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.

ICPRB Commissioners approve updates to the Potomac River Basin Comprehensive Water Resources Plan.

Click here for a PDF of this announcement >>>

The Interstate Commission on the Potomac River Basin (ICPRB) will use this as a tool to protect water supplies, drinking water sources, water quality, and aquatic life, and encourages other organizations to do the same.

Population changes in the Potomac River watershed (2010-2020).

“The Potomac River watershed is home to 6.9 million people. As the saying goes, we all live downstream. Practices and policies throughout the watershed impact our waterways. We expect this updated plan to continue to serve as a catalyst for sustainable water resources management throughout the watershed,” explains Michael Nardolilli, Executive Director of ICPRB.

The plan addresses water resources challenges to the watershed, including sustainable water uses and supplies, protecting and improving water quality, sustainably managing human land use, and protecting ecological health.

The Potomac River watershed includes parts of Virginia, Maryland, West Virginia, and Pennsylvania, as well as the entire District of Columbia. According to Nardolilli, most of the residents in the watershed rely on the Potomac River or its tributaries for drinking water.

The planning document, titled Potomac Basin Comprehensive Water Resources Plan, was originally published in 2018. The 2023 updates were the result of a yearlong review process by an advisory committee composed of regional governments, nonprofit organizations, and academic institutions. It was officially adopted by the ICPRB Commissioners at the business meeting on March 12, 2024.

“The population in the Potomac River watershed has increased by 13% since 2010. With this immense growth, it is more important than ever that we practice smart and sustainable water resources planning. We think this Comprehensive Plan is the tool to achieve that vision,” says Nardolilli.

According to Nardolilli, milestones from the initial 2018 plan are mostly complete. New goals developed in the recent update are expected to begin in the spring of this year.

“It takes a watershed-wide effort to protect and preserve the Potomac River and its related resources. Through a diverse stakeholder group, the plan provides a voice for people and organizations throughout the basin. Further, the update highlights the need to reach the wonderfully varied communities of the basin, with particular focus on those disproportionately affected by water resources issues,” states Dr. Heidi Moltz, Director of Program Operations at ICPRB and an author of the plan.

Dr. Moltz shares some of the ways the public can get involved which are suggested in the new report:

• Encourage and participate in stewardship activities that improve watershed conditions, such as trash pickups, reducing winter salt use, and picking up pet waste.
• Share the plan with local politicians, water suppliers, environmental organizations, and civic organizations.
• Use ICPRB-created tools such as the flow alteration from impervious cover map, the land prioritization tool, and land use webinars to make land use decisions that support sustainable water resources management.

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FOR IMMEDIATE RELEASE – March 13, 2024

Renee Bourassa, Communications Director
Interstate Commission on the Potomac River Basin | Rockville, MD
rbourassa@icprb.org | 301.417.4371 | ICPRB

The 2023 Updates to the Potomac River Basin Comprehensive Plan can be found on ICPRB’s website at: https://bit.ly/basin_plan

The ICPRB is an interstate compact commission established by Congress in 1940. Its mission is to protect and enhance the waters and related resources of the Potomac River basin through science, regional cooperation, and education. Represented by appointed commissioners, the ICPRB includes the District of Columbia, Maryland, Pennsylvania, Virginia, West Virginia, and the federal government. The Potomac belongs to all.