Emergency crews used boats to rescue people trapped in their homes and sewage systems overflowed Monday as rain pounded New England for the fourth straight day in what could prove to be the region’s worst flooding since the 1930s.

The National Weather Service reported more than a foot of rain had fallen in some places by Monday night.

In the Merrimack Valley, north of Boston on the New Hampshire line, the Merrimack and Spicket rivers overflowed their banks and forced the evacuations of hundreds of people.

Firefighters warned roommates Erica Digaetano, 22, and Kelly Malynn, 23, to leave their first-floor apartment in downtown Haverhill. Water had filled the basement up to the ceiling and was still rising.

“My landlord has an office under here and everything is just floating in it,” Digaetano said.

Tens of millions of gallons of sewage spilled into the Merrimack River after pipes burst Sunday in Haverhill, and the flood threatened power at a regional treatment plant in Lawrence, Mass. A shutdown at that plant would force 115 million gallons of sewage into the river each day.

“It’s going to get worse before it gets better,” Gov. Mitt Romney said.

Emergency crews in Lowell took to flooded streets in boats and used bullhorns to urge about 1,000 households to evacuate. Forecasters said the river could rise past 60 feet by Monday night, putting it at more than 8 feet over flood stage.

In New Hampshire, more than 600 roads were damaged, destroyed or underwater. Gov. John Lynch said his own front yard in Hopkinton had become a pond.

The raging Merrimack threatened historic mill buildings in downtown Manchester. The buildings now house a wide assortment of businesses, restaurants and shops.

In Nashua, the overflowing river reached a condominium complex that is normally several hundred feet from its banks. Firefighters rescued about a dozen people and their pets by boat.

Flooding forced the evacuation of St. Paul’s School in Concord. Floodwaters hit some dorms, the library, the health center, post office and performing arts center at the exclusive prep school, which has students from around the world.

A fast-moving storm early today was expected to bring as much as another 1 1/2 inches of rain, threatening to push the Merrimack, Spicket and other large rivers farther over their banks and swamping entire neighborhoods. After that, drier weather was forecast.