Abstract
AbstractBurrowing into riverbanks by animals transfers sediment directly into river channels and has been hypothesised to accelerate bank erosion and promote mass failure. A field monitoring study on two UK rivers invaded by signal crayfish (Pacifastacus leniusculus) assessed the impact of burrowing on bank erosion processes. Erosion pins were installed in 17 riverbanks across a gradient of crayfish burrow densities and monitored for 22 months. Bank retreat increased significantly with crayfish burrow density. At the bank scale (<6 m river length), high crayfish burrow densities were associated with accelerated bank retreat of up to 253% and more than a doubling of the area of bank collapse compared with banks without burrows. Direct sediment supply by burrowing activity contributed 0.2% and 0.6% of total sediment at the reach (1.1 km) and local bank (<6 m) scales. However, accelerated bank retreat caused by burrows contributed 12.2% and 29.8% of the total sediment supply at the reach and bank scales. Together, burrowing and the associated acceleration of retreat and collapse supplied an additional 25.4 t km−1 a−1 of floodplain sediments at one site, demonstrating the substantial impact that signal crayfish can have on fine sediment supply. For the first time, an empirical relation linking animal burrow characteristics to riverbank retreat is presented. The study adds to a small number of sediment budget studies that compare sediment fluxes driven by biotic and abiotic energy but is unique in isolating and measuring the substantial interactive effect of the acceleration of abiotic bank erosion facilitated by biotic activity. Biotic energy expended through burrowing represents an energy surcharge to the river system that can augment sediment erosion by geophysical mechanisms.
Highlights
The active role of animals in geomorphological processes is increasingly acknowledged, in aquatic environments (Albertson & Allen, 2015; Butler, 1995; Fei et al, 2014; Moore, 2006; Rice et al, 2012; Statzner, 2012; Viles et al, 2008; Wilkes et al, 2019)
This study investigates the role of burrowing signal crayfish (Pacifastacus leniusculus) as drivers of sediment supply, bank erosion and geomorphic change in two UK rivers
Bank retreat was different between the two rivers, with mean bank retreat at the River Bain occurring more than ten times faster than at Gaddesby Brook (0.086 m a−1 and 0.008 m a−1, respectively)
Summary
The active role of animals in geomorphological processes is increasingly acknowledged, in aquatic environments (Albertson & Allen, 2015; Butler, 1995; Fei et al, 2014; Moore, 2006; Rice et al, 2012; Statzner, 2012; Viles et al, 2008; Wilkes et al, 2019). Evidence that burrowing can accelerate bank erosion comes from ex situ experiments (Onda & Itakura, 1997; Vu et al, 2017), and physical (Saghaee et al, 2017; Viero et al, 2013) and numerical modelling (Borgatti et al, 2017; Camici et al, 2014; Orlandini et al, 2015). Reviewing this and other research, Harvey et al (2019) suggest that animal burrowing may alter bank erosion processes via several mechanisms.
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