Author(s): Alex Sims; Ian Rutherfurd
Linked Author(s):
Keywords: No Keywords
Abstract: Pulses of bedload sediment raise bed levels, destroy habitat and transform channel morphology. In rivers where sediment supply has decreased, and the tail of the pulse is migrating downstream, managers can intervene to accelerate river recovery. This research compares the effectiveness of several common reach-scale interventions -- fencing to exclude stock, fencing to exclude stock paired with in-stream extraction, and in-stream extraction alone -- at improving the geomorphic condition of a river. We observed an extremely close spatial association between specific reach morphologies and the mapped extent of interventions. We propose that fencing to exclude stock has allowed vegetation to establish on the river bed, transforming the channel bed from a wide, flat, featureless expanse to a well-defined pool-and-riffle sequence. Importantly, vegetation growth and pool development develop even in reaches subject to sand extraction; an intervention known to degrade bed levels and promote bank erosion. These results highlight the importance of sediment-vegetation interactions in shaping river recovery and shows that local-scale interventions (1-2 km in length), can accelerate recovery in rivers affected by pulses of bedload sediment. They also highlight the fact that, in this case, it is stock exclusion that was the critical factor in the geomorphic recovery of this stream. These result contrast with existing models of channel adjustment at the tail of a bedload pulse, which predict stable bars, a meandering low flow channel and pools will develop upstream and then steadily progress downstream. Instead we observe abrupt transitions between different morphologies and pool development in reaches bound upstream and downstream by dissected, degraded reaches.
Year: 2018