This branch left the Southern Pacific (now Union Pacific) Sunset route near Winterhaven, CA, just west of the Colorado River crossing. It headed north, roughly followed the west bank of the Colorado River as far north as the Imperial Dam; it was most likely used for the construction of the Imperial Dam on the Colorado River. There are no signs or other evidence of the physical location of "Potholes", though a particular railroad map places it at the location of Laguna Dam.
The wye area on the mainline is visible on the satellite map, but at ground level it is largely inaccessible because it belongs to the Department of the Interior and Indian Tribal Lands.
Thanks to Mike Palmer for contributing information about this route.