1. Wood density is a key plant functional trait which integrates the trade-offs characteristic to riparian plant ecological strategies. Although high-density wood is costly to construct, it confers mechanical stiffness to stems, increasing a plant's capacity to withstand flooding, and also enables increased tolerance to water stress. For riparian plants, fluctuations in soil moisture driven by surface hydrology should therefore be an important driver of variation in wood density. 2. We asked the following questions in the study: (1) Does wood density increase with increasing frequency and magnitude of flood disturbance? (2) Does wood density increase with increasing unpredictability of water availability in the riparian zone? (3) Does dispersion of wood density peak at intermediate levels of hydrological disturbance? 3. We surveyed wood density of dominant species at 15 riparian sites along flow-gauged rivers across south-eastern Australia. Due to the broad range of hydrological variability associated with Australian river systems, this set of sites functions as a useful model for assessing the response of riparian plants to changing hydrological conditions. 4. We found wood density varied strongly along a single axis of hydrological variability. This axis integrates flood intensity and frequency with metrics of hydrological unpredictability and can be conceptualized as a gradient of environmental harshness, with higher wood density associated with harsher conditions. 5. Synthesis. Our study highlights the importance of hydrological conditions, particularly disturbance and environmental unpredictability, as determinants of ecological strategy in riparian plants. Large, rare flood events in particular appear to favour higher wood density strategies. This is likely to have significant ecological consequences for riparian plant communities in a south-east Australian context, as well as in other regions where increasing climatic variability and frequency of extreme events are hallmarks of climate change predictions.