The Windsor floodplain: the river has risen before …

By BILL CHAISSON
[email protected]
WINDSOR, Vt. — It came as a surprise to property developer Clifton McNaughton to find out that his new project, an old house on Main Street at the north edge of the village, was in the floodplain. That means that the price of his flood insurance will be high, half the price of the house. If you consult the Federal Emergency Management Agency maps at the Department of Homeland Security site, you find out that McNaughton is a bit unlucky. The “floodway” boundary runs north of Runnymede Lane, down the middle of Main Street, and then down Everett Lane. In other words, the Snapdragon Inn, across the street from his property is just outside the official floodplain and his house is in it. You have to draw the line somewhere, but why is it drawn there? Most the town going up Route 5 as far as Hour Glass Country Club is an official “floodway.”

Bends in the river

The town of Windsor occupies a bend in the Connecticut River. Just north of the town the river begins to swing east and somewhere behind the Dollar General it begins to swing back to the west again. About half a mile below the Windsor-Cornish Bridge it begins to bend the other way. These serpentine changes in river course are called meanders. They actually migrate downstream through a river’s course. There are two features in this migratory topography, the cut bank and the point bar, that move the path of the flowing water.

At Windsor the east shore of the river — the Cornish side — is the cut bank; the river is actively removing the sediment there and moving it downstream. Where does it go? Some of it goes to the next point bar. The north end of that point bar is under the agricultural fields between the 12 Percent Solution store on Route 12A and the river. Point bars can be fertile, as they are mixtures of inorganic mineral matter excavated out of the cut bank and organic matter that was floating in the river. There are broad agricultural fields for about a mile down the point bar on that side of the river.

The next point bar is on the Vermont side and the broad farm fields stretch down to just north of the Cedar Hill Continuing Care community. The point bar on the Vermont side north of this one is what some of Windsor was built on.

Floodplain topography

Above Windsor, sediment drops out of the west side of the river as its course swings eastward because the current is slower on the inside of the curve. The accumulating silts create a broad relatively flat area that is sometimes submerged during floods and temporary channels may also be cut through it. If you look at the old U.S.G.S. topographic map for the Mount Ascutney quadrangle, there is long thin pond at the edge of the field just east of where Route 5 and railroad veer away from each other. When you look at the satellite photographs of that spot in Google Maps, you can see that it is now a wetland, not open water. The river cut through, perhaps during the 1973 flood, and then the abandoned channel has filled in since then.

The Hour Glass Country Club sits on a rise above this old channel, but the Windsor Country Club sits on top of the next rise. These steep slopes are old cut banks, perhaps thousands of years old, a record the former course of the river. The narrow terrace that the Hour Glass is sitting on broadens to the south and leads directly to Lake Runnymede, which appears to be an old oxbow lake.

An oxbow is created when a deep meander forms; the river wraps around a very large point bar. Then a flood causes the cut bank above the bar the scour straight through the base of the point bar to the next cut bank. That leaves a U-shaped lake off to the side of a river that has taken a more direct course.

Lake Runnymede is bounded on the south by a large delta built out into the Connecticut valley by Kimball and Mill brooks. The brooks, probably in the early post-glacial period, spewed sand and gravel from the Green Mountain foothills, which were mantled with glacial till and not much vegetation, out onto the flat valley floor. This may have happened when Glacial Lake Hitchcock was present (it drained 12,000 years ago). In any case, this erosive material constitutes the high ground in downtown Windsor.

State Street runs along the height of land between Lake Runnymede and the post glacial valley carved by Mill Brook. Horseback Ridge, the high ground between Mill Pond and the Connecticut River is actually bedrock, not glacial material. This relatively immovable mass of slate and phyllite creates a bottleneck for the river; the meanders cannot propagate between this ridge and the bedrock hills on the New Hampshire side. Instead it must simply rush through.

A 1975 environmental geology report prepared for the State of Vermont by David P. Stewart “Geology for environmental planning in the Brattleboro-Windsor Region, Vermont” notes:

“The Connecticut River … has cut a valley into the New England Upland that shows little influence of the structure of the bedrock. In most sections, the river valley is narrow attesting to the fact that downcutting by the river into the complex bedrock has been a very slow process and that the river is still in the downcutting stage. The tributary streams have kept pace with the Connecticut River in the downcutting of their valleys, at least in their lower reaches, and therefore the stream gradients, in general, are low near the confluence with the master stream. The tributary valleys, nonetheless, have steep sides and rugged topography adjacent to them. The upper reaches of the tributary streams have steep gradients as they flow from the Green Mountains or the foothills on the east side of the mountains.”

The valley is a relatively narrow trough across which the river is free to meander in many places. What is it cutting down into? Mostly Glacial Lake Hitchcock sediments. This huge lake, created by a morainal dam across the valley at Middletown, Connecticut, existed between 15,000 and 12,000 years ago. During that time gravel, sand, silt, and clay was transported by tributaries into the Connecticut valley, where it settled in layers.

The large grain sized sediment gravel and sand, settles quickly when the tributary hits the standing water of the lake. Therefore most of your good gravel pits are in the valleys of the tributaries somewhere above the Connecticut River.

Down in the valley the finer sands, silts and clays collected. This contributes to the point bars as the river slowly redistributes this sediment on its way to sweeping it downstream. This more recent redistributed material is called alluvium. It is made up of the re-excavated glacial sediments and the newly excavated sediments being eroded from the tributaries each day of their existence.

The danger of flooding

The region between Windsor and Brattleboro, according to Stewart, is quite prone to flash flooding:

“These streams have steep gradients in their upper reaches but the gradient is, as a general rule, rather low away from the mountains. A second factor that tends to increase flash flooding is that the surface of the higher elevations of the region are either barren or they are covered with only a thin veneer of glacial till. When precipitation occurs, most of the water runs off into surface streams since very little water can seep into the subsurface. Therefore, when a large amount of precipitation falls in a short period of time most of the water goes into small streams that flow down steep slopes at high velocities. The small mountain streams, in turn, carry large amounts of water into the major drainage lines. When this rapidly flowing, large volume of water reaches the lower sections of the stream, where the gradient is much lower, the velocity of flow is decreased, the water piles up and flooding results.”

Stewart, writing only two years after the event, recalled the catastrophic flooding of 1973. In Windham County over 5 inches of rain fell in four days.(June 28-July 1). The Connecticut River flooded because its tributaries did. The normal elevation of the river at Vernon, Vermont is 218 feet. During the 1973 flood it was 227 feet. During the 1927 flood it was two feet higher than that. And during the 1936 flood it crested at 231.4 feet, over 23 feet higher than normal. Much of the floodplain north of downtown Windsor is between less than 20 feet above the river.