Wednesday, June 24, 2009

Walthenberg Rapid

Some of my favorite beaches on the river were small ones adjacent to rapids. I wish I had had more time to explore and observe, but one of the downsides of an organized trip is that there isn't much unscheduled time, and when there is, it's either not in quite the right place or it rapidly fills with other things.

Here are two contrasting beaches just above Walthenberg Rapid, where a slot canyon on river right spews boulders and gravel into the Colorado. One is a pretty typical sand bar; the other a neat little arcuate gravel berm. Whereas the former is purely a fluvial feature, the latter required waves.

Waves are an interesting question on the river. On open stretches of river, wind waves can form, but something else is going on right around the rapids. Maybe this is obvious to river geologists, but it was new to me. Rapids are not just a bunch of big static standing waves. Sometimes these big rollers grown in size and then collapse, just like surf, except they do it over and over in one place. This pulsing generates waves that travel out from the rapids and seem more than capable of building small beaches in the vicinity. One evening at a camp farther down the river, I watched 5-6 inch waves break on the beach with a fairly regular 4 second period.

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