It is important to remember that bacteria are always present in surface water. This is why it is never safe to drink untreated surface water. But swimming is usually fine because there is a much smaller risk of ingesting bacteria that will make you sick.
Public Health Units carefully monitor E.Coli levels at public swimming beaches. If an average of samples is greater than 100 cfu, a public beach will be closed. Bacteria levels in our lakes, even at single sites, almost never reach these levels. Also remember that a single high result probably does not mean that there is bacterial contamination. If multiple adjacent sites have E.coli readings above 100 cfu/100mL, further action should be taken. In this case, contact the MLA office.