Sunday, February 27, 2011

The Living Arroyo

What’s happening: People pruning trees; Russian thistles breaking loose; Russian olive, juniper and pyracantha berries persist.

What’s still green: Moss, evergreen, yucca, grape hyacinth, Jupiter’s beard, phacelia, pink evening primrose, broom senecio, snakeweed, chrysanthemum leaves; some grasses.

What’s grey, blue-grey or grey-green: Piñon, four-winged salt bush, yellow alyssum and winterfat leaves.

What’s red/turning red: Cholla, Madonna lily, golden spur columbine, small-leaved soapwort, beardstongue, yellow evening primrose, creeping mahonia leaves; rose and young tamarix stems.

What’s yellow/turning yellow: Globe and weeping willow branches.

What’s blooming inside: Cleaned and trimmed plants moved back onto enclosed porch; zonal geraniums blooming; new leaves on deciduous chaste trees.

Animal sightings: Hawk, rabbit; mouse trying to get into house.

Weather: Winds; snow lingers in Jemez and Sangre de Cristo; last snow 2/4/11; 11:01 hours of daylight today.

Weekly update: Sometime during the recent cold spell a chunk of the prairie arroyo wall collapsed. Just before the temperatures fell below zero, there had been a little snow - not much, but apparently enough to freeze in the ground and serve as a wedge.

The same area was disturbed several times last year: once in April after several weeks of alternating rain and snow, then again in late December after our first, and only, real snowfall of the winter.

I suppose it’s not unusual for the soft sides of a bank to collapse, but it’s unexpected because I’ve grown accustomed to thinking of arroyos as static. I know, of course, that rocks change, but over millennia. Continental plates only move about 3 centimeters a year, less than an inch and a half. I simply don’t expect to see alternations in the structural landscape when there are no cataclysmic events, no volcanic eruptions, no earthquakes, no storms, not even any sharply contrasting seasons.

The part of the arroyo I walk is actually a dry, sandy stretch that parallels a fault. Up river, one feeder crosses the fault at the point where that branch meets two others that run from another fault in the Miocene uplands formed before the Rio Grande rift opened during the Pliocene. Below, the arroyo meets another arroyo, the narrow one, in the bottom lands, then flows into the Rio Grande.

The taller walls reveal alternating layers of sandy loam and gravel. The thicker slabs are silica enriched by clay. The thinner layers are silicon dioxide in the form of milky quartz and smaller amounts of granite. Both probably came from the Precambrian rock of the Sangre de Cristo, the one less weathered than the other. Quartz is one of the rocks least likely to age quickly.

Down river from where I enter the arroyo the left bank is steeper, perhaps from harder formations. At one place a folded ridge has been exposed. The land has eroded back a little along that ridge to create a second bottom above the tougher lower strata. The right bank is shallower and spreading.

The floor divides into five sections as the water, when it flows, forks around a central island where chamisa has stabilized the soil. On the soft side, between the water path and the wall, more plants grow, while only gravel and sand exist on the hard side.

Up river, it’s the right bank that’s steeper and the left that’s soft. The floor’s not divided, but again the vegetation is limited to the softer side.

Gravel spills from the opened layers whenever wind or rain dislodge it. Then the rock fragments are carried into the main flow of the arroyo where they skim the surface, marking the borders of the water paths, until they’re pushed into the soil by animals or moved farther along. Whatever actual rocks there are, usually no larger than a hand, also relodge along the water edge.

Last fall, after the summer rains, the floor near the downriver steep bank was glazed by glittering flakes of quartz that probably had floated on the surface of the water that carried away the pebbles, then dropped with the water level. The minute rockiness may be why the plants that live in the sandy arroyo don’t take root there, only Russian thistles and clammy weed.

The actual water channels are smooth like frosting when they’re wet, but molded by wind when the moisture evaporates. Some times, I’ll see no footprints in the sand, only ripples in closely grained arcs or wider, deeper partial chevrons. Other times, I can identify my own shoe prints from the week before.

The sections of the upriver steep wall that fell a few weeks ago are a bit like adobe, cakes of mud and stone bound by sticky clay. With time, the wind and rain will strip them, layer by layer, but the process may not be perceptible. The earlier collapsed section was only smoothed by last summer’s rain, not visibly shrunken.

The arroyo may not change as often as the Mississippi, but it lives at its own pace. In the 1920's, D. H. Lawrence wrote about the local landscape that it “lived, and lived as the world of the gods, unsullied and unconcerned. The great circling landscape lived its own life, sumptuous and uncaring. Man did not exist for it.”


Sangre de Cristo uplift

Miocene: 24.6 to 5.1 million years ago

Pliocene: 5.1 to 2 million years ago

Koning, Daniel J. “Preliminary Geologic Map of the Española 7.5-minute Quadrangle,” New Mexico Bureau of Geology, May 2002.

Lawrence, D. H. “St. Mawr,” 1925; the last part was set at his cabin outside Taos provided by Mabel Dodge Luhan.

Photograph: Arroyo, 19 February 2011. The section in front came down in the past three weeks; the section in back fell last April. A Russian thistle and a clammy weed are all that grew there last summer.

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