It's a shame we don't have satellite imagery dating back to when Europeans first arrived on the continent. I suspect the whole western US was once far more lush and green than it is today. The central valley in California, for example, used to have several natural lakes. Who knows how much impact the damming the Colorado river has on the ecology of the southwestern deserts. I can't imagine that concentrating water into man-made reservoirs combined with intensive agriculture has done much to help the drought conditions we see today across the US.