The dispersal of plants and animals underlies landscape-scale ecological processes such as immigration, gene flow and metacommunity dynamics. Dispersal is important in structuring floodplain-plant assemblages because the plants experience high levels of disturbance. Hydrochory can be an important source of colonists in these systems but there are few studies that examine spatial and temporal variability in propagule supply and relate these patterns to the various models of metacommunity assembly. To address this deficiency, we examine spatial and temporal variability in propagule supply in the lower reaches of two lowland floodplain rivers in south-eastern Australia and relate observed patterns to models of community assembly. Dispersal characteristics were highly dynamic and varied seasonally within a year and over spatial scales of <10 km. Temporal variation in propagule supply was influenced by propagule availability and flow velocity. Four temporal patterns of dispersal were distinguished that are relevant to the various models of community assembly: (i) persistent, propagules disperse in most months; (ii) seasonal, propagules disperse over a few months and are then absent; (iii) seasonally-persistent, propagules disperse over a few months but persist at low-levels for many months; and (iv) pulsed, large numbers of propagules disperse only briefly. Spatial variation in propagule supply was influenced by flow velocity, tributary inputs, a salt wedge, and water-control structures (e.g. sluices). The dispersal dynamics observed suggest that floodplain-vegetation assemblages are structured by strong interactions between physical processes (flow, saline intrusions, water-control structures) and plant traits (e.g. dispersal timing, seed floating duration and life history strategy). Dispersal characteristics consistent with mass effect, neutral, species sorting and patch dynamic models of community assembly were found. We conclude that all models of community assembly are likely to have some relevance in controlling the dynamics of floodplain vegetation.