The Interstate Commission on the Potomac River Basin (ICPRB)’s Section for Cooperative Water Supply Operations (CO-OP) has published the Water Supply Outlook for May 2026. The Water Supply Outlook is published on a monthly basis between the drier months from April to October, and 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.

According to the report, 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 2 inches of precipitation in April, which is 1.4 inches below normal. As of May 1, the 12-month cumulative precipitation is 37.4 inches, or 2.6 inches below normal. Streamflow has reached historical low records for this time of the year. 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.

There is a 23 to 34 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 23 to 34 percent conditional probability compares to the 8 to 15 percent historical probability and is considered the more reliable indicator.

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.

Click here to visit the Water Supply Outlook page for the full report >>>

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.