Herbaceous species comprise most of the floristic diversity in semi-arid region riparian zones, yet little is known about their response to river flooding. We compared cover, richness, and distribution of six functional groups of herbaceous plants after a large fall flood (pre- vs. post-flood year comparison) and after small monsoon floods and rains (dry vs. wet season contrast), and compared richness across a longitudinal (upstream-downstream) gradient of flood intensity. Herbaceous cover and richness increased significantly (p≤0.05, ANOVA) from the pre-flood to post-flood year and from the dry to wet season. Overall, the post-flood increases in richness and cover were related to the combined effects of disturbance (as indicated by strong increases of annual plants) and increased water availability (as indicated by response patterns of hydric perennials and other functional groups). All annuals showed strong increases in richness and cover in the year following the large fall flood, with hydric annuals increasing in richness by 43%, mesic annuals by 52%, and xeric annuals by 75%. Hydric perennials had a small net increase in richness following the large flood, reflecting a positive response to increased flow permanence, countered by low richness at sites with very high flood intensity (total stream power). Mesic and xeric perennials did not change significantly in richness from the pre- to post-flood year. However, across the spatial flood intensity gradient, the richness response pattern of the annuals and perennials alike peaked at intermediate levels of disturbance. In response to seasonal rains and moderate flooding, hydric perennials did not change in abundance, reflecting their primary association with shallow ground water and perennial stream base flows, but mesic perennials increased in cover and xeric perennials increased in both cover and richness. All three annual groups increased in cover and richness and in distribution across the flood plain following the summer monsoon floods and rains: hydric annuals had peak cover in inundated zones, suggesting positive response to river flooding, while xeric annuals peaked in cover above inundation zones, suggesting positive response to increased rainfall; mesic annuals had intermediate patterns. During the dry season, in contrast, annuals had low richness and cover and were restricted to low elevation fluvial surfaces adjacent to the stream channel and/or underlain by shallow ground water. Overall, both disturbance and increased moisture conditions caused by floods, as well as moisture from seasonal rains, contribute to increased richness and cover of herbaceous plants within the flood plain of the San Pedro River.