Schools in the region were closed on Monday, and some were to remain closed Tuesday. St. Paul's School in Concord, N.H., an Episcopal boarding school, suffered extensive damage after the Turkey River spilled its banks.
The Merrimack River in Massachusetts crested Monday afternoon at six feet above flood level and is expected to stay that high until Wednesday, the National Weather Service said. A number of its tributaries in Massachusetts and New Hampshire, including the Spicket River in Methuen, Mass., and the Shawsheen River in Billerica, Mass., were beyond flood stage and had not yet crested.
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Officials were particularly concerned about a weak dam on the Spicket River. If it fails, downtown Methuen could flood.
Gov. John Lynch of New Hampshire declared a disaster in eight of the state's 10 counties, and called out more than 400 National Guard troops to assist with evacuations and to guard the more than 600 roads that have closed. "You can go to a town in any of those eight counties and find flood damage," Mr. Lynch said.
In Massachusetts, tens of thousands of gallons of sewage were spewing into the Merrimack River after Haverhill's main sewage line broke and the Lawrence's sewage treatment facility flooded. Lowell's drinking water plant is in danger of shutting down.
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"We're under siege; water is coming out everywhere," said Frederick Laskey, executive director of the Massachusetts Water Resources Authority, which provides water and sewer services to 43 municipalities. "This is overwhelming."
The first part of the storm system, which hit the area late last week, was fed by moisture that came up from the Gulf of Mexico and sat off the shoreline. The second wave came from a system stuck in the Ohio River Valley. The two essentially set up a conveyor belt of moisture that will not be gone until Wednesday, said meteorologists at the National Weather Service in Taunton, Mass.
In Methuen, Karen Baggetta, a visiting nurse, and her husband, Nick, a state trooper, hauled belongings out of their house next to the Spicket River. The first floor was flooded, and the living area was on the brink of flooding. "Our house is an island in the middle of the river," Ms. Baggetta said in a phone interview.
After the house flooded in 1987, the family raised it five feet. This time, she does not want to go back.
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"It's devastating," Ms. Baggetta said. "All those feelings come flooding back — ha-ha. But I'm out of here. We're so out of here. I can't go through this again."
US Route 1 was shut down due to flooding, between Interstate 95/Massachusetts Route 128 and Massachusetts Route 60.
The New England region of the United States has received the heaviest rainfall in decades leading to flooding and massive road closures. The states affected are Maine, Massachusetts and New Hampshire. A state of emergency has been declared in all these areas. The floods have also led to thousands of evacuations in many cities. Only a single death has been reported.

The main concern in many cities and towns is whether the dams that hold rivers from flowing in cities will hold long enough.

These floods are the worst in New England since 1936 when over 150 died.