Short answers, pulled from the story.
The Merrimack River is 117 miles long. It begins at the confluence of the Pemigewasset and Winnipesaukee rivers in Franklin, New Hampshire, and empties into the Gulf of Maine at Newburyport, Massachusetts.
The name's exact meaning is disputed. One interpretation, drawn from The History of Manchester by Chandler Eastman Potter, holds that it means "the place of strong current." Henry David Thoreau, in A Week on the Concord and Merrimack Rivers, suggested it means "Sturgeon River."
The Merrimack River was one of the ten most polluted waterways in the United States in the 1960s. In 1966, environmentalist Donald Eaton Carr described it as "haunted with the oldest and most hopeless pollution of any in the country," the result of decades of raw sewage, textile mill discharge, and tannery sludge.
The worst recorded flood occurred in March 1936, when the river at Lowell reached 68.4 feet, ten feet higher than the major flood of 2006. Jack Kerouac set part of his novel Doctor Sax during that event.
The Francis Gate is a flood control structure at Lowell, Massachusetts, built in 1850 under the direction of James B. Francis to seal the city's canal system from the Merrimack during floods. Dismissed as unnecessary when first built and nicknamed "Francis' Folly," it saved Lowell from flooding in 1852, 1936, and 1938.
Thoreau wrote A Week on the Concord and Merrimack Rivers, an early American literary classic that recounts a journey along both rivers and includes Thoreau's own interpretation of the river's name.