The Toronto ravine system is a distinctive feature of the city's geography, consisting of a network of deep ravines, which forms a large urban forest that runs through most of Toronto. The ravine system is the largest in any city in the world, with the Ravine and Natural Feature Protection Bylaw protecting approximately of public and privately owned land. The ravine system has been presented as a central characteristic of the city, with the size of the ravine system leading Toronto to be described as "a city within a park".
The ravine system began to take shape approximately 12,000 years ago at the end of the Last Glacial Period when the glaciers that once covered Toronto retreated northeast and left valleys and rivers that eventually formed deep ravines. Due to the topography of the ravine system, limited urban development occurred within it until the mid-19th century. Limited development continued in the ravine system until the occurrence of Hurricane Hazel in 1954. The destruction caused by the hurricane led to a halt to major developments within the ravine system. The Toronto ravine system remains largely undeveloped, with most of its public lands having been designated as parkland.
More than two-thirds of the publicly owned portions of the ravine system are made up of forests, with the other portions being beaches, meadows, open water, successional lands, and wetlands. Although large portions of the ravine system remain undeveloped, parts of the ravine system are used for flood management, carbon sequestration, and recreational purposes by the city. In addition to its human uses, the ravine system also serves as a habitat and wildlife corridor for a large number of animals.
Origin
The Toronto ravine system originated approximately 12,000 years ago at the end of the Last Glacial Period, with rivers and valleys being formed in the wake of the retreating glaciers and the compressed land it left. Meltwater from the glacial deposit at the Oak Ridges Moraine, situated north of the city, flowed southeast towards Glacial Lake Iroquois, carving through nearly of soil, alluvial soft sand, gravel, and clay till to form the ravine valleys. Most of the meltwater flowed southeast along shallow ice-scrapped depressions towards the glacial lake, creating small ponds north of Toronto. However, the early Don and Humber rivers are notable exceptions, with these waterways flowing along the compacted channels of the former Laurentian River System, an ancient river system that was scoured and buried during the Last Glacial Period.
After the glacial ice had retreated north of the St. Lawrence Valley approximately 9,000 years ago, the water level of glacial Lake Iroquois began to recede. The drop in water levels saw new waterways formed as the water meandered through the former lakebed into the newly formed Lake Admiralty, with the two main branches of the Don River joined at the old shoreline of Lake Iroquois, and new streams having been formed. The new lake existed until approximately 8,000 years ago, when the Earth's crust around Quebec and northeastern Ontario began to rebound, having been previously depressed under the weight of the glacial ice that had since retreated from that area. Water levels in the lake gradually rose as the earth's crust near the lake's outlet to the Saint Lawrence River rebounded. Major plateaus in the system, such as the lower Don Valley, were formed during this period, as rising water levels from Lake Ontario pushed into the lower valley, and sediment washed down from the upper valley accumulated in the lower waterbed. The flooding of the lower ravine system also resulted in the formation of several spits, lagoons, and wetlands near Toronto's shoreline.
Before the arrival of European settlers, the landscape from Lake Ontario to the Oak Ridge Moraine was dominated by forests of maple, beech, and hemlock trees, while its low-lying areas were made up of forests and shrub swamps, with extensive marches along the Toronto shoreline. A large number of the trees surrounding the Town of York were logged in the early 19th century, including many trees in the ravine system. However, portions of the ravine system saw limited human developments, with flood-prone areas of the ravine having prevented large-scale urban developments. As a result, the ravine system holds the largest collection of trees in the city that predates European colonization.
The ravine system also saw significant alterations as a result of human activity in the 19th and early 20th centuries, as several natural valleys were filled in, rivers rechannelled or buried entirely, and wetlands drained to facilitate new developments. The Don River was altered near the mouth of the river, with a concrete channel created to prevent the Port Lands from flooding. Creeks and ravines within downtown Toronto were also buried during this period, including Garrison and Taddle Creek. As a result of these changes, the Don River and a small portion of Taddle Creek were the only remaining waterways that flowed into Toronto Bay by the mid-20th century. The surviving portions of Toronto's ravine system primarily surround four waterways—the Don River, Highland Creek, Humber River, and the Rouge River—although smaller ravines can also be found along the other waterways of Toronto, including Etobicoke Creek and Mimico Creek.
Locations
The ravine system is situated on the north shore of Lake Ontario, straddling the Carolinian forest and the Great Lakes–St. Lawrence Lowlands. The landscape of the ravine system includes large river valleys and smaller creeks. Many of the larger waterways are surrounded by narrow steep-sided valleys, with the waterways having eroded the soft deposits around it to reveal large exposures of Ordovician rocks, many of these rocks visible along the stream banks. The ravine system forms a part of a larger network of ravines that extends towards the Oak Ridges Moraine, located north of Toronto. The largest ravines surround the rivers running south from the moraine towards Lake Ontario, the Don, Humber, and Rouge rivers.
The present ravine system is a protected natural landform under the Ravine and Natural Feature Protection Bylaw. The bylaw defines a ravine as a distinct landform with a minimum of change in grade between "the highest and lowest points of elevation," has vegetation cover, and has or once had water flowing through or adjacent to the landform. In addition to these landforms, buffer areas and some other environmentally significant areas are also covered under this act. In total, the act protects approximately 17 per cent of the land in the city, or about of land. The system contains at least 150 ravines, constituting the largest ravine system in any city in the world. The edge of the ravine system throughout the city collectively amounts to over , ten times the length of the Toronto waterfront. The system is primarily surrounded by residential areas.
Approximately 60 per cent of the protected land is publicly owned, whereas the remaining 40 per cent is held privately. Within this area there are 86 municipally recognized "Environmentally Significant Areas" and 1,500 urban parks. Most of the ravine system remains in a naturalized state for flood control purposes, given the high variability in the water levels of the rivers and streams. More than two-thirds of the publicly owned portions of the ravine system are forests, with its remaining portions being beaches, meadows, open water, successional lands, and wetlands. The ravine system accounts for approximately 36 per cent of the city's total tree cover.
Nearly the entire ravine system forms a part of the city's green space and natural heritage system, with municipal bylaws for those areas largely restricting human developments, with exceptions for compatible recreational and cultural facilities and essential public works. Legal ordinances that restrict developments within the ravine system include the Ravine and Natural Feature Protection Bylaw, which restricts the destruction of any tree in the system without a permit from the city, and TRCA Regulation 166/06, which prohibits any man-made alterations or filling of the ravine system's waterways or wetlands without a permit from the Toronto and Region Conservation Authority (TRCA).
Waterways
The Toronto ravine system forms a part of a larger regional watershed system that encompasses the Golden Horseshoe Greenbelt and the Oak Ridges Moraine. The ravine system contains seven watersheds, the Don River, Etobicoke Creek, Highland Creek, Humber River, Mimico Creek, Petticoat Creek, and the Rouge River. The Humber watershed is the largest of the seven watersheds, although the Don watershed constitutes the largest percentage of the city's land area, making up 32.5 per cent of the city. In total, there is approximately of watercourses that flow through the city and its ravine.
Most of the smaller creeks and streams are situated within the city limits of Toronto, although the source for the larger three rivers of the ravine system, the Don, Humber, and Rouge, originates from the Oak Ridges Moraine, a moraine located north of the city. The point where the Humber, Don, and Rouge River watersheds meet is near the intersection of Bathurst Street and Jefferson Sideroad at the Vaughan-Richmond Hill boundary.
Many of the ravines that surround these waterways branch off into smaller ravines, winding through commercial, industrial, and residential neighbourhoods of the city. During the late summer, many of the smaller streams will slow to a trickle or even disappear completely. During the spring and after major storms, creeks in the ravine will often overflow their banks. The shape of streams within the ravine system is subject to constant change, with large storms causing the area to flood, and erosion to occur on river banks and meanders. In particular, waterways in narrow ravines, and straightened/rechannelled waterways are particularly susceptible to change due to erosion.
As a result of heavy urbanization around Black Creek, the water quality of the creek is considerably lower in comparison to the other watersheds of the ravine system.
Biodiversity The ravine system holds the greatest variety of ecosystems, species and genetic diversity within the city. The variety in species is a result of the mosaic of ecosystems that exists within the system, including forests, wetlands, rivers and streams. The connected nature of these habitats helps support the extensive biodiversity within ravines.
Fauna
The ravine system serves as a habitat for approximately 1,700 species of animals and plants, including many locally, regionally, and provincially sensitive species and species at risk. Approximately 90 per cent of animals that reside in Toronto inhabit the ravine system.
In addition to providing a habitat in an urban environment, the ravine system also acts as a wildlife corridor, permitting the animals that live within it to travel from one area of the city to another. The corridors are of particular importance for wild mammals, as they provide the only safe passage for them to traverse from one part of the city to another to find new mates and establish their own homes.
Flora
The ravine system is one of the northernmost Carolinian forests found in North America, consisting of mixed woodlands and southern traces of the boreal forest. Around 1,000 plant species may be found in the ravine system, making up approximately 25 per cent of all plant species that can be found in Ontario.
Most flora that is native to the area, such as the oak, have the genetic diversity that makes them more adaptable and resilient to extreme climate events and pest infestations. Poison ivy and nettles are also abundant.
Invasive species A decline in the quality of vegetation composition was observed between 2009 and 2019, with invasive plant species spreading throughout the ravine system.