Water Supply Outlook and Status

Interstate Commission on the Potomac River Basin

What is the Outlook?

A drinking water reservoir lake with a blue sky. Dam is seen in the background.

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 – July 2025

Summary/Conclusions

The probability of needing water supply releases from the Washington metropolitan area’s backup reservoirs during summer and fall 2025 is below normal. Such releases from Jennings Randolph and Little Seneca reservoirs are typically triggered by a combination of low summer precipitation and low groundwater levels. Above-normal rainfall in May and June has replenished the long-term precipitation deficit, with the 12-month cumulative precipitation now 0.4 inches above normal for the first time since November 2024. As a result, many groundwater levels in the basin are above or near normal at the locations reviewed for the ICPRB low flow outlook. Stream flows remain above normal following record low flows in April, and all major reservoirs are near full to full capacity. Potomac River flows remain well above the environmental flow-by requirements, with June flows averaging 13.9 BGD for the month. Most of Maryland, Virginia, and Pennsylvania are under normal drought conditions, with only localized drought watches and warnings in Maryland as of June 22. At present, there is sufficient flow in the Potomac River to meet the Washington metropolitan area’s water demands without releases from upstream reservoirs. If low flow conditions develop, the Washington metropolitan area is well protected from a water supply shortage due to carefully designed drought contingency plans.

ICPRB’s Low Flow Outlook

There is a 0 to 3 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. Inclusion of the Palmer Drought Index is suspended due to discontinued data. 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 0 to 3 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 for 2024, 1999, and 2002 as of July 1, 2025.

Drought Status:

Recent precipitation and forecasts:

Groundwater Conditions:

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.