Wednesday, October 30, 2019

The River Rises and Falls

From the late 1700s onward the Assabet River was less of a river and more of a series of narrow ponds, each created by dams that first backed up water for seasonal usage by saw mills and grain mills, those dams or their replacements later put into service for factories of the Industrial Revolution. Given the dams, the river was not a useful means of transportation either for people or freight; instead the river’s watershed became crisscrossed by railroads.

Figure from 2011 book "MAYNARD: History and Life Outdoors"
shows water precipitation in inches per month and average river volume,
also by the month. Snow expressed as water inches.
Mill operations were initially all about how much water could be retained. With a sufficient supply, mill operations could be year-round rather than limited to the times of naturally higher water flow (late fall through late spring). When partners Amory Maynard and William Knight bought land in Assabet Village they also bought water rights upriver, include rights to dam up Boon Pond and to the Fort Meadow Reservoir in Marlborough. One nice thing about water power was that once the dam, canal and waterwheel were in place, power was basically free. Within years, however, the demand for power was such that instead of relying wholly on water, coal-powered steam engines were soon supplementing and then replacing water power.    

Ben Smith dam in drought conditions. Click photo to enlarge.
As to how much water flows in the Assabet River, a U.S. Geological Survey station ( located a short distance upstream from the Waltham Street bridge provides depth and flow information. The long-term average volume is 200 cubic feet per second (cfs). As the figure shows, March and April are the high-water months as a result of snow melt plus rain falling on frozen and therefore non-absorbent ground. July through September are the low-water months despite basically the same amount of precipitation every month, because of evaporation and transpiration (water molecules released into the air from plant leaves). A prolonged drought can reduce flow to under 20 cfs. An interesting legality here in Maynard is that while Mill & Main owns the millpond, it is restricted from diverting water into the canal that provides water to the millpond when flow volume falls below 39 cfs. The intent of the law is to prevent the river going dry for the section downstream from the dam. Only when the river rises, as it did after the October 17 storm, is Mill & Main allowed to top up the pond, and perhaps simultaneously release water from the east end, so as to both refresh and replenish the millpond.

The river also rises and falls after each rain storm. Case in point – after that October storm the river rose from 1.5 to 2.8 feet deep at the USGS gauge. Pre-storm volume was 30 cfs, peaking at 260 cfs about a day after the storm ended. There was then a days-long gradual decline toward pre-storm levels, reversed when rain started the night of October 22. Interestingly, a look back at historic floods finds that there was often a previous significant rainfall event that had saturated the ground and raised water levels in the river just before the big storms that pushed the river into flood. For those floods, the most recent in 2010, flood crest levels occurred three days after the heavy rains began. Sometimes the skies had cleared and the sun was shining while the water was still rising.

River depth markers painted on wall below John's Cleaners on Sept 22, 2019.
White paint markers spaced one foot apart were recently painted on the wall below John’s Cleaners, visible from the sidewalk on the north side of the Main Street bridge. These indicate how deep the water is at the wall. Nine feet is optimistic (or pessimistic, depending on hour you feel about floods), as for flood peaks measured at the USGS station, there have only been five that topped seven feet since 1942. Because river width is different at the gauge and the bridge, we don’t know yet how closely the two indicators comply.

There once was, actually, a bit of Assabet River boat transportation. From 1906 to 1914 there was steamboat service from a boat house near the Ben Smith dam, Maynard, and a landing wharf was installed at Whitman's Crossing near Lake Boon, Stow. The one-way cost was twenty-five cents. Disembarking at the crossing, a short walk brought people to a dock on Lake Boon, where a regularly scheduled steam launch would travel to docks along the shore, allowing people to reach resorts, club houses and lakeshore summer homes.

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