Green Lakes murkiness and Stokes Law
Whistler Naturalist Society
After the big October storms one could not help but notice the brown murkiness of Whistlers lakes. The record rainfall had created erosion havoc, bringing tons of debris into our lake basins. Some of the debris was organic, mainly tree and forest litter.
But the bigger component was inorganic derived from the erosion of soil and underlying sediments left in the wake of the disappearance of the great ice sheet which covered Whistlers valleys some 10,000 to 20,000 years ago. Since the cleanup of the debris from the storm event, the general murkiness of local lake waters has subsided in all but Green Lake. Now, the observant are asking questions: Fitzsimmons Creek has been flowing with translucent (clear) water for two to three weeks now, yet Green Lake remains the ghastly, murky, grayish-brown colour. Why?
The answer to this colour conundrum lies with Stokes Law, which states in simplistic terms: the settling rate of particles in a fluid is a function of the size of the particle and the difference in density between it and the fluid. In this case the fluid is water. Thus the smaller and lighter the particle the slower it settles.
So, what are the characteristics of the particles carried into Green Lake? Where did these particles come from? Why are these particles different from those carried into Green Lake during the summer when glaciers are shedding a milky coloured runoff, bringing out a pleasant green hue to the colour of the appropriately named Green Lake?
The explanation is neither simple nor conclusive. Summer runoff from the glacier is carrying silt-sized particles, by and large less than 0.060mm or 60 micrometres ( u m) to 4mm or 4 u m in diameter. It is these small sized silt particles that remain in suspension in the lake waters, which scatter the green band of the visible light spectrum to bring out that colour. The settling velocity of the smallest silt particle is about 1 to 4 metres per day. Therefore in a few days the silt will reach the lake bottom; thus the green colour of the lake requires a daily influx of meltwater coming from the glacier at the head of Fitzsimmons Creek. Once the glacier melting stops during autumn, the lake waters will appear blue in direct sunlight, like the other lakes on the valley floor.
This autumn however, a blue Green Lake is not to be. Hanging in the columns of the lake water is a much finer particle suspension: clay-sized detritus. This is composed of particles less than 4 u m to as small as 0.1 u m. The silt velocity of this material is as low as 1 millimetre per day and if the wind is blowing, agitating the water, it doesnt settle at all. Looking at it another way, up to three months is required for the particles to settle 10 centimetres (4 inches) in the water column!
Where is the clay coming from? Clay minerals come from the alteration of feldspars and micas in granite and metamorphic rocks. These rocks make up the backbone of lithology of the Fitzsimmons and Spearhead Ranges. Clay sized particles can also come from stable quartz, if the grinding action at the head of a glacier is super efficient.
The massive accumulation of clay however, requires a settling basin, or in this case, an ancient lake bottom. Here lies the fly in the ointment. The toe of the Fitzsimmons Creek slide reveals the existence of such a basin, now being eroded wherever the slide pushes into the creek. I would suggest that an unusually large block of clay-rich glacial till and glacial lake bottom sediment had slid into the Creek during the October storms.
Glacial lakebeds perched above valley floors in the best circumstances are suspect for stability. So watch out, there is more to come and regardless of varying authoritative opinions, it could be catastrophic and Green Lake may require a new name.
Monthly Bird Walk Saturday, Nov. 29. 8 a.m. Meet at the bottom of Lorimer Road at the Catholic Church. Join Whistler experts in the monthly update of our feathered locals and migrants and brush up your skills to get ready for the Christmas bird count. For details, contact Michael Thompson: 604-932-5010
Christmas Bird Count Coming soon! Read next weeks Naturespeak for details.
Whistler Naturalists Society Missed the Nov. 13 AGM? Still interested in joining the board? For more information contact Dan McDonald at 604-932-5089, firstname.lastname@example.org
Calling all Aspiring Nature Writers Have an interest in natural history? Want to educate others about your favourite flora and/or fauna? Write your very own Naturespeak article. For more information contact Sorcha Masterson at 604-932-5089, email@example.com