The Ditch - The Dam

The Santa Cruz dam sits near the edge of an uplift of Mesoproterozoic granite topped by early Pleistocene sandy gravel.

Daniel Koning’s geological map for the Cundiyo quadrangle shows the uplift lies between two north-south faults with the western one through the reservoir and the eastern roughly along route 503. Both are moving east. Between them, northeast of the lake, is another, shorter fault that curves and is moving northeast.

The sedimentary land at the top of the dam basin is 6300'. The structural and hydraulic heights of the dam are 151' but only 100' is available to hold water. The width of the dam at its base is 103.6'. That means the crack in the uplifted rock, at its narrowest, is something like 150' x 100'.

The lake covers 121 acres at the top and looks to be roughly 4000' long from the face to the point where the bowls narrows and water is white in the picture below. The opening widens to 600' in places.

From there the Santa Cruz river runs for a mile to the point it forms at route 503. It first goes south where the land above is 6400', then curves to go northeast where the surface land is 6600'. It flows through what Craig Martin describes as a “narrow, rocky canyon.”

The 10' to 15' wide river has “shallow runs and many holes 4 to 10 feet deep” for brown trout. That means, the the crack goes more than 10' below the surface and the river bottom is rock fill.

Martin say’s it’s possible to walk the river, nearly to the dam, if you’re willing to wade through the water and navigate those openings.

You don’t need to venture so far to see the rocks that form the walls of the reservoir. They’re visible on route 503 where the river begins it narrow journey. Daniel Koning says the granite and granitic pegmatite have a “distinct reddish weathered surface” and consist of potassium feldspar, quartz, muscovite, plagioclase feldspar and biotite.

Outcrops can also be seen when you follow the river back from route 520 to the dam at the northern end of canyon fault.

The Bureau of Reclamation says that the “quality designations” of the dam’s supporting rock “ranged from poor to very poor; however, with depth the fractured zones are locked in and will provide adequate foundation.”

That’s as reassuring as it gets in an area striped with faults. A collapsed dam would spread disaster all the way to the Rio Grande.

Notes: Martin, Craig. 2002. Fly Fishing in Northern New Mexico.

Photographs:
1. Santa Cruz lake and dam, 14 February 2012.

2. Santa Cruz dam from the other side, 16 February 2012.

3. Santa Cruz lake looking towards the north opening, 14 February 2012.

4. Walls of the Santa Cruz river canyon from route 503, 14 February 2012.

5. Santa Cruz river from route 503, 14 February 2012.

6. Exposed rock on route 503, 14 February 2012.

7. Exposed rock from other side of dam, near route 520, 16 February 2012.