Future fluvial erosion and sedimentation potential of cohesive sediments in a coastal river reach of SW Finland Eliisa Lotsari, 1 * Juha Aaltonen, 2 Noora Veijalainen, 3 Petteri Alho 1,3 and Jukka Käyhkö 1 1 Department of Geography and Geology, Geography Section, University of Turku, FI-20014 Turku, Finland 2 Freshwater Centre, Finnish Environment Institute, Mechelininkatu 34a, P.O. Box 140, FI-00251, Helsinki, Finland 3 Department of Real Estate, Planning and Geoinformatics, School of Science and Technology, Aalto University, P.O. Box 15800, FI-00076, Espoo, Finland Abstract: This study aims to analyse the combined impacts of future discharges and sea levels on erosion–sedimentation potential, and its seasonal changes, in a ~43-km-long coastal river reach of South-west Finland. To our knowledge, this kind of combined study has not been performed before. In addition to surveying the present erosion–sedimentation conditions, the daily erosion– sedimentation potential is simulated with a one-dimensional hydrodynamic model for the 1971–2000 and 2070–2099 periods by applying four discharge scenarios. Different sea level stages are also employed in the simulations. All scenarios forecast increasing autumn and winter discharges, but diminishing summer discharges. This indicates increasing river channel erosion, particularly during winters and autumns. Although discharge changes have altogether a greater influence on erosion– sedimentation potential, the importance of sea level changes on sedimentation is noticeable in the estuary. The rising sea level scenarios increase the sedimentation potential. In total, by 2070–2099, the erosion potential may increase in most parts of the study area. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS climate change; erosion; fluvial transport; hydrodynamic model; shear stress Received 27 June 2013; Accepted 30 September 2013 INTRODUCTION The projected impacts of climate change on river discharges are non-uniform even within relatively small regions, such as Finland, where hydrological conditions and catchment properties are variable (Andréasson et al., 2004; Veijalainen et al., 2010). As precipitation and temperature regimes change, with subsequent changes in discharge and snowmelt conditions, climate change may also affect the transport of suspended sediments and change the intensity of erosion (Thodsen et al., 2008; Verhaar et al., 2010). Together with hydrological changes, the shoreline shift caused by isostatic land uplift and the global mean sea level rise may affect the fluvial processes on the coastal sections of many rivers in Finland, especially in their estuaries (Ekman, 1996; Ojala and Louekari, 2002; Johansson et al., 2004; Miettinen et al., 2007; Ikonen et al., 2008). According to the Intergovernmental Panel on Climate Change (2007), the range of global sea level rise is 0.18–0.59 m by the year 2100 based on all emission scenarios. This leads to an estimate that many coastal areas of Finland currently experiencing a relative fall in sea level may be subject to a rising sea level and that future change might not be linear (Johansson et al., 2004; HELCOM, 2007). Although global-scale and regional-scale studies on climate change impacts on hydrology, and especially on discharges, by hydrological modelling exist (Andréasson et al., 2004; Beldring et al., 2008; Lawrence et al., 2008; Veijalainen et al., 2010; Sperna Weiland et al., 2012), studies of such impacts on fluvial erosion–sedimentation potential or suspended sediment transport are relatively few (Thodsen et al., 2008; Lotsari et al., 2010). Climate change scenarios based on different global climate models (GCM) and regional climate models (RCM) and emission scenarios exist for Finland (Veijalainen et al., 2010), and the development of different climate change signal transfer methods to hydrological models, such as bias- corrected direct RCM data, has also recently provided possibilities for using more information from the RCMs (Graham et al., 2007; Veijalainen et al., 2012). These direct data sets have not yet been applied in fluvial erosion–sedimentation potential studies in Finland. Even though some simulations of coastal inundation due to sea level changes exist (Purvis et al., 2008), there is a need to study the effects of sea level changes, in combination *Correspondence to: Eliisa Lotsari, Department of Geography and Geology, Geography Section, University of Turku, FI-20014, Turku, Finland. E-mail: eliisa.lotsari@uef.fi HYDROLOGICAL PROCESSES Hydrol. Process. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10080 Copyright © 2013 John Wiley & Sons, Ltd.