Wednesday, May 19, 2021

Assabet River: Floods and Droughts

On May 24, at 7:00 p.m., the Maynard Public Library will present a Zoomed talk titled: “Assabet River Floods and Droughts.” Register (free) at This is the fourth in a monthly series of history lectures produced by the Sesquicentennial Steering Committee as part of Maynard’s celebration of the 150th anniversary of its creation on April 19, 1871. The June talk will be “Digital Equipment Corporation.” A new history book “MAYNARD MASSACHUSETTS: A Brief History” is for sale for $21.99 at 6 Bridges Gallery, 77 Main Street, WED-SAT, 12-5.

The Assabet River is a short (34 miles) river, meaning that its entire watershed of 177 square miles can be impacted by a single storm system. As a result, a flood situation can develop quickly. Data shows that timing from beginning of a storm to peak flow through Maynard is about 48 hours. The Sudbury River is equally short (33 miles), and both have their headwaters in Westboro, but much of the lower Sudbury crosses nearly flat terrain, with a wide flood plain. Thus, at the point where they join to become the Concord River, flood conditions on the Assabet reach the juncture faster, in fact so fast that there are instances when water discharging from the Assabet causes the Sudbury River to flow backwards.        

Main Street bridge, March 2010
The Assabet River is officially declared at ‘flood’ state when the U.S. Geological Survey gauge situated upstream from the Waltham Street bridge reports a depth of five feet, which happens when volume reaches 1,000 cubic feet per second (cfs). Historical records show that about every 10 to 15 years or so either a fall hurricane or spring rains flooded the Assabet. Often the cause was two storms – one to saturate the soil and the second to flood the already partially raised river. At five feet, the river is still entirely within its banks, just moving fast. Six feet means a volume of 1,700 cfs, seven feet 2,400 cfs, eight feet 3,400 cfs and nine feet 4,400 cfs. Eight feet puts water into the mill buildings next to the river. Nine feet, reached in 1927 and 1955, puts water on Main Street, floods the Elks Lodge parking lot, and in 1927, severely damaged the Waltham Street bridge. The last major flood, March 2010, saw 15 inches of rain from several storms, and a flood level of 7.1 feet. The last flood before that was 1987, also a bit over seven feet. Neither caused any significant property damage in Maynard.

Main Street bridge, August 2010

The 1968 flood (8.15 feet) put water over the retaining wall between the river and the mill complex, necessitating frantic sandbagging and pumping to minimize damage at Digital Electronic Company. One-time DEC employee Jack MacKeen recalled, “I have a clear mental picture of Ken Olsen [President of Digital Electronic Corporation] in his suit and rubber boots, helping place sandbags between the buildings.” Afterwards, DEC had the river retaining wall on its side of the river built higher. The wall kept the river out of the mill complex during the equally high flood of January 1979.

Contributing to the fact that the Assabet River has not suffered a catastrophic flood since 1955 are three major flood control dams. The George H. Nichols Dam, Westborough, completed in 1968, created a reservoir of a more than half a square mile which serves as a recreational boating and fishing site. It holds back high water and also serves as a water supply for the Assabet River in times of drought. Tyler Dam, Marlborough, does not obstruct any flow during normal river conditions, but backs water into an otherwise swampy impoundment area during high water. The third major holdback is the Delaney Complex, on Elizabeth Brook headwaters, completed in 1971. There are also more than a dozen minor flood control structures on tributaries. Collectively, these and the historic dams on the Assabet have a holdback capacity of more than four billion gallons of water, thus preventing millions of dollars of property damage. 

Precipitation is inches per month; Assabet
River volume is cubic feet per second, by month
Low water on the Assabet River is not due to less rain during the summer months, as the watershed averages close to four inches of precipitation every month of the year. There are several causes: 1) more of summer rain evaporates into warm summer air, 2) plants take up water and release that into the air via evapotranspiration; 3) surface soil, not frozen, absorbs water and transfers water to the aquifer. The result is that July, August and September average a flow of about 50 cfs. However, almost every summer there are weeks without rain when the flow is reduced to 10-20 cfs. Whenever flow falls below 39 cfs the owners of the mill complex are prohibited from diverting water from the river into the mill pond.

Massachusetts established a Drought Management Task Force in 2001, charged with monitoring drought conditions and advising water restriction policy as status goes from Normal through Advisory, Watch, Warning and Emergency. For our region, the last time Warning was reached was the 2016-17 drought. Locally, Level 3 water-use restrictions were enacted.

There was an exceptionally severe, state-wide, multi-year drought in 1961-69. Assabet flow reached a record low of 2 cfs. Prior to the drought, dating back to 1889, Maynard sourced water from White Pond, which is located on the Stow/Hudson border, south of Lake Boon. Miles of pipe and a pumping station moved water from the pond to a tank constructed atop Summer Hill. Starting in 1963, Maynard began transitioning to sourcing its water from town-owned wells. The town switched to getting all of its water from wells in 1999, after federal water treatment standards for surface water sourcing were made more rigorous. Maynard is perennially considering reviving White Pond as a water supply. A 2019 report estimated the cost of building a water treatment plant and installing miles of new pipe at about $30 million dollars.

A note about summer river flow: upriver from Maynard are now three municipal wastewater treatment plants that discharge water into the river. (Maynard’s discharge is at the Maynard:Acton border.) During summer lows, roughly half of what flows through Maynard represents processed wastewater. Per current state requirements, that water is “...cleaner than the river it is put into.” Overall, the river is far, far cleaner than it was years ago, when mill waste and sewage discharge was poorly regulated. As Ann Zwinger wrote in a 1982 book – A Conscious Stillness – “...the reach above the Powder Mill Dam is closed by joint action of the Maynard and Acton boards of health...the river smell is nauseating, reeking like an unpumped-out campground outhouse times ten."

Click on photo to enlarge
Summer of 2019, Mark painted water depth markers on the wall below John’s Cleaners, visible from the Main Street Bridge. This required permission from the Conservation Commission, Department of Public Works, Town of Maynard Selectmen and the owner of the building. Also, the signing of a liability waiver, and notifying the police department on the day of the painting, that a person would be in the river, near the bridge.


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