Authors: Luna B. Leopold and Claudio Vita-Finzi. Abstract: River valleys in the Mediterranean, the Near East, the southwestern United States, and Mexico have repeatedly alternated between alluviation and erosion, changing the availability of water and agricultural land. The timing and magnitude of the various episodes suggest that the principal cause is a shift in precipitation patterns. Human activity has distorted the severity and the initiation of these changes that have at times been beneficial and at other times detrimental to the local population. Originally published: Proceedings of the American Philosophical Society ; v. 142, no. 1 (March 1998).