Laurentian River


See also: High Park's Hidden Rivers article

See also: High Park Geology

By Karyn Klaire Koski

Left undisturbed for thousands of years, an ancient buried river exists about 50 meters below High Park. It lies within a glacially modified trench that starts 110 km away. Sections of this underground trench are known to be up to 30 km wide.

The buried riverís water is cold, iron rich and very slow moving. The valley is buried beneath a thick sequence of gravel, sand, silt and clay sediments. It also contains pockets of coarser sand and gravel that yield large amounts of groundwater. These pockets are called aquifers.

Proof of the Laurentian Riverís existence as far south as High Park occurred in 2003 when work crews drained Howard and Ridout Ponds in the Parkís northeast section (to enlarge these sedimentation ponds). They discovered previously dug and capped wells from the late 1950s. A deep borehole observation well was dug nearby to explore the bedrock levels and test the waters below. It unexpectedly erupted geyser style indicating that a source of unreleased pressure had been struck.


The Laurentian River / Channel

As High Park has the lowest elevation in the area, all underground creeks and surface waters flow toward it.

The Laurentian Channel is just above bedrock level. Some obstruction is thought to be blocking its smooth exit to Lake Ontario, causing the build up of water pressure.

Black circles on the map are wells that penetrate the bedrock surface.

An Aquifer

Made up of porous rock and sediment layers which allow water to flow slowly, but freely.Can be tapped by drilling a well.


An unconfined aquifer has a natural outlet, a confined aquifer does not.


When drilling into a confined aquifer, tremendous built-up pressure is released. This is what happened at High Park.

Hidden Laurentian Riverís east bank proved to be below Spring Road

July 2003: A work crew drains Ridout & Howard Ponds.


They find 2 previously dug wells that have been capped circa 1959.


They decide to drill an observation well near the curve in Spring Road.


The observation well blows under pressure bringing forth proof of a confined aquifer.

All images provided by Steve Holysh



In the late 1800s, geologists discovered clues that suggested a preglacial drainage network existed that centred on the Great Lakes Basins. This extensive drainage network was a valley system created by the erosive forces of rivers which had cut deep, deep gorges; the result of millions of years worth of scraping and melting from vast ice sheets. The Laurentian River is only one channel within a larger drainage network.

The Laurentian River or Channel stretches from the Wasaga Beach area (on Georgian Bay) to High Park, Toronto (on Lake Ontario). It lies within a 110 km long bedrock trough, that in places, measures roughly 30 km wide. It is also believed to continue north from Wasaga Beach beneath Georgian Bay and also more southeastward beneath Lake Ontario. Its true length is still unknown.

The Outflow Pipe

While the well has been sealed, the riverís clean water, tinged red with iron, continues to flow from an overflow pipe into the north end of Spring Creek.

Look for it below the footbridge at the turn in Spring Road, just south of Ridout Pond.

Frank Remiz

Water flowed along the Laurentian Channel prior to glaciation and also perhaps during some episodes of de-glaciation, (as the ice melted). Then the Laurentian Channel was left buried and infilled with up to 200 m of sediments consisting of a mix of gravel, sand, silt, clay and glacial till. Because it is so deep, it is difficult to observe and research.

The base of the valley lies at an elevation of about 25 m above sea level in the northern part of Toronto (in a borehole drilled near Weston Road and Finch Avenue) and at about 96 m above sea level further north (in a borehole drilled to the west of Barrie). Based on these key locations, the bottom of the valley at Wasaga Beach on Georgian Bay is believed to be about 63 m below the Georgian Bay water level. At Lake Ontario the bottom of the channel is believed to be at an elevation of about 30 m below the Lake Ontario lake level. This drop indicates its southerly flow.

Within the channel are many pockets of subsurface gravel and sand. These are called aquifers, which can also contain vast amounts of slow moving groundwater. One such aquifer was discovered in High Park during the summer of 2003.

After draining Howard & Ridout Ponds, work crews located two capped wells that had been dug in the late 1950s. A discovery borehole was drilled nearby. Expert hydrologists encountered bedrock at a depth of 44.5 m (146 ft), which was at an elevation of about 42 m above sea level. Since the bottom of the valley is believed to be lower, the Spring Road borehole is likely to be positioned on the side of the valley and not in the centre. It was observed that a finer-grained till and silt was in the upper 36.6 m (120 ft). Below that, a coarser grained sand and gravel aquifer was encountered. It quickly rose to some 20 m above the ground and flowed up to 1,800 litres a minute before slowing down to 55 litres per minute after a couple of days.

A surface outflow pipe can be found near the footbridge located along the curve in Spring Road. It relieves hydrostatic pressure. The outlet is stained red indicating a high iron content.

The significant artesian conditions at the Spring Road location suggest that the permeability of the Laurentian Riverís sediments decreases as it travels south. In other words; the pressure built up proves the waters are somehow restricted and do not flow underneath into Lake Ontario.

By Karyn Klaire Koski


Dr. Andy Bajc Quaternary Geologist, Geological Survey

Steve Holysh Chief Hydrogeologist, Toronto & Region Conservation Authority.



Topographical interpretation of the Laurentian valley based on available outcrop, borehole and reflection seismic data

Basin Analysis Applied to Modelling Buried Basins in the Great Lakes Basin


Reprinted from Toronto Water Watch newsletter, Fall 2003

High Park is well known to nature lovers for its green space and ponds. More recently it made the headlines for its newly discovered underground river (aquifer)! City engineers and hydrologists discovered the river running under the park during construction of stormwater ponds. The river has flowed silently for hundreds, perhaps thousands, of years and is part of the underground Laurentian river system. Scientists think that the river winds from Georgian Bay, under Toronto and into Lake Ontario. Until this discovery, no one knew where the Laurentian river system flowed through Toronto.

The underground river is a great find for Toronto and could be a conduit of groundwater from Georgian Bay or the Oak Ridges Moraine. Early tests show the water to be good quality with a high iron content.

The river burst onto the scene during construction of two stormwater sedimentation ponds, which are part of the Cityís efforts to improve water quality in area streams and along the waterfront. While draining the ponds to do the work, two artesian wells (deep-flowing water wells) were discovered. The City capped the wells to comply with Provincial regulations and drilled a new deep monitoring well for groundwater research. While drilling the hole for the new well, the water pressure from the underground river burst, spewing water high into the air. Currently, the artesian flow from the underground river is being used as a habitat for coldwater species.

See also Lost Rivers website, High Park Borehole report, Toronto Star article 2003

See also Basin Analysis Applied to Modelling Buried Basins in the Great Lakes Basin, Sharpe, D R; Russell, H A J; 2004


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