Quantifying floodplain heterogeneity with field observation, remote sensing, and landscape ecology: Methods and metrics

Abstract

Floodplains provide numerous ecosystem services that depend on the spatial heterogeneity, or patchiness, of the floodplain. Direct and indirect human alterations of rivers can reduce floodplain heterogeneity and function, but relatively little is known of patterns of floodplain heterogeneity in natural, fully functional floodplains. We quantify floodplain heterogeneity at four sites in the United States with the objectives of (i) developing a method of combining field measurements and remote sensing data products to calculate integrative landscape-scale metrics of floodplain spatial heterogeneity and (ii) demonstrating which metrics from landscape ecology are likely to be useful for identifying qualities of natural floodplains, differentiating floodplains, and inferring processes, based on a case study of three prairie floodplains and one beaver-modified floodplain in the continental United States. We developed a new unsupervised classification workflow that combines field data, topography, and Sentinel-2A imagery to create classified floodplains for all four field sites that could be used to calculate heterogeneity metrics.We identified six heterogeneity metrics for characterizing natural floodplain heterogeneity: aggregation index, interspersion and juxtaposition index, largest patch index, patch density, percentage of like adjacencies, and Shannon's evenness index, and these metrics capture both intermetric (variation in spatial heterogeneity between the floodplains) and intrametric variation (variation in the patterns of the metrics). Results show that natural floodplains have high evenness and interspersion and juxtaposition of classes, and we attribute this to natural flow and sediment regimes driving channel migration, erosion, deposition, vegetation succession, and active beaver modifications. Colorado floodplains show higher aggregation and lower fragmentation than the Oklahoma floodplain. We attribute this to the greater incision and lower hydrologic variability at the Oklahoma site.

Beyond the Case Study: Characterizing Natural Floodplain Heterogeneity in the United States

“…Patch were delineated along the floodplain transects on the 3-10 m scale and their boundaries were collected with a Garmin GPSMAP 66ST handheld GPS (±3 m horizontal accuracy). Field classifications for specific sites can be found in Table S1 in Supporting Information S1, Iskin and Wohl (2023b), Table 3, and Iskin (2023), Table 3.1.…”

“…Floodplain boundaries were delineated manually in ArcGIS Pro using field transect data, Sentinel imagery, DEMs, and national park boundaries (where applicable). A 10-m geodesic buffer was added to the floodplains to account for field and/or user error (Iskin & Wohl, 2023b). Additional specifics on floodplain delineation can be found in Text S2 in Supporting Information S1.…”

Experimental Observations of Floodplain Vegetation, Bedforms, and Sediment Transport Interactions in a Meandering Channel

“…Studies have identified correlations between topographic heterogeneity and habitat suitability (Bellmore & Baxter, 2014; Stoffers et al., 2022), increased biodiversity (Morán‐Ordóñez et al., 2015), and attenuation of biogeochemical fluxes (Gooseff et al., 2007; Westbrook et al., 2006). Because of this noted importance of topographic heterogeneity, many tools and metrics have been developed to quantify the distribution of physical elements, organization of patterns, or the variation of shapes of discretely classified patches (Dietrich et al., 2005; Iskin & Wohl, 2023; Nelson et al., 2014; Scott et al., 2022; Scown et al., 2015; Wohl, 2016a). These metrics have been used to inform design goals and measures of river restoration success (Hinshaw et al., 2022; Scott et al., 2022), and the physical processes that drive them have also been investigated.…”

Evaluating river morphology change with a geomorphic form variation approach

“…This may provide a fundamental approach to comparing channel forms of all types. Geomorphic form variety may also support decision-making processes as a tool for river restoration planning and design, particularly when geomorphic complexity enhancement is a design objective and may complement quantitative spatial heterogeneity metrics often applied for ecological research (Iskin & Wohl, 2023). Example project objectives may include better understanding of hydrodynamic processes (Wyrick et al, 2014), modelling ecological patterns (Dilts et al, 2023) or understanding the impacts of stormwater runoff (Walsh et al, 2012).…”