Nature Water, Published online: 11 September 2026; doi:10.1038/s44221-026-00699-6 This study presents a globally scalable, seasonally resolved classification of gauged and ungauged catchments into three functional rainfall–runoff types (simple, intermediate and complex), revealing that functional complexity is the norm across most of the world’s rain-dominated streams and rivers and providing a framework to guide streamflow gauging network prioritization and hydrologic model structural design.