Hydraulic drivers of populations, communities and ecosystem processes

Abstract

The combination of climate change and increasing development of land and water resources is imposing pressure on aquatic ecosystems worldwide (Poff et al. 2007; Blann et al. 2009; Arthington et al. 2010; Best 2019; Boretti and Rosa 2019; Reid et al. 2019). Many drivers of fluvial processes are changing today, and many of these changes are expected to accelerate in the near future. Spatial patterns and timing of precipitation are changing globally, thereby shifting water inputs into freshwater systems and potentially producing more floods and droughts via intensification of the hydrological cycle, increasing frequency of extreme events, and increasing duration of dry seasons (Davis et al. 2015; Madakumbura et al. 2019; Koutsoyiannis 2020). Under increasing pressure from water insecurity, both governmental agencies and private landowners are increasing the abstraction of sourcewaters for human use, thereby moving and storing greater amounts of water throughout fluvial systems (Jaramillo and Destouni 2015; Rodell et al. 2018; Best 2019; Boretti and Rosa 2019). Beyond the well-established effects of dams fragmenting river ecosystems, increased damming of headwaters and large-scale water diversions affect downstream river ecosystems by dewatering rivers, shifting patterns of sediment deposition and aggradation, and reducing habitat heterogeneity (Veldkamp et al. 2017; Sabater et al. 2018; Best 2019). Ongoing land development and industrial agricultural practices are also accelerating soil erosion and export of nutrients from the terrestrial landscape to the aquatic environment (Blann et al. 2009; Seitzinger et al. 2010; Borrelli et al. 2017). Globally, the cumulative effects of these changes are altering continental balances of water (increasing evaporation from the continents to the atmosphere) (Jaramillo and Destouni 2015; Rodell et al. 2018; Zhan et al. 2019), eroding and exporting large amounts of soils and sediments (Borrelli et al. 2017; Best 2019), and greatly increasing the delivery of nutrients from the continents to the oceans (Seitzinger et al. 2010; Beusen et al. 2016; Sinha et al. 2017). Increased information on links between watershed management, river flow, river hydraulics and habitats, and ecosystems is needed to ensure the sustainability of water resources and maintain the integrity of aquatic ecosystems. The knowledge needed to effectively protect and restore river ecosystems has proven difficult to obtain and translate into practice for river management and hydraulic engineering. While many fluvial processes have been studied individually, it is extremely difficult to predict the long-term consequences of simultaneous changes in climate, land use, and river management on aquatic ecosystems. Consequently, there is considerable uncertainty in the long-term outcomes of key processes that structure river ecosystems, such as changing river flow conditions; inputs of sediments, nutrients, and terrestrial organic matter; and the spatiotemporal distribution of connectivity between rivers and fringing habitats including hyporheic, riparian, and floodplain environments. Increased recognition of these ecological challenges and the potential for even greater modification of fluvial systems in the future has driven greater interest in conservation, restoration, and resilience measures to protect the biodiversity, ecological functioning, and societal benefits of fluvial systems. Individual river conservation and restoration efforts have a wide range of objectives. Consequently, distinct solutions have been proposed for biodiversity conservation, stormwater retention, seasonal water storage, and nutrient management (Nienhuis and Leuven 2001; Angelopoulos et al. 2017; Roy et al. 2018; Weber et al. 2018). As with the complexity of understanding current challenges to aquatic ecosystems, it is difficult to ascertain the long-term outcome of multiple conservation and restoration efforts within an individual aquatic system (Friberg et al. 2016; Lorenz et al. 2018), and little progress has been made in synthesizing information on key environmental drivers of ecological responses into holistic measures for river management (Palmer and Ruhi 2019; Roni 2019).