Friday, December 16, 2022

Assabet River - a No Flood Year

With 2022 drawing to a close, this was a year with no recorded floods for the Assabet River. Daily data is generated by an automated gauge located about 150 feet upriver from the Waltham Street bridge. It posts river depth and volume in cubic feet per second (cfs) at The default is to show data for the last seven days, but that can be increased. A minor flood is officially defined at water depth exceeding 5.0 feet, which requires a flow of 1,200 cfs. A moderate flood, 6.0 feet; a major flood 7.0 feet. 

Looking back to 1942, which is when the tracking system was installed, in roughly one in five years, there were no official floods, or conversely, in four out of five years there were one or more floods. 

From the footbridge over the Assabet River, looking toward the Main Street bridge, March 2010

Floods do not necessarily equate with property damage. The Assabet River's transit through Maynard is so channeled that at 5.0 feet, 6.0 feet, or even up to 7.0 feet, parts of backyards are under water, but no buildings or streets are flooded. The tracking system recorded only five instances where the river topped 7.0 feet, the last being mid-March 2010 at 7.14 feet and 2,400 cfs. Records before 1942 add severe flooding in 1927, 1936 and 1938 but without depth or flow data. The on-record largest flood for Maynard was due to Hurricane Diane, August 1955, cresting at 8.94 feet and 4,250 cfs. 

Maynard wastewater discharge

Looking to the future, the long-term trend for eastern Massachusetts has been increasing precipitation overall and stronger storms, so we can expect that at some point in the future either a nor-easter or a hurricane will hit big and wet, and deliver the next severe flood despite the existance of three major and several minor flood control dams on the Assaber River and tributaries.

The summer of this year was an extended drought, lowering river depth at the gauge to 1.0 feet and the flow to under 10 cfs. Most of that volume is from the discharge of wastewater from upriver towns. Wastewater treatment specification mean that the discharged water is cleaner than the river. Maynard's wastewater is discharged at the Maynard:Acton border. In the winter, the warmth of the discharge water keeps an area of the river from freezing. Ducks, geese and swans congregate there. 



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