Putting the Emerald in Emerald Lake – Where do the colours come from?
Key Topics
An Introduction to Mountain Lakes
There have been many stories told to visitors over the years
explaining the magnificent colour of the many mountain lakes in the Rockies.
Some were told the colour was because the lakes were drained each fall and the
bottom painted. Others have been told the colours were a reflection of the blue
sky. Where did these colours come from – really?
To understand their origin, we need to understand how we see
colour in the first place. If we look at a table, and say the table is blue,
what is really happening? The light hitting the table contains all the colours
of the spectrum, why do we see only blue? The surface of the object absorbs some
wavelengths of light, and reflects others. If it reflects the wavelengths that
represent blue to our eyes, that is the colour we will see.
In the case of our mountain lakes, every day during the summer
melt, they receive hundreds of tonnes of fine glacial material known as rock
flour. It is very light, and stays suspended for quite some time. When the light
hits the surface of these mountain lakes, the suspended material distorts the
wavelengths of light, reflecting back more of the green and blue end of the
spectrum – hence we see emeralds and blues. Also, since each lake gets a varying
amount of this material, each lake will have its own shade of blue. This shade
will also vary over the course of the year. In the spring, there is little
suspended rock flour, so the lakes are the same colour as lakes elsewhere – deep
blue. As more material flows into the lake with the spring runoff, the lakes
progressively change to their famous colours. Later in the season, as the
glacial melt ends, the rock flour will slowly settle, and the illusion will be
lost – until next season brings a new supply of the Canadian Rockies magical
lake dye.
How do Rivers Challenge Lakes?
The many lakes in the Canadian Rockies gives the impression that
lakes are a rather permanent feature of the landscape. This is not entirely
true. In fact, most lakes are merely a temporary landform that creates a
convenient storage reservoir behind some natural dam.
The lakes may be formed in many ways. High on the mountain
sides, glaciers carved out smooth bowls called cirques. Later, as the ice melted, many of these bowls filled with
meltwater to become the alpine tarns,
or cirque lakes that form the focus of
many of our hiking trails.
In other cases, glaciers flowed down the valleys, and left
behind large piles of debris. These piles became natural dams restricting water
flow in their valleys. This is how Peyto and Waterfowl Lakes were created. In
still other cases, landslides many have crashed into the valley bottoms and
dammed local rivers. This describes the formation of Maligne Lake in Jasper
National Park, and Moraine Lake in Banff National Park.
Regardless of the mechanism of their formation, the fate of all
these lakes is the same – obliteration. Many were formed of glaciers, and these
same glaciers will eventually fill them in. High above the lakes, the action of
weathering and erosion is slowly wearing down the mountains. As boulders and
pebbles alike fall onto glaciers, they become a part of a moving conveyor belt
of debris. Some will hitch a ride on top of the ice, but much more will become
incorporated into the glacier. As the material is scraped and scoured along the
valley floor and walls, it is quickly ground up into the rock flour discussed on
the previous page. Hundreds of tonnes of this flour will flow off the glacier
each day during the peak of summer melt.
Where does all this material go? It heads directly into our many
mountain lakes. A fast flowing river has the ability to carry immense loads.
Unfortunately, the speed of rivers drops rapidly as the river enters a lake.
This rapidly reduces the water’s ability to carry material, and so the
river’s sediments are quickly deposited at the entrance to the lake. Soon
deltas begin to form, and the process of in-filling begins. Already Peyto Lake
has a delta covering almost 1/3 of its former length. Eventually, all that will
remain will be a braided stream flowing across the flat delta left behind by the
deposition of incalculable amounts of sediment. Don’t panic though, it’s
taken almost 10,000 years to create the delta we see today on Peyto Lake. It
will still take thousands more before the lake disappears. A similar delta can
be seen by hiking to the far end of Lake Louise.
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All Material © Ward Cameron 2005