Climate Change and the Changing Water Balance for California's North Fork Feather River

Climate change has likely had a large role the changing water balance on northern California's North Fork Feather River (NFFR) in recent years. In addition to changes in both snowpack quantity and timing of the spring snowmelt, some of its subbasins are also revealing a declining trend in water year runoff, while others do not. NFFR's complex terrain geometry includes a combination of both windward facing slopes and rain-shadowed leeward slopes that result in a mix of climatic gradients. The combined effect of having relatively low elevation and topographic barriers in the form of mountain ridges provide opportunity for both orographic cooling to take place on the windward slopes and compressional warming to take place on the leeward slopes as the airflow of frontal systems pass through the NFFR Basin. On the leeward, rain-shadowed slopes, air descends and warms quickly through compressional heating. Precipitation amounts quickly diminish as the descending air warms and increases its capacity to hold moisture. Both the Lake Almanor and East Branch of the North Fork Feather (EBNFFR) subbasins are two rain-shadowed subbasins that exhibit a declining trend in water year runoff. Trend declines that approach 308 hm3 (250,000 AF)/year collectively from the two subbasins since the early 1960's were analyzed using a water balance approach to help understand the declining runoff trend in terms of changes taking place at the watershed level. The decrease in both the low elevation snowpack and the water year runoff has resulted in a decline in hydroelectric output and less outflow of the NFFR into Lake Oroville.