The distribution of riparian plant species is largely driven by hydrologic and soil variables, and riparian plant communities frequently occur in relatively distinct zones along streamside elevational and soil textural gradients. In two montane meadows in northeast Oregon, USA, we examined plant species distribution in three riparian plant communities¡ªdefined as wet, moist, and dry meadow¡ªalong short topographic gradients. We established transects from streamside wet meadow communities to the dry meadow communities located on the floodplain terrace. Within each of the three communities, we sampled plant species composition and cover and monitored water-table depth and soil redox potential (Eh) at 10- and 25-cm depths through three growing seasons (1997, 1998, and 1999). The study objectives were (1) to characterize and compare seasonal patterns of water-table depth and soil redox potential in portions of the floodplain dominated by the three different plant communities; (2) to compare plant species composition, distribution, and diversity among the three communities; and (3) to relate plant species diversity and distribution to water-table depth and redox potential. Strong environmental gradients existed along the transects. Water-table depth followed the seasonal patterns of stream stage and discharge and was consistently highest in the wet meadow communities (ranging from +26 cm above the soil surface to −37 cm below the surface), lowest in the dry meadow communities (−8 cm to −115 cm), and intermediate in the moist meadow communities (+17 cm to −73 cm). Dynamics of redox potential were associated with the seasonal fluctuations in water-table depth and differed among the plant communities. In the wet meadow communities, anaerobic soil conditions (Eh ¡Ü 300 mV) occurred from March through July at 10-cm depth and throughout the year at 25-cm depth. In the moist meadow communities, soils were anaerobic during spring high flows and aerobic in summer and fall during low flows. In the dry meadow communities, soil conditions were predominantly aerobic throughout the year at both depths. Wet meadow communities were dominated by sedges (Carex spp.) and had the lowest species richness and diversity, whereas dry meadow communities were composed of a mixture of grasses and forbs and had the greatest number of species. Species richness and total plant cover were negatively correlated with mean water-table depth and positively correlated with mean redox potential at 10-cm and 25-cm depths (P < 0.01). Distribution of the 18 most abundant plant species in relation to water-table depth and soil redox potential showed that certain species, such as the obligate wetland sedges, occurred within a fairly restricted range of water-table depth, whereas other graminoids occurred over wide ranges. These results suggest that the biological diversity often observed in montane riparian meadows is strongly related to steep environmental gradients in hydrology and soil redox status.