Winter moraines and glacier monitor — 2002
Observations on the position of the termini of Overlord and Wedgemount Glaciers were carried out at the close of melt season — September, or so we thought. Melting continued on a lesser scale during the warm and dry spell of October which, although not completely unexpected, was not appreciated by any skier or glaciologist.
Last year, NatureSpeak reported on a recession of 7.0 and 12.0-15.0 metres respectively for the above two glaciers. The main cause of withdrawal was a low snowpack over the preceding winter, followed by a hot, late summer. Happily, the snowpack last winter was near or slightly above average, and was followed by a very helpful late spring, that was not appreciated by any resident between here and Inuvik! So, reduced retraction of our glaciers was to be expected, but the ringer was the two hot spells in July and August.
On Sept. 14th, the long, slow hike to Overlord Glacier provided good news. The survey revealed only 2.8 metres of retraction over the year, as compared to the 1990-2002 average of 9.0 metres of recession per year. The bad news was revealed in the Sept. 20th survey of Wedgemount — 18.8 metres of retraction over the year! The snout is now 24 metres above and 159 metres from the lakeshore. In 1991 it was at the lakeshore, although 21 metres of delta growth has taken place since, courtesy of the outwash kicked out by the receding glacier. Thus, 138 metres of recession over 11 years yields an average annual retreat of 12.6 metres, suggesting that year 2002 was 50 per cent above the norm. Whereas Overlord Glacier was 69 per cent below the norm.
The above quite opposing sets of observations are perplexing to say the least. Both glaciers appear to have a sufficiently large accumulation zone lying above the critical elevation of 2,100 to 2,150 metres. This is the so-called "equilibrium line" which separates exposed glacier ice from covering snow at the end of the summer melt season. Those that have don’t have this situation, for example, the recently disappeared glacier in Saudan Couloir, are doomed to extinction. Both glaciers face an advantageous shading aspect of northwest, but to make it more perplexing, the terminus of Overlord Glacier is 200-225 metres lower and therefore in a zone of warmer and prolonged summer temperatures. So, the cause of excessive melting on Wedgemount appears to be a lack of sufficient snowfall during winter; that is, it appears to be in a local rainshadow. This is also borne out by the disappearance of one upper arm of the glacier altogether over the last century. The southeast basin below the summit of Mt. Weart is now virtually bone dry, and the residual ice at its lower end no longer joins the glacier.
Beyond the toe of Overlord Glacier there are a series of arcuate ridges. The ridges are developed by ice shove during the winter while the glacier is advancing without the impediment of surface melting. That is, the newly developed moraine is exposed free of the ice front when the summer melt rate exceeds the daily glacier movement, as generated by gravity and snow load in the accumulation zone. As shown in the photo above, nearly each year the summer ablation of ice exceeds the yearly movement motion, leaving an exposed succession of ridges, each one or two metres high. These "micro" moraines show the maximum down-valley extent of the glacier for each year in roughly April or May, whereas we measure the fully up-valley retracted glacier for each year at the close of melt season (we hope) in September.
In 2002, the distance between the winter moraine and glacier snout was 9.3 metres. The annual rate of ice advance, 12-13 metres per year, was exceeded by melting in the shorter summer season, leaving the glacier in an overall 2.6 metre retracted deficit from September 2001. To generate an advance of the glacier from one year to the next, the glacier must over-ride the present and previous year’s moraines which happened quite often in the period from 1951 to 1986, but only three times since. Overlord Glacier, however, has another 63-64 metres of retraction before reaching its all-time recessed position of 1951. For Wedgemount Glacier, there is no such benchmark. It is already 700 metres up-valley from its 1951 position!
One may ask: "What will happen if climate warming is not curbed through the Kyoto Accord?" We do not really know because conflicting forecast models do not agree on whether our local winters will be wetter or drier than present. If the former, and the winter 0° C isotherm remains on average below 2,100 metres, the changes could be nearly negligible. But if the latter, and the winter 0° C isotherm climbs above the threshold elevation, the glaciers are doomed to drastic recession and probably disappearance of those below the critical elevation.
Upcoming Events :
Friday, October 25th, 7:30 p.m. — The Milky Way and Other Great Galaxies, John Nemy of the Pacific Observatory. Millennium Place. The Whistler Naturalists are pleased to host John Nemy, famed for his astronomy nights in past years at the Rendezvous on Blackcomb. John’s talk is suitable for all ages and includes great images and commentary. Admission by donation. Refreshments available before and after the show. An organizational meeting of the Whistler Astronomy Club will follow the show.
Written by: Karl Ricker