Streamflow

Overview

Streamflow gages were first installed in the basin in the late 1800s. Currently, a network of about 1000?? active gages measure streamflows across the basin, nearly all operated and maintained by USGS and its federal, state, and local partners through the Cooperative Water Program. USGS performs quality control and is the central clearinghouse for online streamflow data, though several partners provide streamflow data through their own websites.

Most streamflow records in the basin are rated by USGS as being within 0-10% of the true value, 95% of the time. Uncertainty in gaged streamflow can arise from

• Errors in measurements of water elevation stage, e.g., due to ice
• Errors in stage-discharge relationship, or rating curve, used to convert the measured water elevation (stage) with streamflow
• Changes in the stream channel, e.g., due to deposition or scouring of sediment

Most gages in the basin have upstream activities that affect the flows (diversions, reservoirs, etc.), and some applications of streamflow data require adjustment or naturalization to remove at least some of these effects, by subtracting or adding back in flow. The two most commonly used variants of naturalized flows are

• Unregulated flows - Reclamation backs out the effects of upstream reservoir operations, but not upstream diversions and consumptive use
• Natural (virgin, undepleted) flows - Reclamation or other agencies back out all upstream effects: reservoir operations, diversions, consumptive use

Natural flows are usually higher than the gaged flows at the same gage, except in basins that receive substantial transbasin diversions, in which case natural flows can be lower than gaged flows. Unregulated flows may be higher or lower than gaged flows, depending on whether there have been net releases of water from upstream storage, and are usually lower than natural flows.

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Figure 1. CAPTION. SOURCE.)

Relevance

Observed streamflows are critical to all aspects of policy and management of basin water supply. Real-time gaged streamflows are used directly in streamflow forecasting, flood warning systems, reservoir operations, diversion scheduling, recreation management, and ecosystem management. Historical streamflow records, often after some modification to correct for upstream uses, are used to depict hydrologic variability and trends in support of planning and management.

Additional resources

State of the Science Report

Chapter # of the State of the Science report, describes...