Geology Overview
The Winooski River begins eastof the Green Mountains, cuts through them, and then empties into Lake Champlain. Most of rivers in Vermont are referred to as “subsequent” which means they flow parallel to a mountain range. The Winooski River, however, is what geologists call a “superimposed” river, which means that it flows perpendicular to the mountain range. This means that the Winooski River predates the mountain range and was able to cut through the Green Mountains at the same rate at which the mountains rose 450 million years ago.
Courtesy of the Vermont Historical Society
Geologic features form as a result of continuous hydrologic processes over very long periods of time. As water flows over bedrock, it erodes and deposits material that, in turn, forms a variety of structures within the channel. The different flow rates form different structures. Where the river flows fast, it erodes sediment from the bottom and carries it downstream. Where it slows down, it deposits the sediment back on the bottom. For example, in a river meander (s-curve) the outer bank is being eroded while the inner bank receives sediment. This dynamic process allows the river to migrate over the entire floodplain. When the natural character of a river is altered, for example by damming, building bridges, road beds and railroad beds, the hydrologic processes governing the river are no longer the same.
Waterfalls, Cascades, and Gorges
Middlesex Gorge
Before Middlesex Gorge, the Winooski River alluvial plain averages over 100 feet in width. Upon entering the gorge, which is about 1,000 feet long with high angled rock walls (20 to 60 feet high), it narrows to a width of 15 to 30 feet. The rocks are vertically bedded schist and contain “potholes” which form in rocks beneath turbulent water.
Bolton Falls
There is a power dam at the bottom of the Bolton Falls Gorge. It is not a natural fall, and the site has been redeveloped to once again produce hydroelectric power.
Allen Brook Cascade
A cascade differs from a waterfall in that the water never falls free from the bedrock. Allen Brook (near Muddy Brook) is a small stream, three to five feet wide, which cascades about 200 feet downstream, dropping a total of 20 feet. Small potholes are common in this area of dolomitic bedrock.
Lime Kiln Gorge
In Lime Kiln Gorge, which extends about 250 feet, the Winooski River flows about 70 feet wide. The walls are nearly vertical and rise 15 to 70 feet. The limestone bedrock forms steep, fractured and jagged walls. There are rippled rocks at the upstream end of the gorge and several caves on the north shore.
Winooski Gorge
The Winooski Gorge is a uniquely formed gorge which is dammed, and relatively inaccessible. The gorge is more than 150 feet across at its narrowest point. The walls range from 50 to 80 feet high, and are composed of dolomitic limestone. Small caves exist along the walls of the river and flooded portions of the gorge may also contain large potholes and sculpted ledges.
The Glaciers
The glaciers that covered Vermont affected how the watershed looks today and the types of soils. Melting water formed streams in the ice. These streams carried soil and gravel down with them as they formed valleys on their way to Lake Champlain. The water dropped the mixed up soil material it had scraped and picked up earlier. The lightest particles of soil (silts and clays) ended up in the Champlain Valley. Generally, the soils are thinner on the peaks of mountains and higher hills and thicker on the sides of these mountains, hills and in the valleys. Geologists estimate that before the glacier began invading this area, the Green Mountains were as high or higher than the Himalayas, today.
At one time a large glacial lake (Lake Vermont) covered all of the lower watershed and went up to the edges of Washington County along the Winooski River into the Mad River, the Dog River, the North Branch, Pekin Brook, Stevens Branch and Thatcher Brook. The heavier “lake bottom” soils and sandy beaches of this lake can be found in towns along these streams. The sandy beaches of an earlier Lake Champlain can be found in the soil in Burlington, Colchester and other towns close to the Lake.
This section draws considerably from a website created for the Winooski Valley Park District by Natures Compass.