Restoring rivers and floodplains: Hydrology and sediments as drivers of change

Rivers and floodplains have a long history of human perturbation, and they have been significantly impacted by land use intensification and physical degradation. Consequently, on a global scale, these ecosystems rank among those that have seen the greatest loss of biodiversity (Sala et al., 2000; Vörösmarty et al., 2010). Furthermore, it is also recognized worldwide that declines in biodiversity, and the loss of ecosystem services provision, cannot be haltered by protection alone and this have led to a set of international targets including the UN Aichi Biodiversity Target 15 of restoring at least 15% of degraded ecosystems by 2020 (Convention on Biological Diversity, 2010). Rivers are ecosystems with a long tradition for restoration, and in the past more than 3 decades, a substantial number of projects have been undertaken (Feld et al., 2011; Friberg et al., 2016; Ormerod, 2004; Palmer et al., 2005). The outcomes have been mixed in terms of ecosystem improvements (e.g., Kail, Brabec, Poppe, & Januschke, 2015; Palmer, Menninger, & Bernhardt, 2010; Roni, Hanson, & Beechie, 2008), and many projects have not been sufficiently monitored to extract consistent knowledge on geomorphic and biological responses to restoration (Bash & Ryan, 2002; Bernhardt et al., 2005). We therefore must continue improving our understanding of how different types of restoration interventions influence fluvial landscapes and their biodiversity. Process‐based restoration focussing on function rather than form is still relatively rare, as is incorporating floodplains into more holistic restoration designs (Friberg et al., 2016). Impairment of natural flow variability (Poff et al., 1997), land use changes (e.g., Pinter, 2005), and various management infrastructure (e.g., Opperman et al., 2009) have physically degraded floodplain habitats and hampered dynamic hydrologic interactions with rivers and groundwater. Flood pulses and their variable characteristics are drivers of developing support a spatially and temporally heterogeneous and dynamic mosaic of habitats in both the river and its floodplain (e.g., Poff, 2002; Tockner, Malard, & Ward, 2000), and restoring riverine ecosystem functions depends on understanding the flows that produce natural floodplain inundation patterns (Benke, 2001). Equally important to understand the flow of water is the issue of sediments. Sediment transport and storage is driven by flow velocities, sediment mobility, and existing instream supplies (Wohl et al., 2015), and this has vital implications for both a dynamic development of geomorphic features and the in‐ stream biota (e.g., Friberg et al., 2016; Garcia de Jalon et al., 2017). The aim of this special issue is to look at the potential of different aspects of flow and sediments in restoring rivers and floodplains and the ecological responses (Figure 1). It consists of seven papers that