Much of the world's population faces significant threats to water security that are likely to be exacerbated under a warmer climate. To understand the impact of climate change on water security, we characterise the relationship between total annual streamflow, a surrogate for water supply, and low frequency flood events. We first calculate the proportion of annual streamflow attributable to flood events at approximately 5000 stations globally, then characterise this relationship as a function of climate and catchment characteristics. To explore the impact of climate changes we investigate trends in these relationships. We find that across the world on average 25% of annual streamflow comes from a single flood event and this proportion is well described by the variability in streamflow and precipitation, which is a function of catchment aridity. In arid to semi-arid catchments, where on average over 40% of annual streamflow comes from a single flood event, we conclude that water availability may be reasonably approximated by focusing on a selection of low frequency events, simplifying the projection of water supply changes under a future climate. Flood generating mechanism is crucial in determining trends of annual streamflow volumes from low frequency events. Where snowmelt is a significant flood process, increasing temperatures are causing a reduction in the proportion of total annual streamflows accounted for by flood events. Where snow is not a significant flood driver, trends are dominated by hydroclimatic variability and aridity, which is expected to be exacerbated with a shift to rainfall driven flooding under climate change.