in the burned area ranged from 5,800 to 10,500 ft (USDA Forest Service, 2002). Many structures survived the fire, but homes, roads, and bridges are potentially threatened by flooding of Willow Creek, which enters the Rio Grande River from the south just east of South Fork, Colorado. The Willow Creek watershed is underlain by volcanic rocks, landslide deposits, and alluvium in the valleys. Soils on slopes are clayey-skeletal, Montmorillonic Typic Cryalfs (USDA Forest Service, 2002). The headwaters of Willow Creek are located in the San Juan mountains above 12,000 ft elevation. The watershed is steep and Willow Creek occupies narrow channels and canyons until approximately the last 2 miles above the mouth at the Rio Grande River, where the flood plain widens and the stream gradient decreases adjacent to pasture land between 8,000 and 9,000 ft elevation. Near its confluence with the Rio