Channel Conveyance Variability can Influence Flood Risk as Much as Streamflow Variability in Western Washington State

Changes in the severity and likelihood of flooding events are typically associated with changes in the intensity and frequency of streamflows, but temporal adjustments in a river's conveyance capacity can also contribute to shifts in flood hazard. To assess the relative importance of channel conveyance to flood hazard, we compare variations in channel conveyance to variations in the flow magnitude of moderate (1.2 years) floods at 50 river gauges in western Washington State between 1930 and 2020. In unregulated rivers, moderate floods have increased across the region, but in regulated rivers this trend is suppressed and in some cases reversed. Variations in channel conveyance are ubiquitous, but the magnitude and timing of adjustments are not regionally uniform. At 40% of gages, conveyance changes steadily and gradually. More often, however, conveyance variability is nonlinear, consisting of multidecadal oscillations (36% of gages), rapid changes due to unusually large sediment‐supply events (14% of gages), and increases or decreases to conveyance following flow regulation (10% of gages). The relative importance of conveyance variability for flood risk depends on the mode of adjustment; in certain locations with historic landslides, extreme floods, and flow regulation, the influence of conveyance changes on flood risk matches or exceeds that of streamflow at the same site. Flood hazard management would benefit from incorporating historic long‐term and short‐term conveyance changes in predictions of future flood hazard variability. Plain Language Summary River flood hazards change through time due to variations in streamflow and variations in the channel's ability to convey flood flows or “conveyance.” We study the importance of both factors to variability in flood hazard over time at 50 river gages in western Washington State. Conveyance variability contributes to shifts in flood hazard at nearly all 50 gages, but rivers do not adjust at the same time nor by the same amount across the region. Conveyance changes steadily and gradually at 40% of gages and varies nonlinearly at 60% of gages. The most common nonlinear patterns are oscillations in conveyance with a period of one or more decades. Less common are rapid changes to conveyance from unusually large sediment‐supply events and conveyance changes following flow regulation. Conveyance variations interact with variations in streamflow due to climate change or human regulations in ways that can worsen or a