What is the Outlook?

Little Seneca Reservoir

ICPRB’s Section for Cooperative Water Supply Operations on the Potomac (CO-OP) was established to serve as a cooperative technical center on water resources in the Potomac basin. CO-OP staff publish the Water Supply Outlook on a monthly basis between April and October of each year. It provides an update on the possibility of water supply releases from the area’s reservoirs based on long-term precipitation data, flows, and other information for the Potomac basin.

Download: Water Supply Outlook – October 2025

Summary/Conclusions

There is an above-normal probability of needing releases from the Washington metropolitan area’s back-up water supply reservoirs through December of 2025. Typically, use of the Jennings Randolph and Little Seneca reservoirs is triggered by low river flows resulting from a combination of dry summer conditions and reduced groundwater levels. September brought continued dryness across the Potomac basin, with only 1.8 inches of precipitation recorded (2.1 inches below normal) and cumulative 12-month rainfall now 4.3 inches below average. Groundwater levels are near long-term averages but slightly less favorable than normal. As a result, drought conditions have intensified in the upper Potomac basin, where roughly 61 percent of the basin is now classified in moderate to extreme drought. In contrast, much of Maryland and Pennsylvania remain normal, and Virginia is under a broad drought watch.

The ICPRB CO-OP is currently operating under its daily drought monitoring protocol to track conditions and coordinate with regional suppliers. No reservoir releases have been required so far this year, and the Potomac River continues to meet all regional demands without augmentation. The Climate Prediction Center forecasts above-normal temperatures and below-normal precipitation through October, with drought expected to persist in the northwestern headwaters. While current water supplies remain adequate, continued dry conditions could increase the likelihood of reservoir operations later in the season.

ICPRB’s Low Flow Outlook

There is a 10 to 24 percent conditional probability that natural Potomac flow will drop below 600 to 700 million gallons per day (MGD) at Little Falls through December 31, 2025. At these flow levels, water supply releases from Jennings Randolph and Little Seneca reservoirs may be necessary. Releases occur when predicted flow is less than demand plus a required environmental flow-by. Drinking water demand ranges from 400 to 700 MGD during the summer months and the minimum flow-by at Little Falls is 100 MGD. Note that natural flow is defined as observed flow at the Little Falls gage plus total Washington metropolitan Potomac withdrawals, with an adjustment made to remove the effect of North Branch reservoir releases on stream flow.
The conditional probability is estimated by analyzing historical stream flow records and considering recent stream flow values, precipitation totals for the prior 12 months, and current groundwater levels. Years with watershed conditions most similar to current conditions are weighed more heavily when determining conditional probability. In contrast, the historical, or unconditional, probability is based solely on the long-term record without adjustment for current conditions. The 10 to 24 percent conditional probability compares to the 7 to 14 percent historical probability and is considered the more reliable indicator.

Adjusted Daily Flow at Little Falls for 2024, 1999, and 2002 as of October 6, 2025.

• USGS Potomac real-time gages

Drought Status:

• US Drought Monitor
• CO-OP Drought Monitoring Updates (link updated July 2022)
• Maryland Drought Status
• Virginia Drought Status
• West Virginia Drought Monitor
• Pennsylvania Drought Status
• Forecasted U.S. Conditions Summary

Recent precipitation and forecasts:

• CO-OP’s Potomac Basin Precipitation Map
• Precipitation maps from the MARFC
• Quantitative Precipitation Forecast, MARFC 3 days
• Quantitative Precipitation Forecast, NWS 1-5 days

Groundwater Conditions:

• Basin groundwater levels

Water Supply Outlook Archive:

2025: April | May | June | July | August | September | October

2024: April | May | June | July | August | September | October | November*

2023: April | May | June | July | August | September | October | November* | December*

2022: April | May | June | July | August | September | October

2021: April | May | June | July | August | September | October

2020: April | May | June | July | August | September | October

2019: April | May | June | July | August | September | October

2018: April | May | June | July | August | September | October

2017: April | May | June | July | August | September | October 

2016: April | May | June | July | August | September | October

2015: April | May | June | July | August | September | October

2014: October

*Due to the persistent dry conditions, the publication period for the Water Supply Outlook was extended.

Please contact us if you would like to be notified when new Water Supply Outlooks are posted.