Motivation: Making Geomorphic Heterogeneity Assessments
Meaningful
Sustainable river corridor restoration and management integrally depend on the assessment and monitoring of geomorphic processes (Hiers et al., 2016; Palmer et al., 2005). Geomorphic heterogeneity provides a framework to facilitate such assessment. Geomorphic heterogeneity is the spatial and temporal variability of geomorphic units. Geomorphic units are patches of the landscape classified by morphology, substrate, vegetation type, and/or other characteristics relevant to the ecogeomorphic processes that reshape the landscape, namely the fluxes of water, sediment, and wood, all mediated by biological activity. Heterogeneity is both the foundation of and response to many of the physical, chemical, and ecological functions river corridors require and provide (Wohl, 2016).
Interconnected patches defined by geomorphic units can reflect physical, chemical, and biological characteristics across multiple scales depending on analysis objectives. The abundance of data and computational tools for assessing geomorphic heterogeneity make it deceptively easy to link heterogeneity metrics to ecosystem complexity and resilience. Spatial heterogeneity, commonly measured as the diversity of geomorphic units (e.g., Williams et al., 2020; Wohl & Iskin, 2019; Wyrick & Pasternack, 2014), is often measured for its own sake, with the argument that higher diversity in geomorphic units will increase the physical capacity to support biodiversity. Although overall heterogeneity of channels and river corridors can indicate their capacity to support local biodiversity (Amoros & Bornette, 2002), more can be gained from targeted applications of various geomorphic heterogeneity metrics to describe specific processes of interest. Whereas geomorphic applications of heterogeneity have thus far focused primarily on diversity, heterogeneity metrics derived by landscape ecologists can also describe geomorphic unit spatial configuration, and when geomorphic units are mapped through time, their turnover can indicate temporal heterogeneity. This paper discusses how geomorphic practice can benefit from lessons learned from both landscape ecology and the authors’ experience applying these methods.
Generally, it is most feasible to measure river corridor form to infer geomorphic processes because direct measurement of those processes (e.g., sediment flux sampling, repeat channel topographic measurements) tends to be costly. For instance, the abundance of habitat-providing geomorphic units can indicate whether a restoration project has created targeted habitat but does not adequately reflect the processes that could sustain such habitat. A measurement of geomorphic forms that would indicate whether geomorphic processes will sustain habitat would need to directly indicate the magnitude of those processes. Geomorphic processes, or the movement of water, sediment, and wood in conjunction with the actions of biota, create and maintain river corridor heterogeneity (Castro & Thorne, 2019; Collins et al., 2012; Gurnell et al., 2005; Williams et al., 2020; Wohl & Iskin, 2019; Yarnell et al., 2006). As such, geomorphic heterogeneity provides a framework for inferring causal links between form-based geomorphic metrics and geomorphic processes for a given river style (e.g., Constantine et al., 2014; Kasprak et al., 2017).
This conceptual discussion paper presents guidelines and ideas for how to quantify and interpret riverine geomorphic heterogeneity. Heterogeneity has long been recognized as a useful framework for understanding and describing river corridor ecology, (Cooper et al., 1997; Erős & Lowe, 2019; Gustafson, 1998; Poole, 2002; Ward et al., 2002), but has not been developed as thoroughly for use in geomorphic applications. As such, this paper aims to: 1) provide guidance to those interested in using geomorphic heterogeneity as a tool to quantify geomorphic processes and forms and 2) spur further thought and inquiry into the use of heterogeneity as a framework for geomorphic investigations.
We focus primarily on patch-based heterogeneity metrics, or those parameterized by the abundance and spatial configuration of geomorphic unit classes that occupy discrete portions of the landscape, as opposed to those parameterized by continuous metrics (e.g., elevation; McGarigal et al., 2009; Scown et al., 2015). A patch-based framework can be directly applied to assemblages of geomorphic units that have direct relevance to geomorphic processes (Fryirs & Brierley, 2021).