Warning: This following essential mass message will probably not very entertaining for most folks, as it is targeted at river rats, that strange breed who shuffle around the halls of Building 10N muttering about attenuation assets, ecosystem services, and pre-disaster mitigation. But if you are a river rat, or have an exceptionally high tolerance for fluvial geomorphic jargon, then forge ahead.
Anyhow, even though I am no longer getting paid to look at and think about mistreated rivers, I seem to be having trouble staying away from them. I was on a rock climbing trip the other week (to Indian Creek, the purest crack climbing in the world) and became acquainted with a small desert stream named Cottonwood Creek. This stream, which is located in Beef Basin near the entrance to the Needles District of Canyonlands National Park, is such a fine example of how a small dam can disrupt bedload transport and lead to dramatic bed degradation and tributary rejuvenation, that I couldn't help but snap a few photos. The following photo album illustrates the situation pretty well:
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Upstream of the small dam, which can (barely) be seen near center of the first photo, Cottonwood Creek is slightly incised, but not enough to make the tributaries unravel. The dam has been trapping virtually all the bedload carried by Cottonwood Creek. Given how much fine sediment is stored just upstream of the dam, it probably used to create a large pool that allowed quite a bit of the suspended sediment load to settle out also. Downstream of the dam, a small diversion structure spanning the entire width of the channel, all hell is breaking loose. The channel is incised over 20' in places, and all its small, ephemeral tributaries are rejuvenating and carving deep gullies.
Sure is easy to see what is going on without all the damn trees. Anyhow, best wishes to all the folks back in the River Management Program in Vermont - keep fighting the good fight!
Ty Mack
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