The Great Lakes comprise the largest fresh water system in the world. The basin is home to Canada's largest urban population, which exacts a toll on the lakes. Chemicals and pharmaceuticals are flushed into the lakes, altering the health of all species in the water. Features the Limnos, a floating laboratory used by the National Water Research Institute to monitor water quality, fish populations and invasive species like the zebra mussel. Inland, looks at the impacts of farming on waterways, and what's being done to clean them up. Examines the freshwater systems, which supply many small towns, and the Walkerton tragedy, which caused E-coli outbreaks, widespread illness and death.