Transpiration (EL) and stomatal conductance (GS) were measured with stem heat-balance and Granier sap flux sensors on the dominant phreatophyte, saltcedar (Tamarix spp.), growing at six sites on a floodplain on the Lower Colorado River, US. Plant-specific leaf area index (LAPS) of shrubs was measured by leaf harvesting and Licor 2000 Plant Canopy Analyzer, and fractional cover (fc) was estimated by aerial imagery. Ground-area transpiration (EG) was calculated as EL × LAPS × fc. The sites presented environmental gradients with respect to distance from the river (0.2-1.5 km), depth to groundwater (2.4-3.5 m), groundwater salinity [1.9-24.0 g L-1 Total Dissolved Solids (TDS)] and soil texture (ranging from sand to clayey silts). EL aried from 1-3 mm m-1 leaf d-1 across sites, while LAPS ranged more narrowly, from 2-4, and fc varied from 0.5-0.95. Due to differences in EL, LAPS and fc, EG ranged from 1.2-9.5 mm d-1, nearly a 10-fold range. Only one site had EG characteristic of unstressed condition. We concluded that saltcedar is capable of high EG rates, but is also a stress adapted species, with E G highly variable over nonflooding riparian zones typical of regulated rivers. Salinity of the aquifer and vadose zone were identified as key constraints on saltcedar EG. Mean EG over the floodplain was only 40% of potential ET, contrary to earlier assumptions that saltcedar is invariably a high-water use plant.