Flooding on the Thames River

Springbank Drive, London, April 1937

The Thames River has been a focus for human activity for centuries. Indigenous peoples and the early European settlers relied on the river as a source of abundant resources and as a corridor for travelling through the forests that covered Southern Ontario. The first settlements along the river were small and transitory, but soon grew to take on a more permanent nature. As human activity near the water increased, it was affected more and more by the seasonal fluctuations of the Thames.

The normally placid Thames was periodically subject to severe flooding which disrupted the new communities built in the river’s extensive floodplains. The first written account of flooding after European settlement on the Thames River dates from 1791 and floods of various levels were recorded regularly after that. However, because of the long time interval between severe floods, residents tended to forget the potential for flooding and built more in the floodplain than was wise. Other activities, such as clearing forests and draining wetlands, also increased the severity of floods.

In 1883, a severe flood on the Thames killed a dozen people in London and caused extensive damage. The 1883 flood helped identify the need to protect people and their properties from repeated flooding. In London, construction began on a system of dykes that would prevent future floods from inundating low-lying properties next to the river. This measure proved to be insufficient, though, when the worst flood ever recorded on the Thames hit in April of 1937.

The devastation left in the wake of this flood convinced many people that the only solution would be to construct a series of dams that, together with the dikes, would control floods and protect lives and property. In 1947, the Upper Thames River Conservation Authority was formed to respond to citizens’ concerns about flood control. The UTRCA undertook a program of flood control, building structures and developing flood forecasting measures for the upper watershed of the Thames River.

Dundas Street bridge, London, April 1937

Historically, the major floods in the upper Thames watershed have occurred from January to April; however, flooding is possible at any time of the year. More recent floods include March 1977, September 1986, July 2000, April 2008, December 2008, and February 2018.

Spring floods are usually the result of major rainfall when the ground is saturated and/or snow melt. In all cases the watershed is usually saturated and any snow pack tends to be well ripened (very moist and quick to melt). Runoff is generally rapid, with major peaks occurring within a day of extreme temperatures or heavy rainfall. The highest flows are generally associated with major rainfall. Floods mainly due to melting snow tend to produce lower, more prolonged flows.

Floods at other times of the year (May to December) are the result of heavy rainfall and problems may be compounded by the lack of storage space in the flood control reservoirs. In addition, intense summer thunderstorms may produce localized flooding problems on smaller watercourses and areas drained by urban storm sewers.