Interestingly enough, a similar painting titled "Light Coming on the Plains" has been determined to be a fake. I do not know if that also applies to this painting. A very interesting article (link below) sheds further light (ha ha) on this matter. [sh]
"...The 28 watercolors comprising the suite were laid out on tables while Mr. Kemper and the Director told their part of the story. When asked if any of the pieces looked suspicious I pointed out that in some the paper and the medium looked too fresh for their supposed age. These works would prove to have been executed on an Italian watercolor paper that wasn’t available in the United States until the 1950's.... First some background on the watercolors. The 28 watercolors comprising the Canyon Suite were purportedly done between 1916 and 1918 while the artist taught in Canyon, Texas....
See also: http://www.independent.co.uk/news/world/okeeffe-paintings-believed-to-be-fakes-1127169.html