A Study of Peak Winter Flow Rates in the Feather River Watershed, California

The snow-dominated Feather River watershed, located in the northern Sierra Nevada of California, is a primary source of water for California’s State Water Project. The state relies heavily on snowpack in the Sierra Nevada to provide water resources for consumptive use. Runoff derived from rain and snowmelt in the Feather River watershed is collected and stored within Lake Oroville. The watershed encompasses 3,200 square miles of land with elevations ranging between 600-3,000 m with most of the basin lying at elevations between 1,200-1,800 m. Brightband heights derived from an upstream 915 Mhz wind profiler at Chico, California and a Frequency-Modulated Continuous Wave radar at Oroville, California for the periods covering water years 2003-2017 and 2011-2017, respectively, have shown median winter snow levels of 1,480 m and 1,620 m, respectively. Since these brightband heights, and most of the basin, lie between elevations of 1,200-1,800 m, small temperature fluctuations during winter storms can cause abrupt snow level changes leading to major flood events. We identified peak winter flow rates at Merrimac, a relatively unimpaired gage on the Middle Fork of the Feather River and examined the temperature changes during these streamflow peaks at nearby hourly surface weather stations. The temperature behaviors bracketing each peak flow by 12 hours was used in an empirical calculation to estimate the percent of precipitation falling as snow. We then estimated the percentage of the Feather River basin receiving rain or snow. These values were compared to the brightband heights at Chico and Oroville. We find reasonably good agreement between empirically estimated snow fractions and brightband heights, indicating that hourly temperature observations can provide a measure of the fraction of a basin receiving frozen or liquid precipitation in the absence of brightband observations. We also identified the most sensitive snow levels that yield high inflows into Lake Oroville.