This mighty river used to be mightier

I love the varying moods of the Credit River in Mississauga, one of Lake Ontario's healthier rivers. A quick snow-melt and several days of rain have left the river high and the current swift, with possibly as much as 3x the volume as usual.

In some years the river is even higher. For example, the long and cold winter of 2014/2015 produced heavy ice that created an ice dam in Erindale Park during the spring, which flooded large parts of the river valley.

River break-up during the spring of 2015

Flooding in the Credit River valley in the spring of 2015

When the summer gets hot and dry the river is low, lazy and warm.

The Credit River in the summer of 2014, with Oscar and Mae Johnson hunting for aquatic invertebrates

As mighty and moody as this Credit River is today, there was a time not long ago when this river was even mightier - MUCH mightier! If you look at the first photo in this post, you can see the banks on the horizon from a time when this river was higher (30-40m higher) and wider - about 400m across at the spot where I took that picture!

When and how did this happen? If we roll back the clock 14-16 thousand years, this river valley didn't exist. There were no grasses, trees, deer or coyotes roaming the valley. Their ancestors were were further south beyond the glacier's reach, south of what is now Pennsylvania and Ohio. The would-be Credit Valley sat under 1-2km of the Wisconsin ice sheet. As the Earth warmed the glaciers began to melt and retreat further north. The melting glaciers gave way to large roaring rivers that that drained into Lake Iroquois, which would later become what is now Lake Ontario.  That is how our Credit Valley and all of the valleys along the north shore of Lake Ontario came to be; they were carved by these fierce rivers.

You can see the signs of this mightier river beyond the high banks in that first photo. As I walk up the valley to University of Toronto Mississauga, I see very young conglomerate rock made of gravel and sand cemented together right at the top of the valley, far beyond the reaches of today's river. These materials were the outwash of the large braided river that once ran here, just a few thousand years ago.

Conglomerate rock at the top of the Credit Valley, deposited by the river several thousand years ago. 

When I stand at the top of the valley, I sometimes like to close my eyes and imagine the river as it was in its glory, a mighty river with its banks lapping against the foundation of my house. When I open my eyes I see the river below as it is now, just a whisper of its former self. I wish I had a time machine.