File:Pawtucket Dam Blowout July 1915 (ace96997-aa46-48d5-8e9e-eae46680ff8e).jpg

A photo showing the Merrimack River flowing over Pawtucket Dam

The Pawtucket Dam at the head of Pawtucket Falls had never failed after it was completed in 1875. But on July 9, 1915, after a period of heavy rain, a 285-foot section of the dam in the middle of the river gave way. The Pawtucket Dam is not a tall dam. It follows rock outcrops in an irregular line for almost 1100 feet across the Merrimack River in Lowell, MA, and the riverbed drops about 30 feet over the next mile. The Proprietors of Locks and Canals responded quickly. A very difficult repair job was underway as soon as water levels dropped enough to begin erecting a coffer dam. A steam-powered derrick crane provided lifting power and a large scow delivered thousands of sandbags to the work site. Workers began recovering the dislodged stones from the riverbed. Some capstones were washed up to 250-feet downstream. Other stones were closer to the dam, but they could weigh up to 2 ½ tons. Why had the dam failed? It had survived larger flows in the past. What was different this time? Blame was soon cast upon the flashboards. The flashboard system comprised iron rods, 4-feet tall, set into holes atop the dam with wooden boards placed on the upstream side. The result was higher water and more energy for the mills. The system was supposed to fail if the water got too high above the dam. The iron rods would bend with the pressure, the boards would wash away, and the water level would drop, relieving pressure on the dam. It had always worked in the past. But now iron pins had been replaced with steel pins, which did not bend as easily as iron.

• Keywords: Lowell; engineering; Merrimack River