1. Introduction
Hydrologic linkages of matter and energy within landscapes are important regulators of physical, biogeochemical (Covino, 2017) and biological processes (Amoros & Bornette, 2002). These linkages, defined as hydrologic connectivity, are a fundamental landscape property that connect multiple landscape components (e.g., uplands, streams, floodplains, hyporheic zones, groundwater). Hydrologic connectivity emerges from complex interactions of topographic, climatic, geologic, biotic, and anthropogenic controls (Leibowitz et al., 2018). Recently, hydrologic connectivity has gained popularity as a conceptual and quantitative framework because it enables description of emergent patterns in hydrologic function without requiring the full quantification of underlying processes and controls (Wohl et al., 2019).
River-floodplain systems are distinct landscapes formed by interactions between rivers and adjacent landforms that support important ecologic and hydrologic services (Opperman et al., 2010). Surficial flow and flood pulses from rivers to their floodplains (Junk et al., 1989; Tockner et al., 2000) as well as subsurface hyporheic exchange shape the geomorphic structure of both the river and the floodplain (Stanford & Ward, 1993). This generates spatially heterogeneous landscapes with a mosaic of habitats each with distinct hydrologic regimes (Poole, 2002). Return flows from floodplains to rivers partially regulate downstream physio-chemistry including fluxes and concentrations of organic matter, sediment, nutrients and heavy metals (Bellmore & Baxter, 2014; Briggs et al., 2019; Tockner et al., 2002; Wohl et al., 2017). Despite their benefits, in many systems, hydrologic connections in river-floodplains have been altered due to water management of river systems and cultivation and development within floodplain areas (Tockner & Stanford, 2002). As efforts have grown to restore connections between rivers and their floodplains, so has the need to effectively characterize hydrologic dynamics, which remains challenging given the spatial and temporal heterogeneity within these systems (Roni et al., 2019).
Measuring hydrologic connectivity in river-floodplain systems can be useful because it describes emergent hydrologic behavior without fully quantifying underlying interactions. In river-floodplain systems, hydrologic connections operate simultaneously across multiple dimensions: vertical (surface-groundwater), lateral (river-floodplain & floodplain-hillslope), longitudinal (upstream to downstream), and temporal (J. V. Ward, 1989). The magnitude and directionality of connectivity can vary depending on the spatial and temporal scale being considered (Covino, 2017). Equally as important as identifying when landscape features are connected by surface and sub-surface pathways is identifying when they are not connected, known as disconnectivity or isolation, which at landscape scales plays a critical role in a suite of important hydrologic and biogeochemical processes (Cohen et al., 2016; Rains et al., 2016), water chemistry (Cheng & Basu, 2017), and in the maintenance of habitat complexity and biodiversity (Amoros & Bornette, 2002).