The Don Valley is notable because of its deep wide valley in the lower reaches.
At the Bloor Street Viaduct, the valley is about 400 m wide while the river is only about 15 m wide.
This is due to its glacial origins.
The Don River and its deep valley were formed about 12,000 years ago at the end of the Wisconsinan Glaciation.
During that glaciation which lasted for 35,000 years, all of Ontario was covered in ice.
As the climate warmed the glaciers began to melt.
As the ice front retreated in southern Ontario, several rivers were formed that drained into Lake Iroquois, a glacier lake which was the precursor to Lake Ontario.
The Don River is now small in comparison to the deep and wide valley that resulted from its glacial origin.
The Don River is now classified as an underfit river.

The landscape at that time was loose glacial till so the large amounts of glacier melt water eroded deep valleys over thousands of years.
As time progressed, isostatic uplift caused the earth's plate to rise and tilt.
This caused Lake Iroquois to drain towards the south.
A remnant of its shoreline can be seen on the north side of Davenport Road in Toronto.
In the Don Valley, the old shoreline is evident just north of Eglinton Avenue.
Today the source of the Don River is the Oak Ridges Moraine, another legacy of the Wisconsin glaciation.

The location of the old shoreline delineates a change in the soils in the Don watershed.
Soils north of the old shoreline are mostly luvisolic Halton Till while south of the shoreline they are sandy glaciolacustrine deposits.

The Don Valley provides an appropriate location for studying the regional geological history.
The Don Valley Brick Works was an old brick making factory with a quarry where they extracted shale.
At the rear wall, local geologists discovered a record of the past three glaciations.
There are nine distinct layers visible dating back 120,000 years.
The Don Valley is notable because of its deep wide valley in the lower reaches. At the Bloor Street Viaduct, the valley is about 400 m wide while the river is only about 15 m wide. This is due to its glacial origins. The Don River and its deep valley were formed about 12,000 years ago at the end of the Wisconsinan Glaciation. During that glaciation which lasted for 35,000 years, all of Ontario was covered in ice. As the climate warmed the glaciers began to melt. As the ice front retreated in southern Ontario, several rivers were formed that drained into Lake Iroquois, a glacier lake which was the precursor to Lake Ontario. The Don River is now small in comparison to the deep and wide valley that resulted from its glacial origin. The Don River is now classified as an underfit river.
The landscape at that time was loose glacial till so the large amounts of glacier melt water eroded deep valleys over thousands of years. As time progressed, isostatic uplift caused the earth's plate to rise and tilt. This caused Lake Iroquois to drain towards the south. A remnant of its shoreline can be seen on the north side of Davenport Road in Toronto. In the Don Valley, the old shoreline is evident just north of Eglinton Avenue. Today the source of the Don River is the Oak Ridges Moraine, another legacy of the Wisconsin glaciation.
The location of the old shoreline delineates a change in the soils in the Don watershed. Soils north of the old shoreline are mostly luvisolic Halton Till while south of the shoreline they are sandy glaciolacustrine deposits.
The Don Valley provides an appropriate location for studying the regional geological history. The Don Valley Brick Works was an old brick making factory with a quarry where they extracted shale. At the rear wall, local geologists discovered a record of the past three glaciations. There are nine distinct layers visible dating back 120,000 years.
Don River (Ontario)