Impacts of European settlement (1840–present) in a Great Lake watershed and lagoon: Frenchman’s Bay, Lake Ontario, Canada

The northern shore of Lake Ontario is one of the longest settled parts of Canada beginning around 1795. Accelerated settlement and deforestation after 1840 resulted in massive soil loss from easily-eroded Pleistocene glacial landscapes and the siltation of creeks and lagoons. Channel capacity was reduced but river flow was enhanced by diminished infiltration resulting in straightening of meandering channels, accelerated erosion of stream banks, increased incidence of downstream flooding and large influxes of mud to Lake Ontario. Conservation measures after World War II were successful but rapid urban sprawl after 1970 hardened watersheds and badly impacted the quality and quantity of surface and ground waters flowing to Lake Ontario. The Frenchman’s Bay watershed (27 km2) 50 km east of Toronto is one of the country’s most urbanized (pop: 100,000; 76 % urban cover) and is crossed by Canada’s busiest highway (Highway 401). The watershed drains to Lake Ontario through a coastal lagoon (Frenchman’s Bay) in which pre-settlement postglacial carbonate is abruptly overlain by a ‘European settlement’ mud layer rich in weed pollen and organic debris; the uppermost ‘urban’ part of this deposit shows elevated level of metals and other contaminants. This layer records soil loss after 1840 and more recently, the influx of contaminated urban waters and sediment. Some 7,600 tonnes of road salt have been applied to the lagoon watershed each year producing spikes of brackish surface runoff during winter thaws. Some 50 % of the total salt applied to the entire watershed is conveyed directly to Frenchman’s Bay Lagoon via surface runoff; the rest enters the groundwater system resulting in year-round brackish baseflow to creeks. Chloride continues to be stored in underlying aquifers such that the system has yet to reach a steady-state discharge. Future salinity of baseflow reaching the lagoon can be expected to increase by about 40 %. Rapid migration of contaminated groundwater is facilitated by the widespread presence of thick (<8 m) coarse-grained and heterogeneous fill materials of the built landscape. The watershed is experiencing ongoing changes in land use as urban infilling proceeds. The aquatic ecology of inflowing creeks to the lagoon has been greatly impacted resulting in major loss of wetlands and submergent vegetation and distinct changes in the structure of fish populations. This is the most detailed study of an urban watershed in Canada; lack of knowledge elsewhere is a constraint on the design and testing of mitigation measures and is a major impediment to assessing the impact of ongoing climate change on urban water resources, and the effects of urban runoff on Great Lakes water quality.