A Battle of the Ages: Horseshoe Bend, Colorado River

Every river has a story to tell.

We don’t often understand the story because it is told in terms of quaternary geology, climatology, hydrology, fluvial geomorphology, aquatic and riparian ecology, and basic chemistry and physics.

This horseshoe bend in the Colorado River is telling a story of the battle of the ages between the force of gravity against the force of continental drift. At any given point in this battle it would seem to be an unwinnable war for the force of gravity. The force of gravity only has the mass of water, delivered at a rate supplied from the atmosphere, to try and wear down the overwhelming mass of a plate of the earth’s crust. It is not a fair fight. However, if you consider the total mass of water hammering on the rock over hundreds of thousands of years, you get to a very large number. This mass, multiplied by the acceleration of the water due to gravity, provides a tremendous amount of force. This force is very strategically concentrated at the weakest points in the lithosphere and as the rock erodes, it creates channels that further concentrate the mass of water. As these channels develop, centrifugal motion concentrates the force at the outside of bends in the channel exaggerating meanders in the stream. The stream channel is continually re-inventing itself in terms of the mass of water and sediments arriving at the upstream end of the stream reach. The stream has to find a channel slope that is ideally suited for transport of this water and sediment through to a downstream reach. Continuity must be maintained so if the downstream reach cannot accept the mass of water and sediments then sediments are stored in the channel resulting in channel aggradation, whereas if the force of the water and sediments is too great for the channel to cope with then the channel degrades, cutting ever deeper into the lithosphere.

In this picture, the story is being told with foreshadowing. We have great anticipation that, at any moment, the channel will cut through the neck of the horseshoe, leaving behind an oxbow lake as a monument to this great battle between water and rock.

National Geographic  |  Photograph by Frans Lanting

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