Figure 2: Two hypothetical
landscapes with identical evenness (50 blue squares, 50 green squares),
but very different spatial configuration.
There are many ways to describe spatial configuration, ranging from metrics of patch type interspersion to metrics of patch shape or contrast between patches (McGarigal, 2012). Many spatial configuration metrics correlate (Frazier & Kedron, 2017), meaning that an investigator must choose between multiple potentially suitable metrics for a given application(for more details, see literature from landscape ecology, e.g., Gustafson, 2019; McGarigal, 2012). The aspects of spatial configuration that tend to be geomorphically relevant include subdivision, or the degree to which the riverscape is broken up (e.g., by multiple channels or patches of vegetation; Figure 3A, B), geomorphic unit edge density (e.g., bank complexity; Figure 3C, D), and aggregation or dispersion, or the degree to which specific geomorphic units are clumped together versus spread out across the river corridor (e.g., wood stored as individual pieces versus large jams; Figure 3E, F). Figure 3 demonstrates high versus low values of each of these types of spatial configuration with multiple metrics for each. Many metrics are redundant, as Figure 3 demonstrates, so it is important to choose one that is well-suited for site conditions, the question of interest, and available data.
Spatial configuration can be measured at the patch (e.g., the shape of a single patch, like a pool), class (e.g., the isolation of a particular geomorphic unit type, like all pools), or landscape level (e.g., the subdivision of multiple geomorphic unit types, like all in-channel units) (McGarigal, 2012). However, most geomorphic applications will likely focus on the class or landscape level.
Class level spatial configuration metrics are useful when individual geomorphic unit classes indicate important characteristics. For instance, the patch density of vegetated islands (i.e., the number of islands per unit area) is a suitable measurement of channel node or bifurcation density. The edge density (i.e., edge length per unit area or unit centerline length) of channels can indicate bank planform roughness (Figure 3C, D). The aggregation (or isolation) of wood can indicate patterns of wood transport and deposition, such as jam formation versus deposition of dispersed pieces (Figure 3E, F).