Wednesday, January 21, 2015

Geo 1095: January 21, Day 751: A Landscape in a Creek Bed

Here we have another exercise like this one from the tail end of last year. "The point is, if all you could see was the sediment in the creek- no horizon, no vegetation, no location- you could still infer much of the environmental setting, just from the nature and arrangement of the sediment itself." We do see some vegetation, and the shadows from the bridge, constructed of 4x4s and 2x4s, give a convenient scale. But by and large, all we can see is the stream sediment.What can we say about the landscape around here? The stream itself is notably absent, which suggests a small drainage area. The sediment visible on the surface is quite angular, which means very little transport, also suggesting a limited drainage area. It's also quite coarse- there's little to no sand or silt visible here- which indicates heavy flow (though there's probably fine sediment buried down under the coarser material) and/or a very steep drainage. But right now, there's no visible flow at all (again, there's probably some transport as groundwater down in that pile). What's up with that? What this suggests to me is that most transport occurs in short-lived storm events, with the flow becoming unable to transport much of anything as the rain subsides. The lush vegetation in the upper corners says there's soil developed here, so why are fines missing in the sediment load? That fraction is probably locked down by plant roots. (Addendum: And as Anne points out in the comments, the finer sediments that are carried through here don't stop here; they just keep gong downstream.) You can also see that all the rock appears to be of the same type.

So that's what a geologist can reasonably infer from looking at the photo. The broader picture? We are, in fact, looking up a small side drainage off the main canyon below North Falls. It's a small catchment area, prone to heavy winter rains, as warm marine air begins its climb over the Cascades, wringing much of the moisture out of the air. The valley overall is quite steep, coming off the same ledges of Columbia River Basalt as we've seen in previous photos of North Falls, which is why all the rock is basalt. Our summers here are quite arid, and by late August, most streams that aren't spring fed are very low or dry. Everything I've noted in this paragraph would be evident to a casual hiker passing through the area, but a person with some geology background can "see" all of it, simply by looking into the bed of a dry creek.

Photo unmodified. August 30, 2012. FlashEarth Location (approximate).


Anne Jefferson said...

Fine as missing because they've been transported further downstream not because they weren't available for transport.

Lockwood said...

I'd have expected *some* to be deposited as flow ebbed, but I'm sure some are trapped between clasts, and this area is very heavily vegetated, so aside from mass movement events and heaviest rain events, I doubt there is all that much fine sediment carried here. What there is, is as you say, carried through this spot. I was thinking about commenting during writing that this *almost* looks more like talus than stream sediment.