We exploit a natural experiment caused by a high-magnitude flood (~500 yr recurrence interval) and sediment slug derived from more than 2,500 concurrent landslides to explore the influence of valley-scale geomorphic controls on sediment slug evolution and the impact of sediment slug passage and emplacement on channel stability and channel form. Movement of sediment slugs is a crucial process that shapes gravel bed rivers and alluvial valleys and is an important mechanism of downstream bed material transport, while changes in bed material transport rate that occur during slug emplacement can trigger channel responses including increases in river channel lateral mobility, channel widening, and increase in alluvial bar dominance. Multitemporal LiDAR and aerial photos bracketing the flood event, which occurred in 2007 on the Chehalis River in SW Washington State, USA, document the channel response to this event with high spatial and temporal definition. The sediment slug behaved as a Gilbert Wave, with both channel aggradation and sequestration of large This is the pre-peer reviewed version of the following article: Nelson A, Dubé K. 2015. Channel response to an extreme flood and sediment pulse in a mixed bedrock and gravel-bed river. Earth Surface Processes and Landform DOI: 10.1002/esp.3843 2 volumes of material in floodplains of headwaters reaches and reaches where confined valleys enter into broad alluvial valleys. Peculiarities of the valley form through which slugs in two separate sub basins moved highlight the important role channel and channel-floodplain connectivity play in governing downstream movement of sediment slug material. Finally, channel response to the high-magnitude flood and increase in bed material sediment transport rate illustrate the connection between bed material transport and channel form. The channel widened, lateral channel mobility increased, and relative importance of bars within the active channel increased in all reaches in the study area. But the response scaled tightly with the relative amount of bed material sediment transport through individual reaches, indicating that the geomorphic effectiveness of the flood was conditioned by the simultaneous introduction of a sediment slug to the channel network.