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 – June 2026

Summary/Conclusions

There is an above-normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2026 summer and fall seasons. Typically, the use of the Jennings Randolph and Little Seneca reservoirs is triggered by low river flows resulting from a combination of low summer precipitation and low groundwater levels. The Potomac basin upstream of Washington, D.C., received 4.9 inches of rain in May, which is 0.8 inches above normal. After a dry start to the month, widespread rainfall brought monthly totals above normal. While this rain provided some reprieve, sustained improvement has not yet been observed. As of June 1, the 12-month cumulative precipitation total was 33.6 inches, which is 6.5 inches below normal. Streamflow is currently below normal. Groundwater levels remain below normal across much of the basin. The U.S. Drought monitor indicates extreme drought to abnormally dry conditions are present in the Potomac basin. The seasonal drought outlook indicates drought Is likely to persist over the coming months. A Drought Watch has been declared for the metropolitan Washington region. Authorities are asking for voluntary water conservation among residents and businesses in the region. Reservoirs remain near full.

At present, there is sufficient flow in the Potomac River to meet Washington metropolitan area’s water demands without augmentation from upstream reservoirs. If low‐flow conditions worsen, the metro area is well‐protected from a water supply shortage through well-established contingency plans.

ICPRB’s Low Flow Outlook

There is a 21 to 37 percent conditional probability that natural Potomac flow will drop below 600 to 700 million gallons per day (MGD) at Little Falls through December 31 of this year. At these flow levels, water supply releases from Jennings Randolph and Little Seneca reservoirs may occur. 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 21 to 37 percent conditional probability compares to the 8 to 15 percent historical probability and is considered the more reliable indicator.

Adjusted Daily Flow at Little Falls shows below historical flows for this time of year (June 10, 2026).

• 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:

2026: April | May | June | July | August | September | October

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.