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The River is a Restless Spirit - Life in the floodplain forest
by Gayle Goddard-Taylor
from Mass Audubon's Sanctuary magazine, March 2006 edition
(see for membership info)

Added to website April 3, 2006

The River is a Restless Spirit

Life in the floodplain forest

by Gayle Goddard-Taylor

In the northeastern corner of Belmont in the floodplain of the Little River sits a pocket-sized forest made up almost entirely of silver maples. A few of its trees are approaching 150 years of age, relics of a bygone era when the land upon which they stood was cropland or pasture that was regularly enriched with the spring floods. Freed of competition from other trees and bathed in sunlight, these trees grew fat in bole and branch and earned their keep by providing shade for cows and sheep.

The privately owned Belmont Uplands—some fifteen acres bounded by Route 2 to the north, Little Pond to the west, Little River to the south, and commercial buildings to the east—is no longer pastureland. The venerable old pasture maples are now surrounded and overtopped by their much younger progeny. Some have lost massive limbs or, in the case of the Mother Tree, have endured worse.

“Half of it fell last spring,” says Ellen Mass, president of the Friends of Alewife Reservation (FAR), inspecting the deeply lobed matriarch. It has been cleaved in two to expose a hollowing core, half lying on its side, but both halves still impressive in their girth. Last year, seven people could encircle the trunk with arms outstretched and just touch hands.

Before it split, the 119-foot-tall Mother Tree was considered one of the three largest silver maples in the state. But what is perhaps more significant is that it is a silver maple in a monoculture of its species that is the only forest of this kind in the heart of metropolitan Boston. Friends of Alewife Reservation and Belmont officials have been leading an effort to convince the state Department of Conservation and Recreation (DCR) to ensure the land remains forested.

That this northern floodplain forest is unique in an urban setting is only one of the reasons FAR wants to keep the Belmont Uplands from disappearing beneath a proposed condominium complex. Another is flooding, always a concern in the Alewife watershed of Belmont, Arlington, and Cambridge, which has become more severe in recent years, due in part to the loss of naturally pervious surface to pavement and drainage construction. As small as it is, the Belmont Uplands still provides valuable floodwater storage, according to preservation advocates.

Additionally, the woods and marshes there serve as an important stopover for migrating birds, as well as habitat for a wildlife population that ranges from muskrats and minks to foxes and coyotes. Also, the site’s merit as an urban outdoor classroom has prompted area educators to bring students there for hands-on learning.

Floodplain forest—a woodland that is regularly inundated with water that moves into the lowlands bordering a river—is a disappearing ecological treasure. While flooding usually occurs in spring when snows melt and rainfall is more abundant, it can also happen at other times of year with major rainfall events, as was evident here in New England last fall. Floodwaters carry sediments from higher up along the river’s course; and, as the force of the flow lessens, the nutrient-laden sediments are deposited in order of weight, with the heaviest rocks falling first and the fine silts last. This cyclical flooding regime continually refertilizes the floodplain and nourishes the well-adapted trees and understory.

“It’s a rough environment,” says Pat Swain, an ecologist with the Division of Fisheries and Wildlife Natural Heritage and Endangered Species Program. “The trees that live there have to be adapted to being flooded for months at a time and having sediment around their bases.”

Floodplain trees tend to be those that mature and set seed quickly. Silver and red maple, willows, river birch, box elder, sycamore, and cottonwood are some of the more common trees in this environment. “The floodplain forest tends to be less diverse than its associated uplands,” says Swain, “because there are simply fewer species that can survive here.”

In Massachusetts, a variety of floodplain forest types have been identified, but they can, for the most part, be lumped into three categories. Major-river floodplain forests exist along such watercourses as the Connecticut, Deerfield, and Housatonic rivers in the western part of the state while small-river forests can be found along the Ipswich, Concord, and Three-Mile rivers to the east. Transitional floodplain forest occurs adjacent to smaller tributaries to the Connecticut and along portions of the Housatonic.

While silver maple trees dominate all three types of forest, the severity of flooding, soil texture, and soil drainage dictate what other types of vegetation grow in each. Cottonwoods and American elms are present along major rivers while, in intermediate-river and small-river forests, cottonwoods are absent, replaced by green ash and elm.

Flooding along the major rivers is usually severe and produces permeable sandy soils through which drainage is rapid. The powerful annual flooding regime means that shrubs find little to welcome them. The habitat is suitable, however, for a herbaceous layer consisting of wood nettle and ferns.

These riverine corridors make for rich birding opportunities; warblers, thrushes and other songbirds feed in the insect-rich habitat and nest in riverside trees. The shady pools along the bank attract wood ducks and hooded mergansers while larger trees also provide perches for fish-hunting raptors.

Shrub layers are still lacking, for the most part, in intermediate-river forests, but the soils are somewhat different. Sandy or silty loam is more the norm, making drainage less rapid, and the number of herbaceous species is greater. In the poorly drained, small-river forest, some shrubs can get a foothold, most notably silky dogwood and buttonbush. The herbaceous layer here is far more diverse and can include water hemlock, water parsnip, and swamp candles.

At one time, the forests that stretched along the rivers in southern New England were a major ecosystem. With farming and subsequent development, floodplain forests have been reduced to isolated patches and narrow strips. But, while the owners of homes built upon floodplains may believe that dams and channels have tamed the rivers, that notion could not be more untrue. The prospect of damage from annual flooding may have been reduced, but the potential for the kind of havoc wreaked by a large-scale flood still remains. In fact, the loss of floodplain forest increases the prospect of damage because it decreases the land’s water-storage and sediment-filtering capacity.

“Putting your house on a floodplain is like putting your house on a railroad track,” says John Kricher, biology professor at Wheaton College and author of A Field Guide to Eastern Forests. “A floodplain forest should not conjure visions of terra firma,” he says, noting that such woodlands are neither aquatic nor terrestrial. “The important thing to remember is that a floodplain forest is part of the river. It exists because of flooding from the river.”

A river is restless. Over time, its waters move its course by grinding away the soil on the windward side and depositing it on the other. Sometimes these changes happen quickly, undermining embankments and toppling trees. Where those trees finally come to rest, soil becomes trapped and builds up, assisting in changing where the river runs. Landscape features, such as oxbows, form as the river slowly carves a serpentine path and then, in times of raging floods, shortcuts across land to leave a meander scar.

“Geologists will tell you that over time a river will occupy every part of the floodplain,” says Pat Swain. “A river is constantly cutting and leaving meander scars and creating oxbows.”

Floodwater detritus that invades the adjacent forest will often snap the trunks of young trees, which, like the Hydra’s head, regenerate by sending up multiple trunks. Where the floodwaters are at their fastest, the river’s banks are scoured—stripped of vegetation, soil, even rocks and trees. And when the water spreads out over the floodplain, it brings not only minerals but also organic matter—from fallen leaves to fish—whose decomposition also boosts the soil’s nutrient content.

A forest that has been scoured of vegetation in severe flooding regenerates through a variety of adaptations. The floodwater actually aids in dispersing seeds, which in some cases produce plants that send down deep taproots. Certain trees such as cottonwoods and birches have short life cycles that allow them to grow to maturity and disperse seeds quickly. Others waste no time sending down deep taproots, and many species seed bank generations to come, waiting until conditions are right, and then promptly take hold.

This flood-blasted area is also vulnerable to invasion by exotic species such as purple loosestrife, Japanese knotweed, and Japanese bamboo, all of which take advantage of disturbed areas. “Along the Housatonic and Connecticut rivers,” says Swain, “Japanese bamboo has gone from hardly there at all to being a major problem. Once it gets in, the natives have a hard time germinating.”

Along the upper reaches of the Connecticut, New England’s largest river, what remains of a grand forest tapestry that once stretched unbroken along the river’s floodplain is slowly being rewoven. The Department of Conservation and Recreation has been acquiring key forested parcels as opportunities arise.

“There’s not a lot of floodplain forest left,” says Terry Blunt, director of the DCR’s Connecticut Valley Action Program. “Much of the agricultural land extends to within 150 feet of the river.”

But in the North Hadley and Northampton areas, the effort has created three large forested areas totaling well over 200 acres. One of these tracts, the 112-acre Floodplain Forest Reserve in Hadley, has earned the designation of Representative Natural Area, a distinction that implements cutting and management restrictions and allows natural processes to take their course.

The bulk of the reserve, a puzzle whose 12 to 14 separate pieces were reassembled over a ten-year-period, is located where the Fort River joins the Connecticut. This forest community—large cottonwoods reaching 30 inches in diameter, silver and red maples, willows, and a sprinkling of sycamore and ash—will now be used for research and limited recreation.

“It’s a prime example of what the floodplain forest would have looked like before people began encroaching,” says Blunt.

“Apart from preserving ecological habitat, a wildlife corridor, and a greenway,” says Blunt, “the DCR has some very practical reasons for restoring these communities.” The broad areas of the floodplain forest are natural water-storage areas. During significant floods—and such events can occur any time of the year—water overspreads the flat wetlands and loses velocity. The trees act as filter strips, holding back sediments and reducing the extent of downstream damage.

“There are still privately held parcels that could be acquired,” Blunt says. “The biggest threat isn’t housing because most communities have at least some kind of prohibitions for building in a floodplain,” says Blunt. “The big issue is recreational sales. Everyone wants their own riverfront campsite.”

For the tiny silver maple forest known as the Belmont Uplands, the only campsites are those occasionally used by the homeless. Housing, however, is a looming threat. In February, a proposal for 300 apartment units on four and a half of the parcel’s fifteen acres received approval for financing from the Massachusetts Development Finance Authority.

Opponents want to see the forest annexed to the Alewife Reservation. One scientific estimate indicates that a development of this size would swell stormwater runoff by 2.26 million gallons a year. Additionally, they also cite a recent preliminary study placing the hundred-year-flood elevation 28 inches higher than previously established by the Federal Emergency Management Agency. This would put the lowest floors of the buildings below state building-code requirements.

Conservation advocates have cited a variety of funding sources that could be tapped to acquire the land and contend that leaving the naturally occurring floodplain forest processes in place better exemplifies smart growth.

“Putting housing here would be totally un-smart growth,” says FAR’s Ellen Mass. “Basically, the people living here would be surrounded by a mosquito marsh.”

Gayle Goddard-Taylor is a field editor for Sanctuary magazine.