In an intermontane basin in central west Iran, centered on Kermanshah (34°, 17'N; 47°, 07'E), Late Pleistocene fan gravels lie below five alluvial units of Holocene age, which are separated by erosional disconformities and soils. These units, labelled I--V in order of increasing age, reflect changes in watershed conditions over a 10,000 year period. The chronology and genesis of these units proves to be of major importance for the interpretation of archaeological data gathered by surface survey. A flood-deposited channel gravel overlying the oldest of these units marks a major change in watershed conditions sometime after 200 A.C.; the change caused the erosional destruction of many sites. Subsequent alluvial deposition has further resulted in the complete or partial burial of many other sites. These events place severe constraints on the interpretation of ancient settlement patterns, population densities, and adaptive strategies in the study area.