Lake Powell is a storage reservoir. Its level is primarily determined by inflows from the Upper Colorado River Basin and releases through Glen Canyon Dam. This chart uses current reservoir conditions and Colorado Basin River Forecast Center (CBRFC) water-supply forecasts to project storage forward under fixed release assumptions.
In other words, the top chart asks what Lake Powell storage would look like if the rest of this runoff season follows the remaining April-July volumes in the CBRFC Ensemble Streamflow Prediction (ESP) guidance or the CBRFC official forecast guidance after observed runoff to date is accounted for. The lower chart then carries one selected July 31 forecast outcome forward using unregulated inflow paths from actual observed water years.
How to use this chart
The white line shows the current WATER YEAR to date. The colored lines extend storage forward from the current forecast anchor date to July 31 using the remaining CBRFC April-July forecast volumes, starting from the reservoir storage that exists today.
Controls
CBRFC mode
The CBRFC mode control switches between forecast sets with the NOAA Global Ensemble Forecast System (GEFS) weather guidance turned off and on. GEFS-on traces incorporate near-term weather forecast information before the seasonal runoff outlook fans out. GEFS-off traces rely more purely on current basin conditions and historical weather sequences.
Forecast source
The forecast source control switches between Ensemble Streamflow Prediction (ESP) percentiles and the CBRFC official forecast percentiles. ESP is the broader model ensemble built from current basin conditions plus many historical weather sequences. The official forecast is the center’s published operational forecast. Only one forecast family is shown at a time.
Benchmark overlays
The benchmark controls add 2002 and 2018 as reference remaining-season inflow scenarios, regardless of whether ESP or Official is selected. These are not forecast traces. They are historical comparison years laid over the current starting storage from the same point in the runoff season.
Annual Release
The release control sets a fixed annual release target. These options span the most important operating levels in the current Lake Powell framework, from 8.25 million acre-feet down to 4.0 million acre-feet.123
Forecast distribution
The remaining CBRFC April-July forecast volumes are distributed from the current forecast anchor date through July 31 using the historical mean daily unregulated inflow shape. This is a storage translation of a seasonal volume forecast, not a day-by-day operational forecast of releases or reservoir operations.
Bottom chart: continuation and analog years
The lower chart starts from one selected July 31 ESP outcome in the top panel and carries that storage condition forward through the rest of the current water year and the next one. It is not a second CBRFC forecast.
The continuation lines use either simple climatology (average and median) or selected historical water years such as 2002 and 2018. Those historical lines are analog-year inflow paths: they take the chosen July 31 storage level from the forecast chart above, then ask what reservoir storage would look like if inflows after July 31 behaved like that historical year. This makes the bottom panel a way of stress-testing the selected forecast anchor against familiar dry, median, or historically specific runoff sequences.
Lake Powell Storage Projections
Data Sources
- CBRFC GLDA3 water-supply forecast guidance and the derived local JSON used by this chart for ESP, official, and GEFS forecast volumes.45
- CBRFC documentation describing the Ensemble Streamflow Prediction framework used behind the ESP traces shown here.6
- CBRFC backend forecast endpoints used to build the local forecast JSON: one call for GEFS off and one call for GEFS on.78
- USBR Lake Powell Hydrodata used for current storage and elevation, with a derived local water-year JSON used for observed storage history, inflow shape, and release context.910
- Lake Powell elevation-area-capacity data used to translate storage into reservoir elevation.111213
- 1
United States Bureau of Reclamation - Colorado River Compact, November 24, 1922. ↩︎
- 2
United States Bureau of Reclamation - 24-Month Study, March 2026. ↩︎
- 3
United States Bureau of Reclamation - Supplement to the 2007 Colorado River Interim Guidelines for Lower Basin Shortages and the Coordinated Operations for Lake Powell and Lake Mead - Record of Decision, May 2024. ↩︎
- 4
Colorado Basin River Forecast Center - GLDA3 water-supply forecast page. ↩︎
- 5
Derived local dataset used by this chart - glda3-2026.json. It was built from two CBRFC API calls to the same backend endpoint, one with GEFS off and one with GEFS on. ↩︎
- 6
Colorado Basin River Forecast Center - Water Supply Forecasts: Ensemble Streamflow Prediction. ↩︎
- 7
CBRFC backend endpoint used for the GEFS-off forecast payload - GEFS-off API endpoint. ↩︎
- 8
CBRFC backend endpoint used for the GEFS-on forecast payload - GEFS-on API endpoint. ↩︎
- 9
United States Bureau of Reclamation - Lake Powell Hydrodata Dashboard, used here as the source for current storage and elevation, the current storage and elevation series to date, and the 2002 and 2018 April-July inflows. ↩︎
- 10
Derived local dataset used by this chart - lake-powell-water-year.json. ↩︎
- 11
Derived local elevation-area-capacity dataset used by this chart - lake-powell-2018-elev-area-capacity.json. ↩︎
- 12
Jones, D.K., and Root, J.C., 2022, Elevation-area-capacity tables for Lake Powell, 2018, U.S. Geological Survey data release. ↩︎
- 13
Root, J.C., and Jones, D.K., 2022, Elevation-area-capacity relationships of Lake Powell in 2018 and estimated loss of storage capacity since 1963, U.S. Geological Survey Scientific Investigations Report 2022-5017. ↩︎