I know they got alot of rain out there last week (when Vegas got all that flooding)...maybe it is runoff into a dry lake bed?
Area 51 - but don't worry, the government has said it doesn't exist.
That's what it looks like to me too. The lower edges matche the profile of the wash leading into it. As well, the upper wash, leading out of it, also follows the same contours. Plus, there is a color gradation where the washes enter and exit the patch. If it was an artifact, I'd expect it to have sharper edges.