Local scale interventions accelerate recovery at the tail of a bedload pulse — ASN Events

Local scale interventions accelerate recovery at the tail of a bedload pulse (#83)

Alex Sims 1 , Ian Rutherfurd 1
  1. School of Geography, The University of Melbourne, Parkville, Victoria, Australia

Pulses of bedload sediment raise bed levels, destroy habitat and transform channel morphology. In rivers where sediment supply is declining, and the tail of the pulse is migrating downstream, managers can undertake interventions to improve geomorphic condition. There is uncertainty as to the most effective type, scale and sequencing of interventions to maximise improvements in geomorphic condition over management timeframes (~50 years). This research compares the effectiveness of several common reach-scale (102 – 103 meters) 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 impacted by a pulse of bedload sediment. We observed a near-perfect spatial association between reach morphology and the extent of interventions, and reach-averaged metrics of geomorphic complexity are greater in those where stock have been excluded. In stock exclusion reaches and in reaches previously subject to instream extraction, the wide, flat, featureless bed has transformed to a remarkably well-defined sequence of deep pools. This is contrary to existing bedload pulse recovery models which predict alternating bars and a meandering low-flow channel will emerge in upstream reaches first, and then steadily progress downstream. Instead, morphology changed abruptly between reaches, and reaches with improved geomorphic condition and stable features (a sequence of pools) were bound upstream and downstream by wide, flat dissected reaches. While emergent macrophytes are associated with pool development, it is unclear whether vegetation drives pool development, or pool development provides the necessary conditions for instream vegetation to establish. These results suggest instream wetlands can be leveraged to improve geomorphic condition and accelerate recovery in rivers affected by pulses of bedload sediment. 

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