Water Supply Outlook and Status

Interstate Commission on the Potomac River Basin

Wide=angle view of a very full Potomac river at Great Falls during winter. Bare trees are seen on both sides of the river.

What is the Outlook?

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 2021


There is a near normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2021 summer and fall seasons. The use of Jennings Randolph and Little Seneca reservoirs is generally triggered by low flows brought about by a combination of low summer precipitation and low groundwater levels. Average precipitation in the Potomac Basin in June was normal. The 12-month cumulative basin precipitation is 2.7 inches below normal as of June 30. Streamflow is currently near normal. Groundwater levels are normal for most of the monitoring wells in the Basin. The Middle Atlantic River Forecast Center’s (MARFC) outlook indicators for water resources and supplies for the Potomac Basin are good. 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 do develop, the Washington metropolitan area is well-protected from a water supply shortage owing to carefully designed drought-contingency plans.

ICPRB’s Low Flow Outlook:

There is an 8 to 19 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 the historical stream flow records and considering recent stream flow values, precipitation totals for the prior 12 months, current groundwater levels, and the current Palmer Drought Index. Past years in which watershed conditions most closely resemble current conditions are weighted more heavily in the determination of conditional probability. The historical, or unconditional, probability is based on an analysis of the historical record without weighing for current conditions. The 8 to 19 percent conditional probability compares to the 8 to 15 percent historical probability and is considered the more reliable indicator.

Drought Status:

Recent precipitation and forecasts:

Groundwater Conditions:

Water Supply Outlook Archive:

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

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