Distribution of logjams in relation to lateral connectivity in the River Corridor

Abstract

We examine the redistribution of logjams from the channel to the floodplain during a large flood following a wildfire in a forested montane catchment, conceptualizing redistribution with respect to changes in connectivity within the river corridor during and after the flood. A 2020 wildfire burned much of the 40 km2 Little Beaver Creek catchment at high severity. A flash flood in July 2022 completely inundated the river corridor and moved substantial amounts of large wood from the channel onto the floodplain. Floodplain logjams were largely absent prior to the flood. Forty-two channel-spanning logjams remained in the channel after the July 2022 flood, compared to 91 before the flood, and only 4 of these created backwater at base flow. Most of the large wood present in the river corridor was redistributed into 700 floodplain logjams, most of which were pinned against tree trunks. Multivariate linear models indicate that floodplain jam distribution density (number of logjams/ha surface area) correlates most strongly with floodplain forest basal area and floodplain/channel width ratio. Prior to the July 2022 flood, channel-spanning logjams limited longitudinal connectivity within the active channel but enhanced vertical connectivity via hyporheic exchange and lateral connectivity via formation of secondary channels and obstruction-inducing overbank flows. The enhanced transport capacity of the flood removed or altered most of these logjams, increasing longitudinal connectivity. Complete inundation of the floodplain during the July 2022 flood represented a transient increase in lateral connectivity. The formation of floodplain logjams and secondary channels, along with localized floodplain erosion and deposition, created more persistent alterations in lateral connectivity. Our observations suggest that logjams can increase lateral connectivity within the river corridor by deflecting flow in a manner that creates persistent secondary channels. Although secondary channels might have formed in Little Beaver Creek during the 2022 flood without the presence of logjams, the flow obstructions created by even relatively transient logjams likely increased the formation of a multichannel planform and the associated persistent lateral connectivity.