by John Hicks-Courant
johnhc@TheWorld.com
I don’t think I’m going too far up the creek without a paddle when I say that we will have floods on the Shawsheen River this spring. I should probably take a moment to make clear the distinction between high water and flooding.
High water is when the river channel is full to overflowing into the floodplains. When there is really high water, the Meadows all look like lakes, even large lakes. The water moves forcefully and fast when it is high, and a normal one-hour paddle can take only fifteen minutes.
When the river is in flood, you can paddle through the trees in the wooded sections. You can paddle across Whipple Road, which you have to do because you can’t get under the Brown Street bridge. When the river floods, Tewksbury’s Shawsheen Street and Route 38 (Main Street) are covered by water. Below that, South Street submerges.
Further downstream, in downtown Andover, North Main Street becomes impassable except by boat. The parking lot at Powdermill Square — a contentious subject between real estate developers and the SRWA — will hold several inches of river water, as will the Brookstone parking lot downstream.
Downstream from Andover, the parking lot next to Route 114 in North Andover will flood. Beyond that is the area we call the “Bayou.” This is the wooded section that floods in high water, where the channel disappears completely as the water spreads throughout the woods. The difference between high waters and floods are nearly indistinguishable in the Bayou but for a couple of things.
The first of these is that the steep walls of the upland edging the river now form the edges of the channel. The second is that the massive chutes that funnel the river under Interstate 495 form a dam. It is often impossible to paddle through those chutes when the river is in flood. Paddling through those enormous chutes with only inches of space between your head and the top of the chute is an experience you will remember for a long time.
Flooding will also occur on the Shawsheen River’s tributaries. Content Brook will undoubtedly overflow Gray Street and the two houses built in its floodplain in East Billerica. In North Billerica, the two houses on Burnham Road built in the floodplain of the unnamed tributary to Content Brook will also flood.
The remarkable thing about flooding on the Shawsheen River is not the dramatic alteration of water volume and depth. Rather it is that, in such an intensely developed and highly populated suburban environment, floods do so very little damage to the homes and civic infrastructure of the eight towns through which the river flows.
In fact, those places mentioned earlier where the flood will alter the flow of motorized transportation for a day or two are all built in floodplains. That is, people consciously constructed roads and parking lots in spots destined to submersion during the inevitable floods.
Floods are inevitable and even somewhat predictable. As the Canadian hydrologist E. C. Pielou commented in her book Fresh Water:
“Given enough data, it becomes possible to describe the flood behavior at a station in terms of the average return period for floods of different heights. For a given flood height this is the number of years, on average, between two successive floods equaling or exceeding that height; for example, one might find that floods of 3 meters or more come at intervals of 10 years, on average. Note the words on average: the average return period, whatever it is, tells nothing about actual return periods, which may have varied widely. Even the average is not an exact figure, but simply an estimate, based on records that may not go very far back into the past, and based on the dubious assumption that no long-term trends are affecting the climate or the river.”1
With this caveat from an internationally recognized hydrology expert, here are the statistics and dates for the three most recent floods on the Shawsheen River:
Date Peak CFS
10/22/96 1850
6/15/98 1240
3/23/01 1580
These were floods that spread beyond the river’s floodplains and submerged primary thoroughfares in Bedford, Billerica, Tewksbury, Andover and North Andover. Because there is a USGS flow gauge at the intersection of Rte. 129 and the Shawsheen River, we could go back much further in time to establish the periodicity of high water (i.e., saturated floodplains) and floods (i.e., overflowing floodplains). We may well do this as project for the Trib, but there is no real need to do so this year to accurately predict flooding in the Shawsheen watershed.
As I write this article on February 17th, the snow is falling at a rate of two inches per hour. Six inches have fallen so far, and the rather imprecise prediction is for somewhere between twelve and twenty-five inches to fall in the Shawsheen River watershed between this afternoon and tomorrow morning. This snowfall is landing on top of more than a foot of old snow.
The last time we saw conditions like this was the winter of 1994. Snow fell on snow for most of January that year, and when it melted, it melted in the rain. The people who live across the street from me — the people who filled in the floodplain in their backyard and planted Kentucky bluegrass — had a pond in their backyard that year. They will have that pond again this year. The peepers’ vernal chorus will sound louder and closer this year than it has in a long time.
But what are the effects of flooding? It is a natural part of the hydrologic cycle of every river and stream.
In her book Water: A Natural History, Alice Outwater wrote:
“Winter precipitation melts in a few spring weeks, and rainstorms deposit large volumes of water in a short period of time, so rivers flood regularly, donating sediment and organic matter to the floodplain. The channel of the river is only a fraction of the total area annually flooded; most of the aquatic productivity in a large river-floodplain ecosystem is in the floodplain rather than the river channel itself. The flood pulse of rivers fed by forested watersheds is timed predictably and maintains a well-established floodplain. During late summer and fall. when the flow in forest streams is scanty or intermittent, deadwood litters the streambed. But when the water runs high in the spring, formerly insignificant flows can move many tons of sediment and driftwood. The stream channel is reworked during the high flows of spring, and the silt that accumulated in the interstices of the gravel beds is flushed out, renewing the sites of fish hatcheries.”2
It is in that last sentence that Alice Outwater touches on one subject that this river activist considers among the most significant about flooding: the affect on the wildlife. In his book Swampwalker’s Journal: A Wetlands Year, David M. Carroll wrote about the aftermath of a flood:
“Here, green frogs relocated well up the terrace of uplands that borders the floodplain. Painted turtles basked placidly on the trunk and tipped-up root mound of a tall red maple, glowingly in bud, that had been taken down by the flood. Things did not go so well for the wood frogs. Their vernal pool was lost at sea. One to three hundred yards from the floodwater shores. The frogs could take to the high ground, but any egg masses they had committed to the pool were surely lost to the sweep of the flood or to predatory fish, to whom every inch of the floodplain was given for a time. Banded sunfish and redfin pickerel, small fish of the vegetation-choked backwaters, must have kept low in the emergent plants that for the time being lay deep under dangerous open water, where big fish could quickly capture little fish. Just beginning to bloom, flowers of the floodplain forest and flanking woodlands — Carolina spring beauty, trout lily, wood anemone, and bluets — had to get by for time as water plants.”3
It is in this same section of the same book that David Carroll touches on the raw force of the flood:
“I circle a deep hollow that is plastered with black leaves and ringed with stately cinnamon ferns and not-so-stately disintegrating remains of skunk cabbage, spent with the season. This silent, waterless bowl in the floodplain forest was brimful and raucous with wood frogs when I came her the second week of April. For weeks before, floodwaters from a foot or two to five feet deep swept through the entire riparian zone, and there was no way to distinguish river channel from oxbow, or surrounding marsh and swamp from either, other than by the winding rows of silver maples or red maples that lined their submerged banks. With the passing of the flood, water dropped back into its channels and pools and the shapes of the floodplain’s individual water-holds emerged: the river’s silvered run, glimmering vernal pools, scroll ponds like ribbons of sky fallen to earth, and the deep, horseshoe-shaped oxbow, bristling with emergent shrubs yet to leaf out, tawny jumbles of standing and crushed cattails awaiting the greening of their arundinaceous beds, and cloud-reflecting open water on the verge of being covered with lily pads.
“A week later, however, following three days of heavy rain, those individual identities were immersed once again in an encompassing flood. The river and its associated wetland, from headwater to the sea, were at full charge and immediately spilled over their banks. Surrounding uplands, many of them steep, were saturated and so contributed heavy runoff to the lowland floodplain. With so much water in the wetlands and in the upland soil, only the floodplain could contain the flood. Although its fury posed challenges for many plants and animals, the return of the flood here in mid-April was a consequence of natural forces and was acted out in a natural arena, with an ecologically built-in give and take. Along many rivers and streams, flood damage to humans and wildlife is greatly exacerbated by runoff from acres of roofs and paved surfaces, and by human alteration and conversion of floodplain wetlands that disrupt their natural regimen and greatly reduce their flood-storage capacity.”3
We are lucky in the Shawsheen River watershed in that the Shawsheen frequently and adamantly restates its claims on its floodplains each year. As a result, few are the houses that suffer immersion when the Shawsheen floods.
This is because high water fills the floodplains at least once and usually twice a year. Floods seem to occur every year-and-a-half or two years. Given that between 65% and 75% of the land through which the Shawsheen River runs is floodplain, much of the flooding we see occurs in the “natural arena” of relatively unmolested floodplains.
There is one aspect to flooding that is of particular interest to those of us in the SRWA who participate in the monthly river cleanups. David Carroll describes it quite nicely in the paragraphs cited above.
The power of large volumes of water reacting to gravity as it flows to the sea effectively scours the bottom of the river channel. When you ponder that a gallon of fresh water weighs eight pounds, that a cubic foot of water is approximately two-and-a-half gallons of water, that at flood levels the water is moving at around 1200 cubic feet per second (CFS), you arrive at a notion of just how much force the flood exerts.
When we pull a tire or a shopping cart out of the bottom of the river, it leaves a hole in the silt that has collected around it. When the water flows sluggishly, as it does most of the year in the Shawsheen River, the silt deposit around that hole remains, despite the fact that the debris that collected it is gone.
When the forces of high water and flood arrive, that silt moves off, lifted and carried away by the current. When the silt is gone, we encounter the next layer of tires and shopping carts. The channel is also slightly deeper.
The flood will typically last twelve to twenty-four hours. That is the amount of time roads will likely stay underwater. Chances are the water will stay high for as long as a week after the flood this year. The reason for this is that the groundwater will have been recharged by the snowmelt, and the river won’t have to backfill the water table as it always does when the sole source of water is rainfall. My guess for the flood is mid-March. Anybody want to start a pool? I’ll put a dollar on March 15.
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Footnotes:
1 E. C. Pielou, Fresh Water (Chicago: Chicago University Press, 1998), p. 147.
2 Alice Outwater, Water: A Natural History (New York: Basic Books, 1996), p. 59.
3 David M. Carroll, Swampwalker’s Journal: A Wetlands Year (New York: First Mariner Books, 1999), pp. 214-215.
