The province says it learned Friday morning of the spill of unknown hydrocarbons on Dodds Lake in Innisfail, south of Red Deer.
Hydrocarbons can include oil and gas.
Officials say the slick stretches about 15 metres and appears to have come from a drainage culvert.
Firefighters are using booms to stop the leak and prevent the slick from spreading.
The town's website says the lake is a popular swimming and water-skiing spot.