Historic surveys record screwbean mesquite (Prosopis pubescens) as widespread and dominant within upland riparian communities of the U.S. Southwest. I compared herbarium records and published surveys with surveys I conducted throughout the former range of P. pubescens. I found that this species has disappeared from 53% of its former range and has declined in density at eight of nine sites compared. Of 112 sites surveyed, only Ash Meadows National Wildlife Refuge has dense groves of P. pubescens over a large area. Prosopis pubescens remains in protected areas but not in urbanized areas. Declines of southwestern riparian species such as cottonwood and willow are well documented, but this is the first example of a declining species that is typically considered drought tolerant and is often found farther away from the stream course. Human alteration of rivers is therefore likely to be influencing plant communities as far away as a kilometer from the river itself.