Lessons learnt from long-term monitoring of river restoration in an English chalk stream

Abstract

River restoration can be difficult to evaluate due to insufficient monitoring over timescales too short to adequately capture physical and ecological response. To better understand restoration outcomes, this study quantified changes in physical habitat (depth, velocity, substrate composition) and macroinvertebrates at two restoration projects on a chalk stream (River Test, UK) over 8–9 years using a Before-After repeated measures approach. Restoration involved coarse substrate addition, tree hinging/woody material placement and planform reprofiling. At both sites, habitat heterogeneity (e.g., cross-sectional depth variability) and macroinvertebrate abundance and diversity increased, whilst non-rheophilic taxa increased in dominance at one site. Substrate composition varied over time but generally became coarser post-restoration. Macroinvertebrate metrics remained relatively stable following restoration, except in 2016 (1- and 3-years post-restoration) when several metrics were lower (e.g., abundance and taxon richness). The reasons for this are unclear but could represent local disturbance (e.g., river management) or variability in data collection between surveyors. This study provides evidence for the effectiveness of restoration and highlights the benefits and challenges of longer-term monitoring. Considering the cost of restoration, there is a need to adequately evaluate project outcomes over the long-term to ascertain reasons for success and failure and overall cost: benefit. Equally, it is recognised that due to the small-scale of many opportunistic projects it is not economically viable to include extensive monitoring. Therefore, we recommend a strategic programme of robust, long-term appraisals be developed assessing exemplar restoration sites to evidence restoration effectiveness and guide future efforts.

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ORCID for Andrew S. Vowles: orcid.org/0000-0001-8253-5938

ORCID for Paul S. Kemp: orcid.org/0000-0003-4470-0589

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