We also stopped at a sand dunes area, east of the Salton Sea. There were sand dunes only on one side of the road, and the other side was more grown in. I was wondering if the sand dunes were possibly created by the people driving their cars over the sand and killing the plants that would turn the dune into a hill. Another possibility, since the area is the ancient mouth of the Colorado River, is that the advent of farming loosened the soil and the sand began to dune up after that. In any case, there were large areas nearby that were not creating dunes and I thought it was human influenced in some way.
I also made a video of the trip in Altair north from La Cruz de Huanacaxtle (in Nayarit) to San Carlos (Sonora). I hope you enjoy it.