Defining a Geomorphic Unit
Schema
As river corridors are reshaped by the erosion and deposition of sediment and large wood and the dynamics of biological communities (Castro & Thorne, 2019), they develop a mosaic of patches defined by their landform classification (e.g., pool, bar), substrate (e.g., gravel, bedrock, soil, wood), and/or vegetation characteristics (e.g., canopy height, moisture tolerance, species assemblage, etc.), which we define as geomorphic units. A geomorphic unit schema is a collection of geomorphic units and their definitions specific to a given riverscape and analysis objectives. Geomorphic units can vary in size and definition depending on the forms and processes of interest, for instance, from a patch of fine sediment covering a square meter to an oxbow lake covering a square kilometer. The spatial scale of the geomorphic unit schema should be appropriate for the processes or characteristics of interest and will set the spatial scale over which heterogeneity can feasibly be assessed. When defined by both physical and biological characteristics, geomorphic (or, ecogeomorphic) units can reflect the interplay of geomorphic (e.g., erosion and deposition of sediment) and biotic processes (e.g., vegetation succession, salmon redd building) that define the ecogeomorphology of a river corridor (Johnson et al., 2019).
The definition of geomorphic units, or the geomorphic unit schema, is key to creating targeted geomorphic heterogeneity metrics. A river corridor can be classified by multiple geomorphic unit schemas, depending on the processes that shape the system (Brierley et al., 2010; Brierley & Fryirs, 2000), questions of interest, and the feasibility of measurements. Geomorphic unit schemas have been developed to help standardize definitions of geomorphic unit classes (Belletti et al., 2017; Wheaton et al., 2015; Wyrick et al., 2014), but for the purposes of geomorphic heterogeneity, it is necessary only to develop a geomorphic unit schema that will represent the relevant geomorphic processes at a site (similar to selecting appropriate geoindicators for monitoring; Brierley et al., 2010).
The multi-channel, forested North Fork Teanaway River provides an example of how the same heterogeneity metric can be used with two different geomorphic unit schemas in the same location to address different objectives (Figure 1). A schema focused on the bankfull channel designed to indicate hydraulic heterogeneity (e.g., to reflect hydraulic complexity beneficial to fishes; Matheson et al., 2017; Naman et al., 2020) might consist of chutes, pools, glides, bars, runs, and riffles. High diversity (e.g., Simpson’s diversity index, a measure of how many different units are present and how evenly distributed they are) of these units would likely reflect high hydraulic heterogeneity. Another channel-focused schema designed to indicate the capacity for wood storage might consist more simply of deep channel (pools and chutes lumped together), shallow channel (glides, bars, runs, and riffles all lumped together), and vegetated bars. High diversity of this simpler, three-unit schema would indicate a more even mix of landforms that tend to trap wood and those that tend to transport wood, and thus a higher wood storage capacity (i.e., enough space to get wood to trapping sites balanced with sufficient trapping sites to store that wood; Okitsu et al., 2021; Scott & Wohl, 2018). Both schemas produce diversity values for the channel, and although these values are similar (both close to 88% of maximum diversity), they indicate completely different processes and are not meaningfully comparable.