Lakes

[I originally wrote this in October 2011, but never posted this series on the local geology. The pictures are still worth seeing. The Las Conchas fire burned through Santa Clara canyon that summer.]

The morning of the day the Las Conchas fire started, the air was so hot, the soil so dry I went looking to see if any prickly pear were blooming anywhere this season.

One place I walked was an open field just beyond the near arroyo. Nothing. Not a flower, hardly a healthy plant in a place they bloom year after year.

The Pacheco fire was still burning towards Tesuque and was sometimes visible from the road in the area. I looked that way from somewhere in the center of the field and saw something I’d never noticed before. The field looked like a great bay that water had washed over from a break in the badlands.

Saturday, when I was in the near arroyo, I wondered how it connected to that open space which it had to border somehow. The bed narrows between five or six foot walls a little beyond the point where water begins channeling itself for the culverts. As I went up stream from there, it turned to the left and suddenly opened into a great, wide expanse with almost no banks.

I remembered something similar in the far arroyo. Yesterday I walked back to refresh my memory. Again, the tall walls make a turn, this time to the right, and a great expanse opens.

I came home to pour over Daniel Koning’s map of the Española quadrangle and I noticed something I’d missed before, that some of the places I’d read as Qay1 were really Qayi.

Qay1 is his code for alliuvial soils laid down in the first phase of the post-glacier Holocene period. The river bottom is Qay2, a younger layer.

Qayi turns out to be some intermediate phase of sand and gravel bands, the very strata I’d noticed along some of the taller walls in the far arroyo. The same pattern appeared in the shallow walls of the great expanses of both arroyos. This is precisely where he’d marked Qayi on the map.

Some passing reference in Ted Galusha and John Blick’s article on Española sedimentation that I’ve slowly been reading sent me to look up the Otowi lava flows they said had dammed the Rio Grande in recent times.

I discovered Steven Reneau and David Dethier’s work on area lakes created when landslides dammed the Rio Grande around the Otowi bridge. One occurred around 43 thousand years ago and created a lake some 15.5 miles long that lasted anywhere from a hundred to a thousand years.

Another landslide created a pool 13 miles long about 17.5 thousand years ago which broke suddenly. The most recent, formed about 12.4 thousand years ago, was also about 15.5 miles long and filled completely with layers of sediment.

During much of this time, the very end of the Pleistocene, there was greater rainfall than now, so the river levels would have been higher. The most recent lake was 100' deep.

This area is well within 10 miles of the Otowi bridge, probably much closer if you’re a crow or a drop of water.

One small mystery has been solved, the origin of those gravel bands. What still is unknown is why the channels that are being opened now in the arroyos and washes were there to be filled by Qay1 and Qayi in the first place.

Galusha, Ted and John C. Blick. Stratigraphy of the Santa Fe Group, New Mexico, 1971.

Koning, Daniel J. Preliminary Geologic Map of the Española Quadrangle, Rio Arriba and Santa Fe Counties, New Mexico, 2002, map and report.

Reneau, Steven L. and David P. Dethier. 1996. Late Pleistocene Landslide-dammed Lakes along the Rio Grande, White Rock Canyon, New Mexico.