Groundwater depletion threatens many riparian ecosystems in arid and semiarid regions of the world. The aquifer that sustains Arizona´s San Pedro River riparian ecosystem, for example, is threatened by regional groundwater declines and localized pumping from the alluvial aquifer. This paper demonstrates the important role of shallow groundwater in structuring the San Pedro River plant commnity, portions of which function as reference areas that indicate site potential for a globally rare forest type (Sonoran riparian Populus-Salix forest). Several ecological indicators varied with depth to groundwater, inclusing a wighted average wetland indicator score calculated for herbaceous and woody plan species, cover of plants within wetland indicator groups, and frecuency of indicator plan species...