Evaluating the potential for natural ecosystem recovery in cut-and-fill wetlands: case study of Pietersielieskloof palmi

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ORIGINAL PAPER

Evaluating the potential for natural ecosystem recovery in cut-and-fill wetlands: case study of Pietersielieskloof palmiet wetland, South Africa S. E. Grenfell

. M. F. Mamphoka . M. C. Grenfell . N. Job

Received: 13 May 2020 / Accepted: 10 September 2020  Springer Nature B.V. 2020

Abstract Cut-and-fill processes, characteristic of many valley-bottom wetlands, present a major challenge to practitioners planning wetland restoration. It is not clear whether the current synchronicity and spatial scale of incision is ‘natural’, and whether these wetlands will naturally recover without intervention during the ‘fill’ phase. The aim of this study, focussed on a Prionium serratum dominated valley-bottom wetland in South Africa, was to evaluate the potential for natural ecosystem recovery in the context of current catchment constraints. Historical rainfall records, aerial photography and a DGPS survey were used to contextualise geomorphic processes. The sedimentology of alluvial fill was investigated by S. E. Grenfell (&) Department of Geography and Environmental Studies, Stellenbosch University, Stellenbosch, South Africa e-mail: [email protected] M. F. Mamphoka Department of Geography, Environmental Studies and Tourism, University of the Western Cape, Bellville, South Africa M. C. Grenfell Institute for Water Studies, Department of Earth Science, University of the Western Cape, Bellville, South Africa e-mail: [email protected] N. Job Freshwater Biodiversity Unit, South African National Biodiversity Institute, Rhodes Avenue, Cape Town, South Africa

sampling sediment profiles. Samples were analysed for particle size and organic content, five were dated using radiocarbon. Sedimentology and radiocarbon dates were used to ascertain phases of incision and aggradation along wetland reaches. While several phases of incision are represented in the record, none were represented in more than one location, suggesting phases of erosion and subsequent deposition were spatially and temporally segregated. The current synchronous phase of incision across the wetland is therefore unprecedented. Analysis of sedimentation rates indicate that infilling of the gully decreases exponentially over time. Infilling of a 3m deep gully in the Pietersielieskloof wetland under natural catchment conditions is estimated to take * 5000 years. Analysis suggests that there has been a reduction in wetland resilience over the last century, and that a change in catchment hydrology and sediment supply has altered this system such that it has crossed a geomorphic threshold. Keywords Cut-and-fill  Resilience  Natural recovery  Palmiet

Introduction Valley floors characterised by either no channel or a discontinuous channel pattern and associated

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Wetlands Ecol Manage

Quaternary alluvial fill (Patton and Schumm 1975) or cut-and-fill sequences (Brierley and Fryirs 1999) are common in dryland regions of the globe (Tooth 2000, 2013; Jaeger et al. 2017). While they are usually associated