Water resource development has had a major impact on the hydrology of the Barwon–Darling River, a large dryland river in southeast Australia. Flows are highly modified through the presence of nine headwater dams, 15 main channel weirs and 267 licensed water extractors. Median annual runoff has been reduced by 42% over a 60-year period. Small flood events (e.g. Average Recurrence Interval of <2 years) have suffered the greatest impact with reductions in magnitude of between 35 and 70%. At a number of stations, the seasonality of flows has also been affected with a distinct shift in seasonal flow peaks relating to irrigation diversions. Overall, flows show a marked increase in predictability and consistency (sensu Colwell R.K. 1974. Predictability, constancy and contingency of periodic phenomena, Ecology 55, 1148–1153). There has also been a change in the shape of the hydrograph. Both long- and short-term hydrological changes in the Barwon–Darling, associated with water resource development, may prove to be critical for the ecological health of the system.