The return of the Pacific salmon every year to the rivers from which they were born, is one of the greatest natural events on the planet. More than half a billion salmon travel up to 20,000 miles to return to the exact patch of gravel in the river from which they were born, to spawn and die.

Using high speed cameras and specially designed digital underwater kits, Nature's Great Events amazingly capture the salmon swimming upstream and against powerful torrents. Salmon have an incredible sense of smell and can detect just a drop of the water from the river they came from among billions of litres of sea water.

On the west coast of Canada and Alaska, over 2,000 miles away, a number of predators eagerly await the salmon's return. Killer whales, sea lions and bald eagles eagerly anticipate the millions of salmon heading their way. In order to survive, bears find themselves eating clams, barnacles and even grass while they wait for the salmon. But they are threatened by coastal wolves, which have been known to kill and eat small bears.

After spawning, the salmon die and yet their decaying bodies continue to feed the animals gathered along the rivers, as well as providing nutrients to feed the developing salmon eggs in the river. The time-lapse cameras demonstrate that the dead fish also sustain the forest itself.