UNRAVELLING THE ROLE OF VEGETATION IN THE AVULSION HISTORY OF GRAVEL-BED RIVERS

Whether meandering through lowland valleys or intertwining through the gravels of a steep mountain front, alluvial rivers have long been known to be influenced by encounters with vegetation. What has been left open to debate is the degree to which vegetation exerts a control on the evolution of alluvial rivers. Within the rapidly changing surfaces of alluvial bars, colonising vegetation only has a narrow window within which to become firmly rooted, ensuring access to water and robustness for withstanding inundation and any associated unravelling of bar sediment. The limit imposed by frequent inundation explains the systematic pattern of vegetation succession observed along aggrading point bars and floodplains. The problem is more acute within rivers prone to avulsion, even at meander scales. Much of our theoretical understanding of avulsion relates to the topography of the riverbank-floodplain interface. Although clearly important, our theoretical understanding does not adequately account for the potential role of floodplain vegetation, which could hinder the ability of overbank flows to mobilise floodplain sediment through flow drag and the strengthening of the floodplain surface. Here, we assess the role of vegetation in controlling both patterns of avulsion and widespread floodplain aggradation along the Yuba River of California. The river has undergone a long history of cumulative anthropogenic impacts, which left behind a perturbed river system, yet one that remains highly dynamic and still functions as salmonid habitat. Repeated high-resolution LiDAR surveys integrated with field measurements of floodplain materials and vegetation revealed an important control of vegetation on meander-scale avulsion. In short, dense vegetation patches facilitated localised aggradation, in some cases by rates up to 50 cm per year. Conversely, non-vegetated patches, particularly when exposed as gaps between vegetated patches, became loci of significant bar degradation, allowing overbank flows to effectively channelise and elevate bounder shear stresses across the bar. In every case of avulsion that we observed, new channels were incised within these non-vegetated patches. Our results suggest a critical role that floodplain vegetation plays in the avulsion history of gravel-bed rivers.