ORE Open Research Exeter TITLE Natural vs anthropogenic streams in Europe: History, ecology and implications for restoration, river- rewilding and riverine ecosystem services AUTHORS Brown, AG; Lespez, L; Sear, DA; et al. JOURNAL Earth-Science Reviews DEPOSITED IN ORE 15 November 2018 This version available at http://hdl.handle.net/10871/34780 COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies. A NOTE ON VERSIONS The version presented here may differ from the published version. If citing, you are advised to consult the published version for pagination, volume/issue and date of publication 1 2 3 4 Natural vs Anthropogenic Streams in Europe: History, Ecology and Implications for 5 Restoration, River-Rewilding and Riverine Ecosystem Services 6 7 Antony G Browna,*, Laurent Lespezb, David A Seara, Jean-Jacques Macairec, Peter Houbend, 8 Kazimierz Klimeke, Richard E Brazierf, Kristof Van Oostg, Ben Pearsa 9 10 a 11 Geography & Environment, University of Southampton, Shackleton Building, Southampton SO17 1BJ UK. 12 [email protected], D.Sear @soton.ac.uk b 13 Laboratory of Physical Geography (LGP) CNRS-UMR 8591, University of Paris-East Créteil, France. 14 [email protected] c 15 Department of Science and Techniques, Universite de Tours, Parc de Grandmont 37200, Tours, FrFrance, 16 [email protected] 17 d Environmental Earth Science and Sustainability, Leiden University College - The Hague, Netherlands 18 [email protected] 19 e Department of Physical Geography, Earth Science Faculty, University of Silesia, Będzińska 60, 41-200 20 Sosnowiec, Poland, [email protected] 21 f Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX44RJ, UK. 22 [email protected] 23 g Earth and Life Institut, Université Catholique de Louvain, Louvain-la-Neuve, Belgium 24 25 * corresponding author Email address [email protected] 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 59 60 61 62 63 ABSTRACT 64 In Europe and North America the prevailing model of ‘natural’ lowland streams is incised- 65 meandering channels with silt-clay floodplains, and this is the typical template for stream 66 restoration. Using both published and new unpublished geological and historical data from Europe 67 we critically review this model, show how it is inappropriate for the European context, and examine 68 the implications for carbon sequestration and Riverine Ecosystem Services (RES) including river 69 rewilding. This paper brings together for the first time, all the pertinent strands of evidence we now 70 have on the long-term trajectories of floodplain system from sediment-based dating to sedaDNA. 71 72 Floodplain chronostratigraphy shows that early Holocene streams were predominantly multi- 73 channel (anabranching) systems, often choked with vegetation and relatively rarely single-channel 74 actively meandering systems. Floodplains were either non-existent or limited to adjacent organic- 75 filled palaeochannels, spring/valley mires and flushes. This applied to many, if not most, small to 76 medium rivers but also major sections of the larger rivers such as the Thames, Seine, Rhône, Lower 77 Rhine, Vistula and Danube. As shown by radiocarbon and optically stimulated luminescence (OSL) 78 dating during the mid-late Holocene c. 4-2ka BP, overbank silt-clay deposition transformed 79 European floodplains, covering former wetlands and silting-up secondary channels. This was 80 81 followed by direct intervention in the Medieval period incorporating weir and mill-based systems – 82 part of a deep engagement with rivers and floodplains which is even reflected in river and 83 floodplain settlement place names. The final transformation was the ‘industrialisation of channels’ 84 through hard-engineering – part of the Anthropocene great acceleration. The primary causative 85 factor in transforming pristine floodplains was accelerated soil erosion caused by deforestation and 86 arable farming, but with effective sediment delivery also reflecting climatic fluctuations. Later 87 floodplain modifications built on these transformed floodplain topographies. So, unlike North 88 America where channel-floodplain transformation was rapid, the transformation of European 89 90 streams occurred over a much longer time-period with considerable spatial diversity regarding 91 timing and kind of modification. This has had implications for the evolution of RES including 92 reduced carbon sequestration over the past millenia. Due to the multi-faceted combination of 93 catchment controls, ecological change and cultural legacy, it is impractical, if not impossible, to 94 identify an originally natural condition and thus restore European rivers to their pre-transformation 95 state (naturalisation). Nevertheless, attempts to restore to historical (pre-industrial) states allowing 96 for natural floodplain processes can have both ecological and carbon offset benefits, as well as 97 additional abiotic benefits such as flood attenuation and water quality improvements. This includes 98 99 rewilding using beaver reintroduction which has overall positive benefits on river corridor ecology. 100 New developments, particularly biomolecular methods offer the potential of unifying modern 101 ecological monitoring with reconstruction of past ecosystems and their trajectories. The sustainable 102 restoration of rivers and floodplains designed to maximise desirable RES and natural capital must 103 be predicated on the awareness that Anthropocene rivers are still largely imprisoned in the banks 104 of their history and this requires acceptance of an increased complexity for the achievement and 105 maintenance of desirable restoration goals. 106 107 108 109 110 111 112 113 114 115 116 117 2 118 119 120 121 122 1. Introduction: stream engineering and natural reference conditions 123 For decades meandering, gravel-bedded, streams with fine-grained floodplains forming their banks 124 125 have been considered as a universal model explaining the morphology and functioning of natural 126 127 streams in temperate lowland temperate European and North American riverine landscapes 128 (Leopold and Wolman, 1957; Wolman and Leopold, 1957). As a logical consequence, the concept 129 130 has also served as template for natural reference conditions (RRC, 2001; Kondolf, 2006). The 131 132 morphological evolution of these channels is typically modelled through shear stress-fields 133 dependant largely upon topographic-steer driven by the alternation of pool and riffles in 134 135 equilibrium with radii of bend curvature and stream width (De Moor et al., 2007). This perception 136 137 and model is increasingly challenged as initially similar-looking stream-floodplain morphologies may 138 involve a considerable variety of inherited floodplain-building processes. This applies even more so 139 140 when the millennia-long record of human interference has been interwoven into what we might 141 142 perceive as classic river landscapes. 143 144 145 The floodplains of European lowland streams are characteristically of very low relief (1-2 m) and 146 147 typically less than channel depth (1-4 m) as revealed by LIDAR surveys (Mann et al., 2007). The 148 149 principal cause is Holocene overbank sedimentation of sand, silt and clay (Brown and Barber, 1986; 150 Dotterweich, 2008; Pastre et al., 2001; Lespez et al., 2008; Macklin et al., 2010; Broothaerts et al., 151 152 2012; Brown et al., 2013; Macklin et al., 2014). It is often an idealised fluvial ensemble of floodplain 153 154 flats, low or no levées, and sinuous (meandering) stream form to which channels are currently 155 being restored in Europe with the re-engineering of meanders, pools and riffles (Moss and 156 157 Monsadt, 2008). Studies of alluvial floodplains in geological sections suggest that fixed-channel 158 159 anabranching or anastomosing channel forms are associated with fully vegetated floodplains from 160 the Carboniferous Period onwards (Davies and Gibling, 2011). The popularity of the high-sinuosity 161 162 single-channel form may owe something to the cultural perception of the tranquil meandering of 163 164 rivers (form rather than the process, or in ecological terms the structure rather than the function) 165 so commonly depicted in both art and literature – a common European aesthetic of perceived 166 167 naturalness – the serpentine form as exemplified by the English 19th Century landscape painter 168 169 Constable, and others (Kondolf, 2006). In addition, further important goals of river restoration 170 171 concern the desire to increase biodiversity and ecosystem functioning through attaining ‘natural’ 172 and sustainable floodplain landscapes. High levels of uncertainty are commonly attached to river 173 174 restoration outcomes (Darby and Sear, 2008) and as this paper shows in Europe this is due to 175 176 3 177 178 179 180 181 complexity created by inherited elements derived from their Holocene evolution and a much more 182 prolonged and gradual transformation of European rivers in comparison to the abrupt 183 184 transformation of rivers in Australia and the Americas (Brierley et al., 2005). The abrupt New World 185 186 transformations, were in some cases associated with mills and dams (Walter and Merritts, 2008), or 187 large changes in sediment supply (Happ et al., 1940; Trimble, 1981). These changes occurred in all 188 189 climatic zones including the semi-arid zone, where