Understanding how Quaternary climate shifts shaped fluvial systems through cycles of aggradation and incision is a central theme in fluvial geomorphology. A widely observed pattern is that wetter climatic phases enhance flood discharge and transport capacity, driving valley incision. Whether this relationship is reversed in sedimentrich, semi-arid environments, however, has remained poorly constrained by direct quantitative evidence. Here we present the first quantitative reconstructions of peak flood paleodischarges from the Late Pleistocene Inaoue`ne valley, northern Morocco. Our results show that minimum peak flood discharges during Last Glacial Maximum aggradation were at least 8487 +/- 1128 m3/s, exceeding those of the subsequent incisional phase (>= 1950 +/- 181 m3/s) by more than four times and the maximum modern recorded flood by over 6.5 times. These findings demonstrate a sediment-supply-dominated regime, where abundant clast production from hillslopes overwhelmed even extreme floods and drove valley-wide aggradation. Incision, in turn, occurred under conditions of much lower discharge and reduced sediment input. This study provides direct, quantitative evidence linking abrupt Lateglacial climate transitions to flood-driven landscape evolution in a semi-arid Mediterranean environment.