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).