The Straightening of the Don River
The lower Don River used to meander through woods and marshes before discharging into both Toronto harbour and Ashbridge's Bay Marsh, one of the Great Lakes' largest coastal wetlands. As the city expanded closer to the river, the Don's natural cycle of flooding became a problem. The slow-moving river and its marshy banks also became heavily polluted and, by the 1870s, were considered a threat to public health.
In response, the City's 1886 Don Improvement Plan channelized the lower Don River to both speed up and contain its flow, creating new industrial and transportation corridors along its banks. By 1892, the river had been straightened, widened, and deepened from near Gerrard Street to the lake.
As a result of this plan, more river valley land became available for development. The plan, however, did not fulfill its promise of bringing shipping to the Don. It also failed to improve sanitary conditions and prevent flooding, largely because it did not consider the full complexity of the Don River's natural hydrology. Beginning in 2006, the extensive mound of earth behind you was created to prevent the possible flooding of downtown Toronto through the West Don Lands.
Plaque via Alan L. Brown's site Toronto Plaques. Full page here.