The Mackenzie Delta / by Sam Cornish

 The Mackenzie Delta, NW Territories

The Mackenzie Delta, NW Territories

Flying by helicopter above the Mackenzie River Delta, you bear witness to its vastness and almost unfathomable complexity. Lazily meandering channels feed into lakes like stems into leaves; movement of water through these plant-like systems becomes clear when rain fills the channels with silt. These silty waters act as a tracer for fluid flow, slowly feeding into clearer lakes and ponds.

 Silt delineates active flow routes

Silt delineates active flow routes

The main stem of the Mackenzie is fed from the west arm of the Great Slave Lake, more than 1,700 km upstream, but only 156 m higher in elevation. In total, the Mackenzie watershed drains an area of tundra and taiga forest more than seven times the size of the UK. It is a vast and wild landscape, where black spruce trees make shallow roots in permafrost ground. The annual frost heave and thaw of the upper soil gives rise to ’Drunken Forest’  - tilted trees, propping each other up.

The delta itself branches out just downstream of the confluence of the Mackenzie with the Arctic Red River at Tsiigehtchic. Upon opening it is joined by the Peel River, flowing northwards from the Ogilvie Mountains in Yukon.

As it empties into the Arctic Ocean’s shallow Beaufort Sea and the watercourse widens, the water slows considerably. With this loss of kinetic energy, much of the sediment - derived from mountain regions that fringe the watershed - settles out of the water. The accumulation of sediment further decreases the slope of the river channel, and gravitational instability leads to the propagation of new channels with more efficient courses.

 Sediment load suspended in a layered flow

Sediment load suspended in a layered flow

Since the retreat of glacial ice roughly ten thousand years ago this process has been arranging and rearranging the pattern of watercourses in the delta, which now spans more than 200 km north to south, and 100 km east to west at its mouth.

Some local people know how to traverse this labyrinthine delta. By boat it is an exercise in navigation which requires significant skill, and the willingness to jump out into the soft sediment and push when the water proves too shallow.

 A labyrinthine network of ponds and channels

A labyrinthine network of ponds and channels

Discharging nearly 10,000 cubic m each second into the Beaufort Sea, waters from the Mackenzie River feed the distinctive freshwater lens of the Beaufort Gyre, maintained by anticyclonic winds driving convergence of buoyant surface fresh waters towards the centre of the gyre. The fate of freshwater in this gyre has critical implications for the stability of sea-ice, marine ecology and wider ocean circulation.

The delta islands and riverbanks are home to grizzly bears, moose, muskrats, minks and beavers. The channels see the passage of fish like the arctic cisco venturing upstream to spawn. In the spring, beluga whales come to calve. Migratory birds - snow geese, sandhill cranes, tundra swans - make the delta their summer home, and bald eagles breed.

This is by nature a changing landscape. But in recent decades, the permafrost which underpins the structural integrity of the land dissected by the watercourses has been thawing, and in places black thaw slumps degrade the banks.