On Monday March 3, the Windham Historical Society presented its first program of the 2025 season, ‘Why is Sebago Lake so deep?’ led by Don Wescott of the Presumpscot Regional Land Trust and Irwin Novak, professor emeritus of Geology of the University of Southern Maine.
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| Sebago Lake and the Presumpscot River are very important to the State of Maine and the Lakes Region community, providing water and aquatic life. COURTESY PHOTO |
Presenters said that the roots of the word “Sebago” come from the Abenaki people meaning, “great waters.”
Sebago Lake is the drinking source for Portland Water District, stretches over 14 miles, and covers 45 square miles. It stands at 270 feet above sea level, has a volume of 995 billion gallons, and has the deepest point at 316 feet, making it the deepest lake in Maine and perhaps New England, with the exception of Lake Champlain.
According to the presenters, the current Sebago Lake originally formed at the edge of the continental ice sheet over 14,000 years ago. During the later stages of glaciation, ice melted from the highland areas and a great ice block filled the lake basin.
They say that as the ice melted, water and ice filled a basin that had been scoured out by a combination of glacial ice and stream erosion. Post-glacial streams formed and shaped the surrounding landscape.
Moving ice and meltwater steams deposited massive amounts of sand and gravel at the southern end of the lake northeast of Sebago Lake Village forming an end moraine and delta complex. This end moraine and delta complex acted as a dam causing a large lake to form behind it.
Unable to flow in its old path to the south, a new lake outlet formed over bedrock outcrops at the east side of the lake, near Whites Bridge.
The presentation examined construction of the Eel Weir Dam beginning in 1820, just below Whites Bridge, which raised the water level in Sebago Lake about 12 feet to its present full pond level of 270 feet.
The rocks that are visible now were originally deeply buried and long periods of erosion exposed the deepest rocks.
The Presumpscot River plays a big role in Sebago Lake, being the main outlet of the lake, draining its water into Casco Bay. During the last ice age, glaciers depressed the land surface below sea level and when the glaciers melted, sea water flooded the coast.
Meltwater from the glaciers carried silt and clay to the seafloor and the weight of the ice sheet caused the crust of the earth to curve, the presenters said. As the ice melted, the crust did not immediately rebound, allowing the sea to flood inland.
The melting of the last Ice Age carved out the river valley and deposited glacial sediment, what we know today as the “‘Presumpscot Formation,” along its path. This sediment was largely composed of fine particles like clay and silt carried by glacial meltwater and deposited as the ice retreated, creating the river's current course through the landscape that was previously depressed by the weight of the glacier.
Today, Sebago Lake and the Presumpscot River are very important to the state and particularly our community. Sebago Lake provides water to about 16 percent of Maine with exceptionally clean water that requires minimal filtration, and the Presumpscot River acts as the main freshwater input to Casco Bay, supporting both aquatic life and recreational activities in the region.
While this information might not have a direct effect on us today, both Wescott and Novak shared that it is nice to know the information and history of where you live.
“Some may be interested in where our rocks came from and how Sebago Lake formed,” said Novak.
If you are someone with an interest in history, these facts and information might leave you with a different point of view next time you visit Sebago Lake, Wescott said.
“Next time you go to Sebago Lake station, it is likely that you will think and wonder what was happening here 15,000 years ago,” said Wescott.
For more details about upcoming Windham Historical Society presentations, visit https://www.windhamhistorical.org/programs as well as the information contained on its Facebook page. <
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