Despite over five decades of research, recent state-of-the-science reviews illustrate that we still lack the rigorous, quantitative, physically-based process understanding and modelling capability required to understand how rivers respond to environmental change. Our failure to develop such models to date has major environmental, social and economic ramifications, because most rivers on Earth are affected by natural or human-induced disturbances, the consequences of which we cannot yet predict robustly. Dam construction, water abstraction, river engineering, deforestation and changing flood regimes are recognised as major catalysts of dramatic channel pattern change and associated bank erosion, loss of land and infrastructure, river bed scour or aggradation, habitat degradation and enhanced flood risk. These issues are a particular concern in the case of sand-bed rivers, because they are present at a range of scales from relatively small channels (widths <100 m) up to the largest rivers on Earth (widths 5-10 km). For example, the world’s 10 largest rivers (draining 17% of global continental land and delivering 33% of the terrestrial sediment supplied to the oceans) are all sand-bed channels. Given the global significance of these issues, why have we struggled to make rapid progress in our ability to model and understand sand-bed river morphology and evolution?