Situational Awareness (Friday, 2026-09-18)
According to the Middle Atlantic River Forecast Center (MARFC), the basin saw between 0.01 and 0.5 inches of rain during the previous 24 hours. They predict the next three days may see an additional 0.1 to 0.75 inches of rain, with a projected seven-day total of up to 3 to 4 inches.
Special Conditions
Intra-daily flow fluctuations at Potomac River gages, including Point of Rocks, are occurring due to upstream dam operations associated with eel passage.
Daily flows:
Little Falls gage flow 2026-09-17: 847 MGD (1310 cfs)
Little Falls gage flow 2026-09-18: 840 MGD (est., based on recently available real-time data) (1300 cfs)
Note: Gage flow at Little Falls is measured after water supply withdrawals.
Point of Rocks flow 2026-09-17: 1053 MGD (1630 cfs)
Point of Rocks flow 2026-09-18: 1086 MGD (est., based on recently available real-time data) (1680 cfs)
Yesterday's estimated daily flows:
Great Falls estimated flow 2026-09-17: 847 MGD (1311 cfs)
Note: Estimated flow below Great Falls intake is flow at Little Falls plus Little Falls water supply withdrawal.
Yesterday's Washington metropolitan area Potomac River withdrawals and discharges (2026-09-17):
Fairfax Water Corbalis withdrawal - Potomac: 115 MGD
WSSC Water Potomac withdrawal: 162 MGD
Washington Aqueduct withdrawal - Great Falls: 142 MGD
Washington Aqueduct withdrawal - Little Falls: 0 MGD
Loudoun Water withdrawal: 12 MGD
Loudoun Water Broad Run discharge: 7 MGD
Total Potomac withdrawal: 430 MGD
Total net Potomac withdrawal: 423 MGD
Loudoun Water Potomac River (PR) flow values for drought operations protocol (based on yesterday's flows):
QPR: 1630 cfs
QPR, obs: 1630 cfs
QPR, WS: 0 cfs
Note to Loudoun Water: See NWS ensemble flow forecasts (HEFS, GEFS, NAEFS) for Point of Rocks flow for the next 7 days – https://www.weather.gov/erh/mmefs_marfc?id=PORM2&model=NAEFS. See US Army Corps of Engineers Potomac River Basin page for observed releases from Savage and Jennings Randolph reservoirs - https://water.usace.army.mil/office/nab/; See US Army Corps of Engineers North Branch Potomac River and Savage River page for scheduled releases - https://water.usace.army.mil/office/nab/releases/north-branch.
Note: Potomac River flow at Point of Rocks has dropped below the low-flow threshold of 2,000 cfs. When this occurs, per the Water Supply Coordination Agreement of 1982, CO-OP begins daily monitoring and reporting of Potomac flows and withdrawals on behalf of the CO-OP suppliers, Fairfax Water, Washington Aqueduct, and WSSC Water. Suppliers can review data at CO-OP's Data Portal at www.icprbcoop.org. Reports formatted in approved templates should be submitted by email to coop@icprb.org by 8:00 am for automated processing. Published monitoring reports are at http://icprbcoop.org/products/monitoring-reports.