Yup. The whole area was much wetter then. The rivers were flowing with water from glacial melt that had accumulated for 100k years. Once all the glaciers melted, the region dried to a crisp.
People once (as recently as early in the twentieth century) widely believed that "rain follows the plow," that human agriculture tended cause more rain in dry regions. I've lately noticed that, as part of the tendency to blame humans for all evil and all change, the more fashionable current variation is that "desert follows the plow." That is, in places like the Sahel of Africa and other regions once blooming but now dry, humans have caused something called "desertification" which otherwise would not happen.
Well, here and there humans do reroute the water from point A to point B, such as in the Aral Sea basin, changing things at least for a time. Also, humans are pumping up water from the Oglala undergound reservoir to (for time, until the finite supply of water runs out) make western Kansas, Nebraska, and Oklahoma more fertile than they used to be. But I suspect that the net influence of humans on this kind of thing has not been large or permanent.