ABSTRACT Rainfall-induced surge waves in rivers are still today considered as an elusive and non-realistic phenomenon that has continued to mystify researchers and practitioners alike since the kinematic shock theory introduced in 1955. This is mainly due to scarce sightings of real cases, as well as physical and numerical uncertainties and sporadic calculations. In order to overcome such indefiniteness, a first set of documented real cases of surge waves in canyon rivers has been collected and their dreadful consequences highlighted. Then, the physical mechanism of surge wave development and propagation and the role of wave celerity differences are examined. A novel parameter (relating wave celerity and cross-section geometry) is introduced and used to highlight few relevant characteristics of rainfall-induced surge waves. The role of floods faster than ordinary flash floods (here defined as instant floods) is demonstrated. The conclusions are confirmed with a series of numerical tests for ideal and natural channels. Finally, differences from ordinary flash floods are identified in the framework of risk assessment, as well as the perspective to revisit real cases of surge waves in canyons and other streams.