Just how were the Kawartha Lakes formed?

The Peterborough Examiner

25 Nov 2023

DREW MONKMAN

It’s hard to over-emphasize how fortunate we are in the Peterborough area to have such a wonderful chain of clean, beautiful lakes right on our doorstep.

It’s easy to take them for granted. I learned this lesson when I lived in Quebec City and later in Edmonton. The absence of easily accessible, high-quality lakes was one of the things I missed the most. But how much do we really know about our lakes? How were they formed?

The Kawartha Lakes are the headwaters of the Trent River system which drains into Lake Ontario. They include Balsam, Cameron, Scugog, Sturgeon, Pigeon, Little Bald, Big Bald, Buckhorn, Chemong, Upper Chemong, Lower Buckhorn, Lovesick, Stoney, Clear, Katchewanooka and Rice. There are also many other smaller lakes such as Sandy, Coon Lake, and Big Cedar, and the many lakes that make up Kawartha Highlands Provincial Park.

Shield-contact lakes

If you look carefully at a map of the Kawarthas, you’ll notice that the more northerly lakes — Stoney, Lovesick, Lower Buckhorn and Bald — follow an irregular east-west orientation along the edge of the Canadian Shield. On the northern shores lie extremely hard, 600-millionto three billion-year-old shield rocks like granite. On the southern shores, we find much softer, 450-million-year-old Ordovician limestone.

The lakes essentially lie in a groove or valley at the junction of the limestone and shield rock types. Well before the first glaciers, the initial valley probably formed as a result of rivers flowing east along this junction. The flowing water found it much easier to cut down and southward through the relatively soft limestone than through the extremely erosion-resistant shield rock on the northern shore. This process deepened and widened the lakes.

A curious feature in areas along or just back from the southern shores of Stoney, Lovesick and Lower Buckhorn is the presence of limestone cliffs, some measuring 30 metres high. The easiest ones to see run east, starting at the junction of Highway 28 and Stricker’s Lane, just south of Burleigh Falls.

The presence of the cliffs is connected to the nature of the limestone itself, of which there are two kinds — a layer of strong, more erosion-resistant limestone lying on top of a softer form of the rock. Millions of years ago, these layers were uplifted by the movement of the Earth’s tectonic plates and now angle slightly downward to the southwest. Because of this angled uplifting, part of the lower, weaker bed of limestone was brought to the surface.

It is this layer that makes contact with the shield and was largely eroded away by the ancient, eastward-flowing rivers described above. However, as the river cut laterally and southward through this softer limestone, the harder bed of limestone lying above it was eventually undermined by the water. Large blocks of this harder variety eventually broke off and came crashing down into the valley below. The result was a steep cliff face. Niagara Falls was formed in much the same way.

The more southern lakes

The other large Kawartha Lakes such as Clear, Chemong, Buckhorn, Pigeon and Rice occupy valleys that were also cut into the limestone bedrock by ancient rivers. The valleys lie on a northeast–southwest orientation which is the same orientation that the glaciers followed. These ice sheets deepened the valleys as they moved southward and also left behind thick deposits of rock, gravel and sand on the uplands between the valleys.

Of special interest is Sandy Lake, located just west of Buckhorn on County Road 37. The first thing you notice is its slight turquoise colour. The lake bottom is different, too. It’s made up of soft, white “marl,” a mixture of silt, clay and calcium carbonate. Sandy Lake is also unique in being totally spring fed and usually clear of turbidity. The turquoise colour results from the minerals present in the surrounding limestone and carried by the springs into the lake.

The drainage system

The water draining into the Kawartha Lakes comes largely from the Canadian Shield through tributaries such as the Mississauga River (drains into Lower Buckhorn) and Eel’s Creek (drains into Stoney). However, a few rivers that feed the lakes flow from low-lying areas of limestone bedrock and glacial till located to the south. For example, the Scugog and Pigeon rivers actually flow northwards into the lakes of the same names; Jackson Creek flows eastward from the Cavan Swamp into the Otonabee River; and the Indian River flows into Rice Lake, delivering water from Stoney.

The Indian River is famous for the Warsaw Caves. They were formed by the flow of glacial meltwater when the glaciers began to retreat 12,000 years ago. The ancient Indian River was more like the modernday Niagara River than the shallow, placid river of today. The deep, swift, glacier-fed water shaped the landscape bedrock in the Warsaw Conservation Area leaving behind caves, limestone cliffs and kettles. The latter were formed when granite stones trapped in the river current were spun around in place, grinding their way into the underlying limestone.

The lakes have changed

If you were to travel back 200 years, the first thing you’d notice is how much smaller the lakes were. This is especially true for Chemong, Pigeon and Clear. Because of the need for water power and transportation links between the various settlements, dams and locks were built in what was to become the Trent-Severn Waterway. The highly controlled water levels made these lakes both deeper and wider. What were once low-lying forests around the perimeter of the original lakes are now shallow bays, often full of submerged logs and tree stumps.

Maybe the least recognizable body of water in pre-settlement times, however, would have been Rice Lake. Huge beds of wild rice grew around the perimeter of the lake and were harvested by native people. They also attracted multitudes of waterfowl which were also a critical source of food. The beds were destroyed, however, when the dams and locks caused water levels to rise.

The Otonabee River was nearly unrecognizable, too. Before the construction of the waterway, the stretch of the river between Peterborough and Lakefield was characterized by extensive sections of rapids. Rather than carry their canoes around the many rapids, local Indigenous peoples and the first Europeans often preferred to take the portage route that used to run between Little Lake and Chemong.

The next time you’re out for a drive or boat ride in lake country, keep in mind how our heritage of lakes and rivers was formed and how much these waterways have changed.

DREW MONKMAN IS A RETIRED PETERBOROUGH TEACHER AND COAUTHOR OF “THE BIG BOOK OF NATURE ACTIVITIES.” REACH HIM AT DMONKMAN1@COGECO.CA. TO SEE PAST COLUMNS, RECENT NATURE SIGHTING AND HIS OTHER BOOKS, GO TO DREWMONKMAN.COM.