Lessons from a Stream

I love streams, especially the small ones. They contain so many lessons in hydraulics and fluvial forms. This little one tumbles down a mountain side with classic boulder step pools. But by random geological chance, this short reach has a flatter slope and switches to a gravel bed with alluvial meandering. It’s a good reminder that streams follow patterns but there is endless randomness from time and lithology that further shape a stream. You’ll find all kinds of fluvial processes and stream types throughout the watershed in places that you wouldn’t expect. Steep rocky brook may temporary relax along a flat valley reach controlled by a bedrock outcrop. Or a meandering lazy river might suddenly have to squeeze through a terminal moraine, changing its speed and bed type.

Here’s a large-scale example from another part of the world on how underlying lithology may suddenly change fluvial form. New Zealand is known for its braided rivers, this one near Christchurch. They are wild, unique rivers. Draining from the Southern Alps across a flat plain to the ocean. But look at what happens to the Rakaia River in this image. Some type of geological feature in the centre of the image, a moraine perhaps, has confined the river, causing it to narrow and has inducing meandering. Then, just as suddenly it is back to open plans and is once again braided.

The more I learn about fluvial geomorphology the less I am certain about any of it. But at least unlike geology, you can always get a lesson in fluvial processes at your local creek. Its all right there in the open, no need to imagine what is 1000 feet underground.

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