It’s almost as big as Manhattan, contains trees more than 1,300 years old and is visible from space. Now it hides a ticking time bomb

When we think of rivers, we imagine water flowing freely from source to sea. But for some of the world's great waterways, their natural state is to be blocked or dammed by fallen trees – on a massive scale. Today, the world's largest tree-blockages or logjams can be found in the delta of Canada's mighty Mackenzie River .

.While the word 'logjam' is a byword for life's annoying obstacles, recent studies have shown that river logjams are vital for wildlife and human communities , and are crucial stores of carbon.

The Mackenzie River, named after Scottish explorer Alexander Mackenzie (1764-1820), is the largest river that flows entirely within the boundaries of Canada. From its source in the Great Slave Lake of Canada's Northwest Territories, it flows almost 1800km north through sparsely inhabited, forested lands to the Beaufort Sea and the Arctic.