Streams, rivers, lakes and reservoirs - collectively referred to as surface water - are important natural resources for irrigation, public supply, wetlands and wildlife. Surface water is also measured as annual runoff, which is the amount of rain and snowmelt drainage left after the demands of nature, evaporation from land, and transpiration from vegetation have been supplied. It supplies most of our basic water needs.

The California Water Science Center measures the flow of surface water, and annual runoff, using a network of more than 500 streamgages that collect real-time data at locations across the state. These gages are part of the larger USGS national streamgage network that contains about 8,000 continuous-record stream-gaging stations. The stream-gaging network provides a continuous source of well-archived, well-documented, and unbiased water data. Streamflow data assist water managers in making daily operational decisions for dealing with water requirements for municipal, industrial, and agricultural purposes, demands for hydroelectric power generation, and for controlling water storage in reservoirs to ensure adequate water supply.