Streamflow

Overview

Streamflow gages were first installed in the basin in the late 1800s. The primary stream-gage network is the over 500 active gages 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. State and local water agencies also operate and maintain their own gages, typically on diversion structures and at reservoirs.

Most USGS 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

Gaging errors also tend to be higher when flows are extremely high or extremely low.

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) after the fact 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 adjusts gaged flows to account for upstream reservoir operations, but not upstream diversions and consumptive use
• Natural (virgin, undepleted) flows - Reclamation or other agencies adjust for 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 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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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.

Data and tools

Gaged streamflow - Real-time and Historical

USGS Water Watch interactive map

This interactive map shows over 500 active gages across the Colorado River Basin, and hundreds more in adjacent basins. Zoom in to see all gages, mouse-over a gage to see a yellow popup with its name and current streamflow statistics, and click on that gage's name within that popup to open the real-time data page for that gage. On the real-time data page, click "Change time span" to plot and retrieve historical gaged flow data.

Additional resources

State of the Science Report

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