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.