Channel response to an extreme flood and sediment pulse in a mixed bedrock and gravel-bed river

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.