The installation I originally asked about seems to be the Gallegos Pumping Plant of the Navajo Indian Irrigation Project, or NIIP. It is possible that I am wrong about that, and it is the Kutz Pumping Plant, but it is quite difficult to tell for sure. The plant entrance is completely unmarked, as is evident from using the Google Maps street view on the nearby Highway 302, from which you can see the various power lines going to the pumping plant, as well as just the top of the large white tank that's adjacent to the north end of the pump building.
After some perusal of various PDF documents available on line, it would appear that the NIIP is something the United Stated Government did for the Navajo Nation in return for their agreeing to share their water resources with non-nation users. This initiative looks to have been started in the early 1960s, and may go back further than that. The NIIP was apparently the first large-scale project of the BLM (Bureau of Land Management) initiated during the Kennedy administration.
The plant I located may indeed lift water up so that it can flow west through an underground pipe, which emerges to an above-ground canal at a point about three miles to the west of the plant, as suggested by freeper az_gila (very good eye there, az_gila).
The scale of this NIIP project is really astonishing. The system comprises about 70 miles of main canals, and more than three hundred miles of "lateral" canals. The main canal is conducted underground in ten separate tunnels at various intervals along its length. Pumping stations that move water through the system consume about 50 MW of electricity. The system is entitled to draw 508,000 acre-feet of water from the San Juan river every year; that's about 0.15 cubic miles of water!
As I examine the size and scope of this project, I am in awe of the incredible wealth of the United States, that can make such a project possible. Also of the value of a commitment of the United States Government, which has steadily worked on this project for more than fifty years.
Thanks again to all those who helped figure this out.