Wednesday, June 16, 2010

Evidence of change - Relict channels and wetland soils in the Dry Cim and Gold Creek, NM

In the last couple of weeks, I've been able to see some relict fluvial features that are indicative of a former landscape condition. Sometimes there is enough local history/knowledge to get a very good idea of when major landscape change occurred (as in the case of the Dry Cimarron), in others you can make inferences from historical activity, but it's difficult to derive specific dates of change.

In both these cases, the specific change has been incision due to anthropogenic changes to the landscape combined with flood/erosion events. The Dry Cim has been heavily grazed in the past, and had specific flood events that caused channel avulsion/incision to bedrock in the location where my photos were taken.

This photo shows the relict channel, with fine-grained sediment accumulation. One might think that it is a far-in-the-past feature, until you start talking to the landowner and/or folks who know the local hydrograph. Though one would need to age-date material in the relict channel to get an actual date range of the channel deposits, it's quite possible that it may be quite young. There was a ripper flood in 1910 or so, and the rancher indicated that floods in the 40s and 50s caused an extensive amount of incision - maybe 15-20 feet or more (down to bedrock in this location). So, much of the landscape change isn't actually in the far distant past.

The wider view of the above. It is interesting to note that the sediment in the relict channel is mostly clay-to-sand. Just below the relict channel in this photo is a large gravel bar deposited last year, mostly cobble-sized grains. I saw nothing that size in the relict channel.

The rancher here has reduced his herd from about 1200 to 500 head, and has mostly rested this reach for the past 4 years. The resulting riparian and fluvial changes from the release of grazing pressure have been phenomenal. He has beaver dams/ponds and a well-wetted, vegetated channel bottom, increasing bank storage and aquifer recharge. It's unlikely that the system will aggrade back to its original, pre-grazing, agriculture and logging elevation, but it is in much better condition than I had hoped before I went out there. It helped to have a very wet winter for this region.

We also talked a lot about wetlands, and what anoxic environments are and the soil columns/profiles that occur in these environments. It just so happened that there was a big grey blob of soil on the opposite slope/bank of the relict channel. It was a splendid opportunity to discuss hydric soil indicators such as gleyed soils and oxidized root channels, even if relict. In this photo, you can see the oxidized root channels that occur when roots of hydrophytic plants carry oxygen down into the reduced environment. Rust then occurs along the root channels. In this instance, they are pervasive through the entire exposure. This blob of former wetland is also about 15 feet up from the stream bottom. Again, little grain size larger than sand. There was also a lens of shells near the bottom, and frequent scatterings of charcoal throughout, indicated upstream or adjacent fires. The three of us there that day loved the fact that even without a lab, a handlens, or a rock hammer, we could infer such a great deal about the landscape.

Speaking of relict wetlands, at Gold Creek in Valle Vidal, I got this great bank shot that's actually about 8 feet tall from stream bed to top-of-bank. Below the most recent soil column at the top, you can see a well-reduced horizon. Though I wasn't able to get a good close-up, it too has relict oxidized root channels and other hydric soil features.

Gold Creek had intensive logging in the past, as well as intensive grazing pressure prior to its acquisition by the USFS (it's still grazed, but I don't think as heavily). Looking at the surrounding landscape, we all surmised (me, especially) that it was likely that this drainage and others like it likely had fens at the extreme headwaters, that have now been incised and drained due to the landscape changes. This creek is now incised about 10-12 feet, but the channel bottom seems to be recovering to the point that it is forming an E-type channel. [We kind of puzzled about the gradient, though. Seemed too steep for an E, but my thought is that the source material grain size is too small to support any kind of a step-pool environment that the gradient would suggest. Crappy weathering volcanics, I think.]
Looking downstream towards Comanche Creek. There's probably about 200 or so feet of drop between the photo point and the confluence with Comanche Creek.

I'm amazed everytime I venture into the field about how much of an impact we have had on our environment, even on a little stream up at 9500 feet. I'm also amazed at, and thoroughly appreciate the dedication of the people I work with to restore these landscapes, from the rancher on the Dry Cim, to the folks working here. I may get frustrated on a day-to-day basis, but this, and they make it all worthwhile.

2 comments:

Anonymous said...

Nice blog.

> We kind of puzzled about the gradient, though. Seemed too steep for an E, but my thought is that the source material grain size is too small to support any kind of a step-pool environment that the gradient would suggest. Crappy weathering volcanics, I think.

Other streams up there appear to be Rosgen Eb – I think it’s a local trend. I also think that E channels, with their “evolutionary endpoint” status, can tolerate step valleys.

I think Es often form when the valley beltwidth exceeds about five channel widths, regardless of grain size. This valley width seems to represent a threshold from riffle-pool channels (Rosgen Cs) into a Rosgen E form. I think this evolution is related to secondary cells (typically associated with C-channels) bumping into terraces – when the terraces get too far apart, there’s nothing to support those “secondary-cell” pools. This triggers a C-channel to evolve into an E, such as steepening (and eventually losing) point bars and the loss of the C-familiar controls over meander geometry (radius of curvature, meander wavelength, &etc.).


> I'm amazed everytime I venture into the field about how much of an impact we have had on our environment, even on a little stream up at 9500 feet

I also think that there were non-anthropogenic causes, like climate change and that Italian volcano. I mean, we see those Holocene terraces all over the place, so there must be long-term incision.

But I think the turn-of-the-century impacts were enormous. I also think things are much better now – a general healing phase.

Lots of reason why getting into the field is a good thing!

sbo said...

Wonderful blog! I found it while searching on Yahoo News. Do you have any tips on how to get listed in Yahoo News? I’ve been trying for a while but I never sbobet
seem to get there! Many thanks.