At the heart of the US-Canada border sit The Great Lakes – the world’s largest freshwater ecosystem, containing as much as one fifth of the planet’s supply. The water’s journey from source to sea spans half a continent, two thousand miles, and takes more than three centuries to complete. These immense lakes each have their own character and moods. Cold, deep Superior can rage like a furious sea with waves 30ft high. Michigan, plunging far to the south, contains one of the busiest shipping lanes in the world. Huron, riddled with islands, is strewn with shipwrecks. Erie picks up the pace as the water hurtles
over Niagara Falls with a force so powerful it grinds 12 inches off the rock face each year. Then Ontario, the most densely populated at the end of the line, bears the brunt of pollution flowing downstream. Finally, the St Lawrence River drains into the largest estuary on Earth, with the mighty Ottawa River and its subterranean caves providing a final flush of freshwater.

This vast watershed nourishes a rich, diverse web of life, above the surface and below. Endangered loons and piping plovers flock here to breed. The surrounding forests and wetlands are home to beavers, wolves and great horned owls. Delicate monarch butterflies and millions of birds must cross the expanse of open water on their epic migrations south. And in the nutrient-rich waters of the Gulf of St Lawrence, fourteen species of whale come to feed. Learn more in GREAT LAKES UNTAMED (Wednesday 19th, 8pm).