Streamflow

Relevance

Observed streamflows are critical to all aspects of policy and management of the basin's 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 both planning and management.

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 most important variants of naturalized flows are

• Unregulated flows (Reclamation) - Gaged flows are adjusted to account for upstream reservoir operations, but not upstream diversions and consumptive use
• Unregulated flows (NOAA CBRFC) - Gaged flows are adjusted to account for reservoir operations, and measured diversions
• 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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Data and tools

Gaged streamflow - Real-time and Historical

USGS Water Watch interactive map

This interactive map shows over 530 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.

USGS NWIS: Map View - Colorado River Basin

This interactive map is similar to the above but shows both the ~530 active gages (dark gray) and the ~380 inactive gages (light gray) across the basin for which daily streamflow over some period has been archived in the USGS National Water Information System. Click on a gage to get see the name; click "Access data" to go to the (old-USGS-style) data page for that gage.

Reclamation Boulder Canyon Operations Office (BCOO) - Data Explorer

Reclamation's BCOO provides near-real-time access to flow data from about 15 gages on the lower Colorado River, below Davis Dam, that are not part of the USGS network.

Colorado DWR Stations - Current and Historical

This interactive map shows over 360 active gages, and over 600 inactive ('historical') gages, within the 4 water divisions that comprise Colorado's portion of the Colorado River Basin. These gages include all those shown on the USGS map, along with others operated by DWR and local agencies. Zoom in to locate a gage of interest, mouse-over to see its name, and click to open a popup showing the length of the record. Click "View more details" to open the data page for that gage.

Natural streamflow (gaged flow corrected for all human modifications)

[Reclamation - Current Natural Flow Data https://www.usbr.gov/lc/region/g4000/NaturalFlow/current.html Name_Tool3]

The Excel file linked on this page contains estimated monthly natural flows for the 20 locations in the Upper Basin, and 9 in the Lower Basin, that are the inflow points for the Colorado River Simulation System (CRSS). Most prominent among these is the Colorado River at Lees Ferry. These flow data are updated every year or so by Reclamation to extend the record forward, but they are always 1-2 years behind the most recently completed water year. Currently (April 2022) the data extend from 1906-2019. Provisional data for the Lees Ferry gage only for the most recent water years are found on this page.

Additional resources

State of the Science Report

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