Monday, August 8, 2016

Assabet River - Low Water During Drought

This is a rewrite of a 2010 column, written during a milder drought summer than this one.

So little rain has fallen in 2016 that the reservoir at the head of the Assabet
River now has its water level below the outlet, meaning that the riverbed is
dry until it gets to where the Westboro wastewater treatment plant
discharges into the river. Photo shows the dried up riverbed. 
A river with next to no water excites the river viewer far less than high water – no sandbags, no roads impassable, no sense of invasion. Plus, understanding cause for high water is as simple as “rain runs off,” while comprehending the causes of low water leads to my-eyes-glaze-over nuances about groundwater, wastewater, evaporation, evapotranspiration, aquifers, etc. And yet, and yet, there are things to know as the Assabet River slows to a trickle.

First the facts: the peak of the 2010 flood saw 2,500 cubic feet per second (cfs) rushing through downtown Maynard. That’s 18,700 gallons a second, 1.1 million per minute, 67 million per hour, or 1,616 million gallons of water per day. For comparison, Maynard’s water usage is just under one million gallons per day, drawn from Town wells, most of which gets put into the river as cleaned wastewater. August 2016 finds the river at times under 10 cfs. This in comparison to U.S. Geological Survey records which show an August median of 40 cfs. Record lows for this time of year are under 5 cfs.

Low water on the Assabet River is not normally due to less rain during the summer months, but this year the drought is definitely is a contributing factor. Decades of record keeping show this area as averaging close to four inches of precipitation every month. This year, June, July, and so far August, have been under one inch per month. 

Ben Smith Dam (Maynard, MA) with a trickle of water over the top.
Click on photos to enlarge image.
The Assabet River is low in summer because more of summer rain goes to replenishing local groundwater without ever reaching the rivers and streams that make up the river's watershed. Green plants take up water, and via evapotranspiration release that moisture into the air. A single large tree can release several hundred gallons of water per day, an acre of grass far more. An eighteen hole golf course might irrigate 500,000 gallons per day, with much of that lost from the local watershed to evaporation and evapotranspiration.

Low water reveals trash. Downstream of the Elks building there are still scores of old car tires visible in the river despite the Organization for the Assabet, Concord and Sudbury Rivers (OARS) having conducted annual clean-ups that removed literally hundreds of tires from just that section. Clearly, once upon a time someone in the tire business thought dumping in the river meant “away.” Tobin Park shoreline is littered with broken glass. Clean up glass, rake a bit, more glass. Rake more, more glass. It’s glass all the way down. Post-flood finds back in 2010 were an unbroken “Hans Eriksen” pint sized milk or cream bottle, circa 1940’s and a glass, 6.5 ounce Coca-Cola bottle, with “LOWELL  L  MASS” on the bottom, date unknown. This year’s OAR clean-up, scheduled for September 17, is posted at Volunteers welcome.

Greyish line is river volume; raindrops and snowflakes symbolize
precipitation per month in inches, so all close to 4"/month except for Feb.
Low water reveals fish. Maynard’s section of the Assabet is home to white suckers, golden shiners and various types of sunfish. Late morning to early afternoon are good times for fish sightings from the Main Street bridge or the footbridge. Spotting fish from above can be difficult. One trick is to scan the bottom for a moving shadow, then look above the shadow for the fish. Great Blue Herons stalk these shallows for fish, frogs, crayfish and the occasional duckling. Snapping turtles lurk on the river bottom.

Back in the day when mills operated on water power some rivers were shut off nights and Sundays. These no-flow times allowed mill ponds to refill with water to power the next work shift. With multiple mills operated on the Assabet and its tributaries there had to be cooperation among the mill operators so that everyone had water when they needed it.

A not-so-secret secret about the Assabet River is that by mid-summer much of the water flowing through Maynard is cleaned water that was discharged by three upstream wastewater treatment plants. When the river is this low, upstream discharge contributes more than half the flow you see. According to the treatment plants, the water being put in is cleaner than the river it is being put into. Maynard’s cleaned discharge is added to the river just before it enters Acton.

Low water can end quickly. In August 1955 Hurricane Diane brought torrential rain to eastern Massachusetts. Within 48 hours of the storm's arrival the Assabet River went from 1.1 feet to 8.96 feet at the gauge upstream of the Waltham Street bridge. Volume went from 20 cubic feet per second to 4,500 cfs. Main Street near the Main Street bridge, and surrounding buildings, were flooded. 

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