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A River Runs Through It – River Mechanics
Key Topics
An Introduction to River Mechanics
When asked about local rivers and what they
mean to them, most people bring up recreational aspects like fishing, canoeing
and kayaking. However these rivers have much more important duties as they begin
their long journey to the ocean. The river is the sculptor of its valley, and
provides the major water-source for all the animal and plant life in the area.
Rivers begin rather unpretentiously. They are
lazy. They follow the force of gravity until they find the most convenient way
to flow downhill. This is governed by the topography of the area in which they
flow. In time, the river begins to modify this landscape until it creates a
valley. The average river system, if viewed from above, looks like a many
branching tree. Hundreds, perhaps thousands, of small rivulets flow into
slightly larger branches, which in turn flow into tributaries. Eventually the
water makes its way into the main body of the river.
What does all this mean? While we patiently
sit along the river’s bank, it's easy to imagine this single entity, lazily
meandering as it begins its long journey. However, exactly the opposite is true.
The river is the product of thousands of miles of tiny rivulets that completely
dissect the surrounding terrain. As a result, anything that takes place within
the rivers watershed can affect the quality of water carried within the river.
When the glaciers entered the scene, they
adopted the pre-existing river channels. Since glaciers are little more than a
slow moving river of ice, it seems logical that they would follow existing
pathways. As they flowed down valleys like the Bow, Kicking Horse and Athabasca,
they altered the pre-existing valleys until they seemed very different from the
valleys that had existed prior to the ice. As the ice melted, the rivers
reclaimed their valleys and began to flow once again.
Over the past decade, we have had seen
serious flooding, and this must be carefully considered as we continue to
develop the mountains. Rivers carry a certain amount of water based on an
intricate relationship between average rainfall and runoff from the surrounding
mountains. However, the forest cover on those mountains absorbs large amounts of
that water long before it reaches the river. By bringing in excessive
developments, and removing that large storage reservoir, we risk increased
flooding as the reduced forest cover allows ever increasing amounts of water to
enter the river channel.
Nature reacts to change very slowly. We
initiate it at a rate far too rapid for the ecosystem to respond. We must be
prepared for the unexpected changes that result from our impatience.
The Life Of A River
Water is the most powerful force on Earth. It
impacts every aspect of our life, and rivers provide its principle transport
mechanism. These rivers begin as rain and snow high up in the mountains. As the
rain hits the ground, it stirs the soil, splashing fine sediments and dissolving
minerals. Some of this water penetrates the surface either through cracks, or by
percolating through the thin mountain soil. Some of this will later re-emerge
and enter our rivers. Some may percolate deep down into the Earth, heat up, and
resurface in one of our hot springs. This capturing of rainfall and releasing it
later is essential. If all the rainfall in the mountains were to go directly
into our rivers, flooding would be a regular occurrence. In area’s where
overcutting of trees has occurred, we often see increased flash flooding. For
this very reason, logging is carefully regulated.
Not all of the water seeps below the surface.
As the soil becomes saturated, water begins to flow over the surface. As it
feels the pull of gravity, it picks up speed, and at the same time, increases in
energy. It picks up fine surface sediments loosened by the pounding raindrops,
and will also carry dissolved minerals. Gravity will take the water downhill,
taking advantage of any depression to aid its descent. In this way, the
landscape begins to be divided by small rivulets that over time will be
enlarged. Eventually, the water makes its way into a small stream and begins its
long trip to the nearest ocean.
The faster the water flows, the larger its
ability to carry sediment. As the streams flow into rivers, the water picks up
increasingly large material. Raging torrents have been known to carry immense
boulders, cars – even entire buildings. Very little can stand in the way of a
powerful river. Material carried with the water adds to its erosive power. The
material acts as an abrasive rapidly carving through rock, deepening its
channel, and cutting through obstacles. As the river rounds a corner, water on
the outside of the corner will move much faster, eating away at the outer bank
of curves. On the inner bank, the river may deposit some of its sediment. Over
time, the river may take on a meandering pattern.
Trellis Patterns
As rivers flow through the mountains, they
tend to follow a specific route. This route is largely determined by the
landscape. In most cases, they follow the base of a wide, glaciated, shale
valley. Periodically, as the Bow River does near Bow Falls, they will cut across
a limestone ridge, and then begin to follow a parallel valley. This pattern is
called trellis drainage and is typical of the Front Ranges.
Braided Channels
In some area’s, particularly in glacier country, rivers may carry an immense load. As glaciers melt, their runoff includes hundreds of tonnes of fine and coarse material. On steep gradients, the rivers can carry vast amounts of gravelly debris. As the gradient decreases, they begin to deposit this material. Crossing such flats, the river creates dozens of small, meandering channels. These become interwoven with each other into a braid-like pattern. This distinctive appearance is known as a braided channel, and is extremely popular along the runoff from glaciers. Since the amount of discharge varies almost hourly, the number of channels in a braided area will also vary. From the Peyto Lake viewpoint, the runoff from Peyto Glacier enters Peyto Lake through a long braided channel. Graveyard flats, to the north of the Saskatchewan River Crossing, along the Banff/Jasper Highway, is another excellent example. As is the Sunwapta River downstream of the Columbia Icefields.
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All Material © Ward Cameron 2005