Abandoned ring-shaped island is all that remains of failed Toronto engineering project

Nestled among the quiet refuge of the Toronto Islands, an unassuming and completely vacant ring-shaped island now home to a calm, picturesque pond is all that remains of a failed engineering project to bring fresh drinking water to a growing city.

The small island's official and somewhat utilitarian name, the Settling Basin, hints at its fascinating past: a story of ambition, urgency, and ultimately, failure. 

Many kayakers and canoers passing the island daily have no idea that it is all that remains of a 19th-century public works project aimed at quenching the thirst of a rapidly growing city, which later fell into obscurity.

Situated in the Toronto Islands' Long Pond across from Mugg's Island, the Settling Basin stands out from the surrounding archipelago in aerial views and maps for its unique shape and self-contained pond.

Eagle-eyed perusers of city maps may have noticed the island's curious ring shape and wondered if it was a natural or artificial landform.

Anyone guessing the latter would be correct, as this island was actually built as part of an ambitious but poorly executed project to deliver clean drinking water to Toronto in the 1800s.

As its name implies, the Settling Basin is an artificial (but naturalized over time) pool constructed to allow larger particles suspended in water to settle to the bottom as part of the water purification process — but the modern-day ring-shaped island was only just one visible element of this mammoth public works project.

The fascinating history of this engineering project is laid out in a Dec. 1929 issue of the Journal of the American Water Works Association, in an article by R.C. Harris, namesake of Toronto's masterpiece water treatment plant, which would later open in 1932.

Demand for a City-operated water system was growing in the mid-19th century, as Toronto experienced unprecedented population growth.

The city's fledgling population almost tripled from just 20,000 people between May and October of 1847, when 38,560 immigrants arrived, fleeing a crippling famine in Ireland, driving demand for a steady supply of clean, potable water.

According to R.C. Harris' 1929 article, the city was already plagued by a lack of supply by the mid-1850s, and the inability of private enterprises to keep up with demand spurred proposals for a municipal water system.

Despite the selection of a plan and a subsequent report in favour of the system's construction, Toronto would struggle with inadequate water supply and even worse water quality provided at what Harris described as "exorbitant rates" for another decade and a half before action was finally taken.

The construction of a water works system was finally approved in 1872, with a planned capacity serving three times the city's then population of 68,000. 

But there was some debate over where to source the city's drinking water. The Don and Humber rivers were among the options considered, along with the city's heavily polluted harbour. Even the distant Lake Simcoe to the north was on the table, though the infrastructure to move potable water over these great distances would have been costly.

The decision was eventually narrowed down to various sites along the shores of Lake Ontario between Scarborough and Humber Bay due to the lower water turbidity and distance from pollution sources. 

A plan to build an intake kilometres offshore was deemed too expensive for engineers of the day, and planners ultimately scaled back their ambitions, instead settling (no pun intended) on an infiltration basin on Toronto Island.

The Settling Basin is visible at the north end of a long channel in a 1903 report on park improvement plans for Toronto Island.

Constructed from 1873 to 1877, the basin was designed to allow water in via natural infiltration, where the still conditions would allow pollutants to settle out of solution. 

Despite plans calling for a capacity of 20 million Imperial gallons, the ever-scaled-back vision was ultimately built with just one-fifth of this capacity.

Site of the Settling Basin marked as a Water Works Department plot in an 1884 Atlas of the City of Toronto.

The settling basin was constructed attached to a 48-inch diameter wooden stave pipe across the Island, we well as its main connection to the mainland — a 36-inch diameter pipe made of cast iron, engineered with flexible joints, that carried water to a pumping station at the foot of John Street.

The Settling Basin marked on an Oct. 1918 plan of Toronto Island produced by the Toronto Harbour Commissioners.

While the pumping station on the current site of the Rogers Centre no longer exists, a relocated modern pumping station built in 1987 is located to the immediate south of the present-day stadium.

But even after a significant scaling back of the original vision, the resulting system still proved quite problematic. The 48-inch wooden stave pipe across Blockhouse Bay was not properly laid with a cover, resulting in it rising to the surface just a year after construction began.

The filter basin itself was also deemed inadequate, requiring channels to be dug in 1877 to better connect it with the lake.

Storms regularly clogged the system with debris, and by 1878, just a year after the system was built, it was already targeted for mothballing due to the mounting issues.

In a defeat for the engineers, the basin was abandoned after a new six-foot wooden stave pipe was installed in 1881, feeding a supply of water directly from the lake from a depth 25 feet roughly two kilometres offshore.

Even after it was phased out as the city's primary drinking water source, though, the basin remained in use as an emergency water supply, last employed in 1911 before finally being abandoned.

Over a century after its abandonment, Toronto now draws its drinking water from intake pipes deep below Lake Ontario, as far as five kilometres offshore from the city. The city's modern water treatment process involves disinfecting using agents like chlorine or ozone, before the water is treated with poly-aluminum chloride to encourage clustering of suspended particles.

Similar to the principles behind the long-abandoned Settling Basin, modern facilities include their own indoor — often underground — settling basins where larger particles settle out of treated water, leaving clear water at the top to proceed through further filtration and chlorination to remove suspended impurities and microorganisms.

So, next time you are drinking a cold glass of tap water, think of the abandoned donut-shaped island that paved the way for Toronto's modern water purification system.