"...In the larger version, to the west of the lower Grand Canyon and southeast of Lake Mohave and a small mountain range, there's a flat light-brown patch..."
That is a playa, flooded by runoff from recent rains.
That's what the discussion was indicating. Pretty big playa. Examination of Arizona maps does not indicate that there's anything in this area (i.e., it's empty).
My goal is to learn something new every day. Looks like I did today.
Kingman to Williams on Route 66
"Beyond Kingman, US 66 follows alongside the Santa Fe railway across the rather barren landscape of the Hualapai Valley, a very wide flat plain that stretches northwest for over 50 miles towards Lake Mead. There is a long sandy side-track (Antares Road), fine for normal vehicles, that passes many scattered ramshackle dwellings typical of the Arizona desert, all surrounded by rusty machinery, abandoned outbuildings and old cars. Other features of interest include Red Lake, a colourful dry lake bed, and large forests of Joshua Trees around the north end of the road, near the junction with the main route to Pearce Ferry."
Not to be confused with Red Lake, AZ, north of Williams. I found one other site that indicated that a salt cavern under the playa may be developed for natural gas storage -- and this confirmed that the playa is indeed Red Lake.
I even found a map of Red Lake! I promise that you'll be impressed. ;-)