Over The Lip - Mountain Waterfalls
Few landforms showcase the power of water better than
waterfalls. Standing beneath the 380 m (1,249 ft) drop of Takakkaw Falls in Yoho
National Park, the power of waterfalls is instantly apparent. Waterfalls are
more than just a ledge over which water falls. They are the result of a specific
set of conditions that allow water to maintain its vertical cascade. In the case
of Takakkaw Falls, the mighty Daly Glacier, only a few hundred metres above the
falls, releases immense amounts of water into its narrow channel. This ends up
cascading over the lip of the Waputik Range. This type of waterfall, where there
simply happens to be a cliff in the way of a river, forms a minority of
waterfalls. In this case, over time, the river will cut into the cliff, reducing
the gradient of the waterfall, until eventually, only a rapid will remain.
The majority of waterfalls result from layers of rock composed
of different degrees of hardness. Hard layers are more resistant to erosion.
Soft layers are quickly removed. This is visible all around us in the mountains.
Most of our summits are made of resistant limestone or dolomite. The softer
shale layers remain protected beneath these resistant caps. If a river flows
over a lip composed of a layer of resistant material (like dolomite), that lays
atop a softer layer of shale, then the water will remove the soft layer beneath
the dolomite at a faster rate. In this way, waterfalls often become undercut as
this soft material is worn away. This results in the caves found beneath many
waterfalls. Eventually, the undercut becomes so large that the weight of water
on the unsupported layer of dolomite will be sufficient to collapse the layer.
At this time, amidst a catastrophic collapse, the cave disappears, and the cliff
face moves further up the valley. In many cases, the hard-soft layer
relationship remains, and the falls simply migrates upstream. This creates a
canyon in front of the waterfall. As long as the hardness differential remains,
the waterfall will persist. Common examples of this type of waterfall are
Athabasca Falls, Sunwapta Falls, and Johnston Canyon.
All Material © Ward Cameron 2005