The Potomac River supplies drinking water to some 5 million people in the D.C. region, and right now, the river is running low. Water managers announced they may have to use water from backup reservoirs upstream — it would be the first time in more than a decade.

So far this year, there has been about 5 inches less rain than normal, with an unusually dry spell in the late spring and early summer. According to the U.S. Drought Monitor, parts of the region are currently in a moderate drought, including Montgomery County and Loudoun County, while other areas have been downgraded to “abnormally dry.”

“People have a tendency only to think in terms of drought in the West, but we have had historic droughts here in the D.C. area,” says Mike Nardolilli, executive director of the Interstate Commission on the Potomac River Basin, which monitors water supply in the area. The worst droughts on record were in the 1930s and 1960s.

In June, the commission began conducting daily drought monitoring, the first step when there’s concern that water supply could run low. Potomac River levels returned to normal in July after a series of storms, but the river’s level has dropped again since. Historically, late summer and fall are the most drought-prone times of year in the region.

The current flow on the Potomac River at Point of Rocks, Md. is about 665 million gallons per day. The region’s drinking water suppliers withdraw about 400 million gallons a day, according to the latest data from the commission. That means there are about 265 million gallons of extra water flowing in the river after daily withdrawals. The absolute minimum flow required to maintain the Potomac’s aquatic ecosystems is 100 million gallons per day.

These low flow levels triggered what is called “drought operations” at the Interstate Commission on the Potomac River Basin. Officials there put out two reports a day comparing river flow to drinking water withdrawals, and they start discussing measures to keep enough water in the river, including releases from Jennings Randolph Reservoir and Little Seneca  Reservoir, both located upstream in the Potomac watershed. Jennings Randolph is in West Virginia, and water released there takes about 9 days to reach D.C. pipes. Little Seneca, in Montgomery County, takes just a day, but the lake is much smaller.

The commission may also ask drinking water suppliers to draw more of their water from other sources besides the Potomac watershed — Fairfax Water can use the Occoquan reservoir, and in Maryland, the Washington Suburban Sanitary Commission can withdraw water from the Patuxent River.

Should dry conditions continue, water conservation measures may become necessary.

“There are a lot of backups,” says Nardolilli. “The D.C. metro area water supply is well-protected due to decades of careful planning and preparation.”