Unimpaired and Regulated Discharge in the Sacramento River Basin, California

Storage of winter runoff in 43 dams and reservoirs is the cornerstone of the water redistribution system in the Sacramento River Basin. Reservoir storage supplies nearly one-half of the surface water used by agriculture and urban areas in California. Analysis of regulated and unimpaired discharge for the Sacramento River reveals the extent to which water storage and diversions alter both the discharge quantity and regime. Average annual regulated discharge is smaller and less variable than average annual unimpaired discharge, but the differences are not statistically significant. Average monthly regulated discharge peaks two months earlier than unimpaired discharge. Differences between mean monthly unimpaired and regulated discharge for April through October are statistically significant, monthly regulated discharge variability is greater, and the differences in monthly variability are statistically significant for four of these months.