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?

The water supply outlook is published by ICPRB’s Section for Cooperative Water Supply Operations (CO-OP) staff 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 for October 2018

The next Water Supply Outlook will be published April 2019.

Water Supply Outlook, October 2018


There is a below normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2018 fall season. Generally, the use of Jennings Randolph and Little Seneca reservoirs is triggered by low flows brought about by a combination of low summer precipitation and low groundwater levels. Precipitation for the month of September ended with greater than 75 percent above normal rainfall totals across the Potomac basin. Current (September 28) data from the U.S. Geological Survey shows that streamflow are much above normal. In particular, the Potomac River flow near Little Falls (gage 01646500) was above the historical October 1 maximum flow value of 63,400 cubic feet per second, which occurred in 2005 based on 88 years of record. Groundwater levels are above or much above normal. According to the Middle Atlantic River Forecast Center, the outlook for water resources and water supplies is very good or excessive for most areas in the Potomac Basin. At present, there is sufficient flow in the Potomac River to meet the Washington metropolitan area’s water demands without augmentation from upstream reservoirs. In the event that low-flow conditions do develop, the Washington metropolitan area is well-protected from a water supply shortage because of carefully designed drought-contingency plans.

ICPRB’s Low Flow Outlook:

There is a minimal (<1 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. 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 giving consideration to 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 minimal (<1 percent) conditional probability compares to the 3 to 5 percent historical probability and is considered the more reliable indicator.

Drought Status:

Recent precipitation and forecasts:

Groundwater Conditions:

Water Supply Outlook Archive:

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.