Arroyo Seco - Natural Influences

After I read Luis Granillo’s description of La Cañada with houses destroyed by an arroyo, I thought it time to see the rest of Arroyo Seco. I’d seen upstream parts that were accessible by car, but never ventured downstream from the bridge I cross to get into town.

The thing that has always struck me about this arroyo is that beyond the bridge it becomes the prettiest of the arroyos in the area, a wide sandy road winding between tree lined banks. Compared with the others, there is no deep erosion.

That may, in part, be due to human factors. This arroyo goes through residential, rather than pueblo land. The land owners are present, and probably much more likely to report people on ATVs breaking through fences and tearing by their houses. When I was walking in the arroyo, someone’s security system started screeching.

While early settlers must have grazed their cattle in the area upland from their irrigated fields, there may not have been the density of livestock there was in my area where ranchers were active and so less trampling disturbed the surface.

Perhaps more important, there are no major irrigation ditches dumping into the arroyo. Instead, waters are directed toward the Río Grande. The only one I know is the runoff from a lateral that drains from the acequia that comes to my area. The ditch is now buried and comes under the road, then the pipe just stops, leaving the water to find its way to the arroyo.

A large cottonwood has grown and directed its path.

The natural ditch is narrow and deep, but has little impact on the arroyo. The most it has done is expose some of the underlying rocks and pebbles.

When I looked at Daniel Koning’s geological map of the area, I realized there were also some historic reasons for the differences in this arroyo. Most of the settled land in the area is alluvium from the middle to upper Holocene that “generally supports junipers and grass” but with limited soil development.

Near the river most areas are more recent deposits that “supports sparser plant growth, particularly in regards to grass and trees, but more abundant woody shrubs.”

This arroyo is different in two ways. The confluence with the Río Grande is more subject to annual deposits, so, Koning says, the surface is “not vegetated and there is no soil development.”

The upstream banks are an upper Holocene deposit that Koning thinks “is probably related to regional arroyo incision and stream complex response phenomena that episodically occurred over the last several hundred years (since 800 to 2,000 years ago in the Rio Tesuque drainage).”

One forgets this arroyo has its origins in the Tertiary badland watershed with the Pojoaque River, while the arroyos near my house arises in closer Tertiary sedimentary ridges. Whatever those differences in rocks, which Koning defines in detail, the effect is the banks do not erode as easily.

The Ditch - Chimayó

When you pass beyond the badlands, the land opens out. You’ve passed the northeast trending boundary between lithosome B from Peñasco and lithosome A from the Santa Fe section of the Sangre de Cristo within the Tesuque formation. Ridges of sediment still exist to the north, but they’re farther back from the road, on the other side of the Rio Chiquito. They’re now Quaternary alluvium.

The road no longer follows the Santa Cruz which lies farther to the right. The 1940 WPA Guide to 1930's New Mexico says the area then was “planted with fields of corn, chili, frijoles (Mexican or pinto beans), apple and peach orchards.” Today all you can see are houses, many announcing themselves as studios or galleries.

Here and there signs warn of possible water on the road. When you look north, you often see an arroyo that’s been turned into a road.

On the other side of the road you see steeply declining roads where water runs from arroyos and route 76 itself.

Dams were built on several of these washes in 1962, to stop silt from running into the Santa Cruz and the irrigation ditches downstream. In Sabrino’s Map, Don Usner suggests they may have had the unintended consequence of encouraging settlement along the arroyos. He says early settlers in Chimayó “took care to build their homes well away from the arroyo beds” where “spectacular floods once churned,” but since the dams were built “random arrangements of mobile homes lay boldly in the path of some of the temporarily constrained washes.”

You don’t actually see the Santa Cruz again until you turn towards the Santuario in El Portrero southeast of the old Chimayó plaza. For the most part, the rocks again are smaller than they were downstream.

In front of the dam face, the river is dry or nearly so. The dam is releasing no water right now.

Between the two points, the Rio Quemado has joined the Santa Cruz and snow is continuing to melt from the recent flurries.

Photographs:
1. Rio Santa Cruz in El Portrero, west of the Santuario on route 520, 16 February 2012.

2. Arroyo to north off route 76 in Chimayó area, 16 February 2012.

3. Road leading off 76 toward Rio Santa Cruz in Chimayó area, 16 February 2012.

4. Rio Santa Cruz in El Portrero, west of the Santuario on route 520, 16 February 2012.

5. Rio Santa Cruz soon after it leaves the dam, off route 520, 16 February 2012.

6. Snow melting into Santa Cruz river from route 503, 14 February 2012.

Lakes

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, besides smoke, 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.

The River Runs

Wednesday it rained, day and night.

Thursday noon when I went to the post office the river was running higher and faster, the color of caffè latte.

I thought, ah yes, of course, the river’s running. It rained. I didn’t think, I’m seeing the great shaping force of this part of the country roused from a long seasonal slumber.

I didn’t connect even though I’d spent the morning looking at Daniel Koning’s “Preliminary Geologic Map of the Española 7.5-minute Quadrangle” which shows a lopped triangle with the Rio Grande and Route 285 on the sides, the Rio Pojoaque to the south and the Santa Cruz river on the north just above where I was driving over the Griego bridge.

He shows the road near my house skirts what he labels a “geological contact.” The soils to the river side have recent alluvial origins. The ones to the east date back to an earlier Tertiary period. Since the time before the great glaciers when the river began to connect the discrete basins of the rift valley, water has been digging and padding its channel.

It’s removed or redeposited the existing tertiary sediments, or perhaps both at the same or different times, and left a boundary area that needs no geologist to recognize.

I went back to today to the Griego Bridge to see the river at the point the Santa Cruz enters. The current was slower, but the water was still carrying dirt. Gravel and sand have been deposited where the dammed and controlled water flow of the one meets the less tamed Rio Grande.

Then I drove home and looked again at that “geological contact” out the car window. You could imagine the grass as some great sea lapping against dunes. And like ladies of a certain age who once were rivals and now nod when they meet, you can only guess their pasts from the differences in their outerwear, their vegetation.