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1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition.

2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U. S. A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River.

3. Patterns of species richness varied with temporal scale of analysis, that is between single-year and multi-year time frames. At the annual timescale, quadrat species richness (m(-2)) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single-year pattern, the highest long-term richness occurred at intermittent sites.

4. Quadrat species richness, total species richness at a site (per 18 1-m(2) plots) and cover were more variable year to year at non-perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter-annual compositional variance, while the perennial sites had the lowest.

5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types.

6. High long-term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i. e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non-perennial sites.