Watersheds in Southern Ontario - Part 1

Since I retired in 2022, I’ve taken the time to learn an open-source software package called QGIS, which is a geographical information system (GIS) that allows me to create maps from publically available data. This new skill has led me to draw maps of stream networks and watersheds and I have developed an appreciation, even fascination, of watershed maps.

I’ve previously defined a watershed as: “An area of land that catches rain and snow and drains or seeps into a marsh, stream, river, lake or groundwater” (Reference: Watersheds 101 // Conservation Ontario)

Watershed boundaries are drawn on the basis of where water flows, and since water flows downhill, we really only need a good ground elevation map to draw watersheds. I like to think of it this way: if you pour water on the ground, where is it going to go? The answer: water will flow downhill (or into the ground).

I’m going to present three of Ontario’s watersheds in this article. I’ll post different watersheds in future articles.

This first is the Thames River watershed in southwestern Ontario. Flowing from the northeast to the southwest, there are two main tributaries: (1) The North Thames River, which flows north to south from near the town of Mitchell, through St. Marys and (2) the main Thames River, which starts north of Innerkip and flows through Woodstock. These two branches converge in London, Ontario and the Thames River continues southwest through Chatham, Ontario before discharging into Lake St. Clair. Maybe it’s the red colour, but the stream network for this watershed makes me think about the complex network of blood vessels and capillaries in our bodies.

The second watershed, shown below, is that of the Nottawasaga River. I love the shape of this watershed, as it resembles an inverted tree. The Nottawasaga River flows south to north. Headwaters in the west are on the Niagara Escarpment, as tributaries send water from west to east to the main river. In the east, water flows to the west from near Lake Simcoe. The main channel of the Nottawasaga River discharges into Georgian Bay at Wasaga Beach. The stream network for this watershed, shown below, makes me think about what our lungs might look like.

The third and final watershed I want to present is the Ganaraska River watershed. The headwaters for the Ganaraska River are up on the Oak Ridges Moraine. Tributaries flow from the moraine in the north to the main branch of the river, which flows east to west before turning to the south towards Port Hope, where it drains into Lake Ontario.

When I first isolated the stream network for this watershed, it reminded me of the iconic pine tree in Killbear Provincial Park:

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