The Mill Flat Fire Hydrologic and Flood Potential Evaluation

The Mill Flat Fire near New Harmony in southern Utah in 2009 posed a substantial risk of flooding and possibly mud/debris flow from the steep terrain and nearly 200% of average snowpack in the area. This hydrologic evaluation and aerial photographic survey by NRCS Snow Survey quantified in general terms what that risk was based on the snowpack inside the fire area and on the contributing watershed versus outside the area where NRCS has a SNOTEL monitoring station. The assessment included the area's ability to infiltrate both snowmelt and a potential rain-on-snow event based on current soil moisture characteristics as well as measured infiltration rates. We were able to define at each sample location the current snowpack, soil moisture content, and an estimate of infiltration rates and to relate this information to the same information at the SNOTEL site. Large portions of the steep south facing aspects throughout the watershed, up to as high as 2590 meters in Dam Canyon, were bare of snow cover at the time of the survey even though snowpacks were much above average. However, areas that had even moderate slopes as low as 2255 meters on south east, east, north or northwest facing aspects still had substantial snowpacks. Data and analyses from the fire area indicated that the watershed could easily infiltrate much if not all of the snowpack and that the greatest danger of flooding would be a large rain-on-snow event.