The St. Lawrence drainage region includes areas surrounding and draining into the St. Lawrence River, including the cities of Brockville, Montréal and Québec. Its 5,971 km2 of built-up area, accounting for 5.0% of the total area, was the second highest in the country in 2011. Natural and semi-natural area accounted for 81.3% of the area, followed by arable land (12.8%) and smaller amounts of natural land for pasture (0.8%).
This drainage region had the highest density of linear infrastructure including roads, rail lines and transmission lines at 1,093.9 m/km2 in 2011.
The population was 6,583,552 in 2011, with a population density of 59.9 persons/km2, the highest in the country. Population was up 33% compared to 4,941,807 in 1971.
Surface freshwater intake from drinking water plants, manufacturing, mining, irrigation and thermal-electric production was 2,045.2 million m3 in 2013, the second highest in the country.
The average annual water yield was 72.3 km3 over the period 1971 to 2013. The water yield per unit area was 0.61 m3/m2, compared to 0.35 m3/m2 at the Canada level. Monthly water flows peak in April.
Sources of nitrogen and phosphorus residuals in the environment include fertilizer application, livestock and poultry production and industrial emissions. On average residual soil nitrogen on agricultural land was 51.8 kg/ha in 2011 compared to 23.2 kg/ha at the Canada level. The risk of phosphorus release from agricultural soils was high, with 3.5 mg/kg of water extractable phosphorus potentially released in 2011.
Turbidity levels of source water for drinking water plants were among the highest in the country at 9.8 nephelometric turbidity units (NTU) in 2013.