Bulk of Flooding Expected in Old Town, Washington Harbour

Although streams and creeks can flood quickly, a major river such as the Potomac "integrates the whole watershed," Kolva said.

By contrast, Hurricane Agnes in June 1972 dropped six inches or more over much of the watershed, triggering a major Potomac flood that registered 15.45 feet at the Wisconsin Avenue gauge and 22.03 feet at Little Falls.

Although much of this week's rain came Sunday and Monday, the Potomac is not expected to crest until early tomorrow because it takes time for rain across the watershed to reach the Potomac and flow downriver. "Small streams flood much more quickly because the distance the water has to travel is much less," Kolva said.

In addition to rainwater flowing downriver, officials were keeping a nervous eye on a storm surge that could cause coastal flooding on the tidal Potomac and western shore of the Chesapeake Bay. The tropical disturbance that blew into Washington last night could drive water ashore. "You have an upriver wind," Hitchens said. "That's what makes it extra challenging to predict."

The last storm surge during Hurricane Isabel in 2003 caused more than $10 million in damage in Alexandria and Fairfax County after a nine-foot wall of water coursed through Old Town and the Belleview and New Alexandria neighborhoods in Fairfax County, damaging 2,200 homes and more than 60 businesses.

Alexandria officials were monitoring levels of the Potomac yesterday from an emergency command center that has been open since Sunday evening and will stay open until the crisis has passed, said Brian Hannigan, a spokesman for the city.

City officials sent leaflets to businesses in Old Town yesterday, warning that the river could rise one to two feet above normal at peak high tide during the next few days.

Bob Lorenson, owner of the Virginia Shop on South Union Street in Old Town, was carting stock from the first floor of his business to the second floor yesterday. Even though the end of his block often floods during high tide, he hadn't taken the extra precaution of moving all his wine bottles and gift items upstairs since Hurricane Isabel, he said.

"I'm not that concerned," Lorenson said. "I think the floor might get wet."

Staff writer Annie Gowen contributed to this report.