The weather, ever-serviceable as a commonplace, becomes the principal topic of conversation when it closes our roads. When people hear that flooding would have been much worse along the Assabet River except for the mitigating effect of flood control dams, some ask, “What’s the difference between flood control dams and other dams?” The answer is that while most dams store water all the time, flood control dams only work during periods of high flow.
Over the years, residents of eastern Massachusetts have found many reasons to store water in ponds. They’ve used dams to capture nighttime flow for turning water wheels the next day. Other dams made ponds in which ice could be cut, or to supply water for drinking or fighting fires. A dam at Billerica diverted Concord River water into the Middlesex Canal, which floated freight and passengers from the Merrimack River to Boston Harbor. Long before beavers returned to eastern Massachusetts, our friendly little rivers and streams were thwarted by many dams.
Dams are usually built where the stream profile is steep. Boaters do not run into dams on the lower Sudbury or the upper Concord rivers because their gradients are too flat. The ideal location for a mill dam is where a stream narrows between solid banks at the upper end of a fall. Few such locations in this region have gone undammed over the past three centuries.
Colonists built dams to turn machines for grinding grain, sawing boards, and fulling cloth. (“Fulling,” derived from an old word for baptism, was a process of washing and beating cloth to clean and thicken it.) During the industrial revolution, water wheels gave way to turbines, but dams were more important than ever. The energy of falling water powered mills that made textiles, paper, and chocolate, and, more recently, generated electricity.
During periods of unusually heavy rain or snow-melt, ponds behind mill dams fill quickly. When the water reaches the top of the dam, it just spills over. If a certain number of cubic feet per second reaches the pond, the same amount hurries over the dam, so it no longer affects the volume passing downstream. That leaves mill towns such as Hudson, Maynard, and Waltham vulnerable. In the 1950s, multiple hurricanes resulted in damaging floods, which prompted flood control planning. A mix of remedies was proposed, which were implemented over the ensuing decades. Conservation of stream-side wetlands and flood plains was mandated, especially in the upper Charles River.
In the drainage of the Concord, Sudbury, and Assabet rivers, ten flood control structures were designed and built. An excellent example is Tyler Dam, which is crossed by Robin Hill Street in Marlborough. Usually Tyler Dam makes no pond. It just sits there, with a culvert in its base, letting the Assabet River pass freely through its massive bulk. But when the Assabet becomes a torrent of charging runoff, Tyler Dam goes to work. Only so much water can flow through the culvert. Denied quick passage down to the streets of Hudson, Maynard, and Concord, the extra gallons are held safely behind the dam’s high, widespread earthen wings, and back up onto a flood plain preserved for this purpose.
Tyler Dam and 26 other flood control structures statewide are owned and maintained by Massachusetts Department of Conservation and Recreation. Though their designs vary, most are intended to be self-regulating and don’t require DCR personnel to scurry forth on rainy nights. Instead of making news, these dams quietly prevent it, leaving us to talk more about flooded roadways and less about flooded downtowns.
Ron McAdow is Executive Director of Sudbury Valley Trustees.