Islands in the stream: Wood-induced deposition and erosion in the river corridor

Large wood causes and responds to deposition and erosion within a river corridor. We focus on the anastomosing, gravel-bed Swan River and two meandering, gravel-bed tributaries in northwestern Montana, USA to explore the temporal dimensions of deposition and erosion associated with channel avulsions and island formation and to introduce the concept of wood levees. Channel avulsion represents isolation of part of the existing floodplain and formation of an anastomosing channel planform, with wood-induced deposition at the point of channel bifurcation. Islands form at a wood jam that migrates upstream with time as sediment accumulates in the lee of the jam. The island creates only a local interruption of the single-channel planform. We use tree-ring and ¹⁴C dating to constrain wood-induced island ages. We interpret the three wood-induced forms of deposition and erosion that we describe here as reflecting a temporal continuum. Wood levees have primarily non-woody vegetation and may be transient relative to the other features. Tributary islands appear to persist from a decade to over a century. Tree ages of 100–200 years at the floodplain avulsion site and the characteristics of the secondary channels suggest that these wood-induced avulsion features can persist for more than a century. Understanding the temporal dynamics of wood-induced features and spatial variation in erosion and deposition provides insight into the dynamics and spatial heterogeneity of natural river corridors, with implications for river restoration.