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Figure 1. The most important stream gage in the basin: Colorado River at Lees Ferry, AZ (USGS ID 09380000). At left is the ~25-foot-tall gage house, built in 1921, as seen from the opposite bank in September 1923; at right is a similar, present-day view of the gage house. Source: Left: Topping, Schmidt, and Vierra (2003),; Right: USGS Arizona Science Centers.

Streamflow observations

Observed streamflows are critical to nearly 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 observations, often after some modification to correct for upstream uses, are used to depict hydrologic variability and trends, in support of both planning and management.

Streamflow gages were first installed in the basin in the late 1800s (Figure 1). 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

These sources of error tend to be greater at times when flows are extremely high or extremely low.

Most gages in the basin have upstream activities that affect the flows (diversions, reservoirs, etc.), and many 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 adjusted to account for upstream reservoir operations, but not upstream diversions and consumptive use
  • Unregulated flows (NOAA CBRFC) - Gaged flows adjusted to account for reservoir operations, and measured diversions (Note: For the inflows to Flaming Gorge, Navajo, and Powell, CBRFC uses Reclamation's unregulated flow values rather than their own definition and calculation.)
  • Adjusted flows (NRCS) - Gage data are adjusted for reservoir operations and diversions, where adjustment data are available
  • Natural (virgin, undepleted) flows - Gaged flows are adjusted for all upstream effects: reservoir operations, diversions, and estimated consumptive use

Natural flows and CBRFC unregulated 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. Reclamation 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. The flow adjustments themselves also have some uncertainty, especially the estimates of broad-scale consumptive (crop) water use that are part of the calculation of natural flows. Reclamation is looking at changing their methodology for these estimates, which could substantially change the natural flow data.

Modeled streamflows

Because of the relatively high density of streamflow observations (i.e., stream gages) available in real-time and as historical records, modeled streamflows are not as prevalent or important for water-supply monitoring as modeled soil moisture values or modeled snowpack values. But modeled streamflows--typically generated by a hydrologic model or land-surface model (LSM)--do have a key role in some applications. Modeled streamflows are inherent to the streamflow forecasting process (e.g., seasonal streamflow forecasts); the forecast model must accurately reproduce historic observed streamflows (i.e., calibration) as a precondition for forecasting. Modeled flows are also developed for real-time and historical applications in catchments or stream reaches where there are no gages, or to represent historical periods before gages were installed.

Figure 2. Active stream gages in the Colorado River Basin, highlighting those that were in drought conditions for April 2022 (monthly gaged flow <25th percentile). Source: USGS Water Watch interactive map.)

Data and tools

Gaged streamflow - Real-time and Historical

USGS National Water Dashboard

This US interactive map shows in the default view all active USGS gages and current streamflow. It can also show many other hydrologic parameters measured at USGS gages and stations, including those related to surface water, groundwater, and water quality, and can also overlay current weather radar and recent precipitation. Mouse-over a gage to see a popup with its name and the data for the selected layer; click on that gage to open the real-time data page for that gage.

USGS Water Watch interactive map

This interactive map is similar to the one above; the linked view of the map highlights over 530 active gages across the Colorado River Basin, and hundreds more in adjacent basins (Figure 2). 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 two 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.

Unregulated inflow - Reclamation

24-Month Study Report - Archive (back to 2010)

Reclamation's 24-Month Study Reports, in addition to projecting system inflows and conditions ~24 months into the future, includes the observed (unregulated) inflows to each system reservoir for 12 months prior to the report date. These observed flows are shaded in gray and labeled "Historical".

Unregulated streamflow and inflow - NOAA CBRFC

CBRFC Streamflow Volumes

From the basin map shown, click the gage of interest so that the popup appears, then the link within the popup to open the webpage for that gage. For the current year and recent years, select the Water Year of interest at lower left (back to 2011); the forecast evolution plot will show that year's daily cumulative observed (unregulated) flows, in orange. For the full record (30-100 years), click Historical Volumes under "Data" in the lower right. A list of all historical April-July unregulated flows, in order of increasing flow, will be shown.

Adjusted streamflow - NRCS

NRCS Interactive Map - Observed Streamflow Volume

This link shows the NRCS Interactive Map customized to show the adjusted streamflow for the current water-year-to-date, as a % of the period-of-record average. Click on a gage of interest to bring up a popup, click on the popup to expand it, and then click on Data Reports and either Water Year Table or Water Year Chart for the current year's monthly flows. For historical flows at that gage, open the Water Year Table, click on the Create/Modify Report tab in the upper left. When the tab opens, click the first checkbox for 'stream volume, adjusted' then below that, change Time Period to "Period of Record", then click on the View Report tab at upper left.

Natural streamflow

Reclamation - Current Natural Flow Data

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. 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 5 of the State of the Science report, Section 5.3 describes streamflow observations and measurement and adjustment procedures in much greater detail.

Streamgaging Basics - USGS Water Resources

This webpage covers the methods of streamflow gages: stage measurement, discharge measurement, and stage-discharge relationships, with links to many related USGS resources.