Large expanses of riverine marsh are rare in the desert Southwest, given the dry surface of many floodplain soils. Along the Hassayampa River, riverine marsh underwent a 5-fold increase (from 2% to 9% of the floodplain-channel area) after a large winter flood in 1993. Flood waters eroded terraces that had aggraded during frequent, smaller floods, widened the channel from about 3 to 50 m, and recharged the floodplain aquifer. The net effect of these changes was a lowering of the floodplain surface relative to the water table, a variable of critical importance to riparian plant composition in arid-land rivers. Olney’s bulrush (Scirpus americanus Pers.), southern cattail (Typha domingensis Pers.), jointed rush (Juncus articulatus L.), and other obligate wetland species were abundant in 1993 and 1994 on areas with saturated surface soils or shallow water tables and often were intermixed with seedlings of early-seral tree species, including Fremont cottonwood (Populus fremontii S. Watson), Goodding willow (Salix gooddingii Ball), and salt cedar (Tamarix chinensis Loureiro and related species). The gain in riverine marsh and young cottonwood-willow stands occurred at the expense of mature cottonwood-willow forests and deep-rooted, velvet mesquite (Prosopis velutina Woot.) woodlands. Another large flood in 1995 scoured the channel of most existing vegetation and aggraded the 1993 flood channel. Early-seral tree species again established in moist soils exposed by the slowly receding flood waters. However, redevelopment of extensive marsh habitat was precluded by sediment deposition that increased the elevation of the floodplain surface relative to the water table. These changes highlight the transitory nature of riverine marsh and other vegetation patch types in the dynamic floodplains of alluvial, arid-land rivers and underscore the importance of maintaining flood flows of varying magnitude to maintain patch type diversity.