Driftwood along the shores of Great Slave Lake is not only morphing the geology of the area, but geologists’ understanding of how wood carried by ice and water is used to construct landforms.
Findings stemming from ongoing geological research on the bays of Great Slave Lake near Fort Resolution have helped coin a new term being used by geologists to describe the way in which driftwood builds up over time to form islands and build bigger shorelines.
“Driftcretion” – a combination of driftwood and accretion – is a new term developed by Colorado State University PhD student Natalie Kramer Anderson, whose National Geographic-funded research has brought her to the Northwest Territories for the last two years to study the movement and volume of driftwood entering the territory via its river systems.
The word, which will be presented for the first time at the Geological Society of America meeting this month, describes the way that driftwood, cobble and boulders become new land as they are pushed up onto the shore, or into island forms, over many years by ice and water, and comes from her research into where wood goes once it enters Great Slave Lake – research that won her the award for best student proposal in geomorphology in the US this year.
“Wood delivered from the Slave (River) stays in the lake and contributes to land growth,” Anderson said. “What we’re talking about here are driftcretions where big wood mats or wood berms get shoved up on the shorelines and then stay there permanently, and what happens is they allow new trees to grow and then eventually the land grows outwards.”
Looking at two islands near Fort Resolution – Paulette Island and Moose Deer Island – Anderson found that the soil on the island was strictly the product of decomposed wood.
“If you dig into any of the soils on those islands, there isn’t any soil. It’s straight organic matter down to cobble. Usually when we talk about soils, we talk about two things: mineral soils and organic materials. Mineral soils would be rocks when they weather…But the organic matter is just the decayed logs and leaf litter.”
Surveys of the lake shores done from airplane and sea kayak also clearly revealed the driftcretion process, highlighting the constant growth of the lake shore, she said.
“You can see these linear lines of trees that correspond to these decaying lines of wood. It is really neat,” Anderson said. “Before the forest matures, for the first 100 metres or so from the shoreline, you can see these lines of trees that probably correspond to these lines of old driftwood that have been thrown up on shore by the ice or lake tsunamis.”
She said driftcretion will likely cause the lake, which was carved out by glaciers and built on bedrock, to shrink over time – albeit, a very long time.
“The Slave’s been throwing a lot of sediment into the lake, and wood. That’s probably why the South Shore’s so shallow while the East Arm’s so deep. All of the rivers draining into the East Arm aren’t really carrying any sediment or wood because they drain out of the Canadian Shield, and there’s just not a lot of trees toppling,” she said.
Anderson said the lake serves as an important basin for cleaning debris and potential toxins out of the rivers.
“The Great Slave Lake is keeping the water and the sediment in the Mackenzie River corridor really clean.”
Cameras set up on river system
Anderson’s research on the lake coincides with, and is aided by, another branch of work she’s been carrying out using time lapse photography at various sites on rivers throughout the NWT, including the Mackenzie, Slave, Liard, Hay and Peel Rivers.
Photographs taken on the Mackenzie at Fort Providence using a camera planted there all winter confirmed Anderson’s hunch that most driftwood remains trapped in the lake, while the small amount that’s exported down river is mostly due to ice break up.
Though it’s too early to identify solid trends on the river systems, observations so far have allowed her to conclude that wood transport hits a maximum threshold timed with the highest peak of water volume each year, at around 4,500 m3/s.
Based on her photos from this spring on the Slave and Liard Rivers, she said she’s able to identify a gap of approximately one day between the ice jam flood and a large pulse of wood coming down river, as well.
Preliminary results from the time lapse photography – a new methodology she developed to estimate volumes of wood being transported down river – will also be presented at the American Geophysical Union meeting in San Francisco in December.
Anderson said the research, as it unfolds over the next several years, could have important implications for those considering hydro development on the Slave or other rivers in the NWT.
“That story is going to be really interesting to anyone studying the deltas and anyone who’s interested in what could it do if you block all this wood,” she said. “What if you severed this artery of wood being delivered to the lake? What role does that wood play around the lake margins? I think it’s a really big one.”