Why do beavers build dams?

Nestled in the forests of Canada sits the world's longest beaver dam. This 850-meter-long structure is large enough to be seen in satellite imagery and has dramatically transformed the region, creating a pond containing 70 million liters of water. But dams of any size can have huge impacts on their environment. So how exactly do beavers build these impressive structures? Glynnis Hood investigates. [Directed by Keegan Thornhill, narrated by Alexandra Panzer, music by Salil Bhayani, cAMP Studio].