Short answers, pulled from the story.
The name Merrimack derives from a Native American term meaning the place of strong current according to Joseph B. Walker citing Chandler Eastman Potter in 1856. Henry David Thoreau implied the name signifies the Sturgeon River in his classic book while French explorers first pronounced it Merremack in 1604.
The river begins in Franklin New Hampshire at the confluence of the Pemigewasset and Winnipesaukee rivers. It flows south through Concord Manchester and Nashua before entering Massachusetts at Tyngsborough and turns northeast past multiple towns until reaching the Gulf of Maine near Boston.
Environmental pollution intensified after 1951 when many alterations and pollutants were dumped into the waterway. The river ranked among the ten most polluted waterways in the United States during the 1960s with Phthalates identified in high concentrations by 1973.
Rainfall raised the river more than six feet above flood stage on the 15th of May 2006 causing around 1,500 people to evacuate their homes in Haverhill Massachusetts. A main sewage pipeline broke that day dumping raw sewage waste into the river while most areas received around a foot of rain.
The city of Lowell installed a modern steel beam flood control gate at the historic Francis Gate site under the direction of James B. Francis. This Great Gate was built in 1850 and first saved the city from flooding in 1852 before subsequently protecting it again in 1936.