8 August 2013 (CalEPA) – Since 1906, the fraction of annual unimpaired runoff into the Sacramento River that occurs from April through July (represented as a percentage of total water year runoff) from the accumulated winter precipitation in the Sierra Nevada, has decreased by about 9 percent. The Sacramento River system is the sum of the estimated unimpaired or natural runoff of the Sacramento River and its major tributaries, the Feather, Yuba and American Rivers. “Unimpaired” runoff refers to the amounts of water produced in a stream unaltered by upstream diversions, storage, or by export or import of water to or from other basins. This decreased runoff was especially evident after mid-century; the recent two decades seem to indicate a flattening of the percentage decrease. There is no significant trend in total water year runoff, just a change in timing of runoff.
Large accumulations of snow occur in the Sierra Nevada and southern Cascade Mountains from October to March. Each winter, at the high elevations, snow accumulates into a deep pack, preserving much of California’s water supply in cold storage. Spring warming causes snowmelt runoff, mostly during April through July. If the winter temperatures are warm, more of the precipitation falls as rain instead of snow, and water directly flows from watersheds before the spring snowmelt. Other factors being equal, there is less buildup of snow pack; as a result, the volume of water from the spring runoff is diminished. Lower water volumes of spring snowmelt runoff may indicate warmer winter temperatures or unusually early warm springtime temperatures.