Summary/Conclusions

There is a near-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 3.9 inches of precipitation in July, 0.1 inches above average; as of August 1, the 12-month cumulative total was 29.2 inches, 10.9 inches below average. Groundwater conditions are generally near normal basin-wide, though above- and below-normal readings occur at individual wells. Recent rainfall has increased Potomac River streamflow, prompting ICPRB to suspend daily drought monitoring on August 9 after initiating it on August 3 in response to declining flows.

Drought conditions remain across portions of the basin: as of August 4, about 18 percent was in severe drought (D2), 54 percent in moderate drought (D1), and 22 percent abnormally dry (D0), with no area in extreme drought (D3). The Climate Prediction Center’s seasonal outlook indicates drought conditions across much of the Mid-Atlantic should improve or be removed by the end of October. A Drought Watch for the metropolitan Washington region remains in effect. Reservoirs remain near full.

At present, Potomac River flow is sufficient to meet metropolitan area water demands without upstream reservoir augmentation. The region remains well protected by established contingency plans should conditions worsen.

ICPRB’s Low Flow Outlook

There is a 5 to 16 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 5 to 16 percent conditional probability compares to the 7 to 15 percent historical probability and is considered the more reliable indicator.