Toronto ravine system

The Toronto ravine system is one of the most distinctive features of the geography of Toronto, Ontario, Canada. It is a network of deep ravines that form a large urban forest that runs throughout much of the city. For the most part designated as parkland, the ravines are largely undeveloped. Toronto's slogan: "The city within a park" partially stems from the extensive ravine green space.

The terrain that the city of Toronto sits on was formed after the end of the last ice age some 12,000 years ago. The glaciers flattened the terrain, and deposited a thick layer of loose sand and soil over the region. Over the millennia, small rivers and creeks eroded this soil cutting deep ravines through what is today the Toronto region. To the north of Toronto is the 1,900-square-kilometre (730 sq mi) Oak Ridges Moraine. The largest ravines are home to the rivers running south from the Moraine to Lake Ontario: the Humber River, the Don River, and the Rouge River. Smaller creeks and streams rise within Toronto from rain and melt water. The Don River is categorized as an underfit river, as it is too small for its relatively steep and broad valley.

Despite the dense population of metropolitan Toronto, many of the ravines have been left close to their natural state. The most important reason for this is the danger of flooding. The rivers and streams that flow through the ravines are highly variable. During the late summer, many of the smaller ones will slow to a trickle or even disappear completely. During the spring and after major storms, the creeks often overflow their banks. Every few decades, a massive flooding event will occur where the ravines are almost totally flooded. The most recent such event was in 1954 when Hurricane Hazel arrived. That storm dropped over 12 cm (4.7 in) of rain on the city in a day onto soil that was already waterlogged by a week of rain. In the post-war boom years, several developments had begun to encroach on the ravine lands, and these neighbourhoods were badly damaged by the storm. Whole blocks were washed away and 81 people killed. The damage was most severe along the Humber River, where a part of a street was destroyed completely. This disaster led to an almost complete ban on development in the ravines, and a new Region Conservation Authority was created to maintain them as open spaces.

...
Wikipedia