Product vs. process? The role of geomorphology in wetland characterization

Abstract

Wetland classification has become a primary tool to characterize and inventory wetland landscapes, but wetlands are difficult to classify because they straddle the terrestrial and aquatic boundary and occur in a variety of hydroclimatic and topographic settings. Presently, many ecological wetland classification schemes are focused on the 'hydrogeomorphic' unit, which attempts to account for the physical setting of a wetland. In many cases topographic terms (e.g. flats, slopes) rather than geomorphological terms (e.g. oxbow, floodplain) are used to characterize landforms, and little attempt is made to characterize the processlandform relationships within wetland landscapes. The current misrepresentation of product geomorphology (i.e. topographic rather than landform description) and underrepresentation of process geomorphology (i.e. lacking process-landform relationships) means that many current wetland classification schemes represent an incomplete and static attempt to characterize geomorphologically dynamic wetland landscapes. Here, we use examples from wetlands in the drylands of Africa, Australia, and North America to identify the capacity for adjustment (i.e. form and timescale of adjustment) of wetland landforms and we relate this capacity to the

A Genetic Geomorphic Classification System for Southern African Palustrine Wetlands: Global Implications for the Management of Wetlands in Drylands

“…As a result, wetlands usually occur in areas of flow accumulation concentration, whether from surface flow (channel or surface runoff) or inter-flow (within soil and bedrock), or occasionally at locations of groundwater discharge. In dryland regions (MAP:PET < 0.65; UNEP, 1997), long-term geomorphological processes and fluxes of earth surface materials shape valleys and concentrate surface and near-surface flow accumulation, providing the conditions necessary for wetland formation Tooth et al, 2015;Lidzhegu, 2019;Lisenby et al, 2019).…”

“…In this context, wetlands can be considered geomorphic systems that are dynamic in form as well as in their fluxes of water and all constituent matter (sediment, solutes), and are responsive to natural and anthropogenically-induced changes in water, sediment, and solute supply (Tooth, 2018;Lisenby et al, 2019). Wetland dynamics may respond to a combination of autogenic and allogenic forcing Larkin et al, 2017).…”

“…In this paper, we propose a new genetic geomorphic classification system that acknowledges the role of geomorphology in wetland formation (Tooth et al, 2015), and aims to connect observable geomorphic features in the landscape with the processes that created them (Buffington and Montgomery, 2013;Lisenby et al, 2019). Wetland classification systems are usually based on a selected wetland definition, the most commonly used being that of Cowardin et al (1979), which requires that a wetland meets one or more of the following criteria: (1) it supports hydrophytes at least periodically, (2) the substrate is composed of undrained hydric soil and/or (3), the substrate is non-soil and is saturated or covered by shallow water at some time during the growing season of each year.…”

“…A failure to appropriately conceptualise natural and human induced trajectories of change in wetlands, in tandem with a lack of appreciation of the importance of longitudinal (dis)connectivity, can result in poor outcomes for wetland restoration in dryland environments. For example, the hydrogeomorphic classification scheme currently in use in South Africa, developed by Ewart- Smith et al (2006) and Ollis et al (2015), incorporates very little geomorphological insight beyond consideration of the landscape setting, and emphasises "product" (landscape setting) over "process" (dynamic feedbacks between process and form) geomorphology (Lisenby et al, 2019). The result is that short-term hydrological processes are considered the master variable in wetlands with developmental histories that in many cases extend over several millennia, and have been shaped by a highly complex interplay between a geological template and dynamic flows and fluxes of earth surface material Lisenby et al, 2019).…”

“…For example, the hydrogeomorphic classification scheme currently in use in South Africa, developed by Ewart- Smith et al (2006) and Ollis et al (2015), incorporates very little geomorphological insight beyond consideration of the landscape setting, and emphasises "product" (landscape setting) over "process" (dynamic feedbacks between process and form) geomorphology (Lisenby et al, 2019). The result is that short-term hydrological processes are considered the master variable in wetlands with developmental histories that in many cases extend over several millennia, and have been shaped by a highly complex interplay between a geological template and dynamic flows and fluxes of earth surface material Lisenby et al, 2019). As Simenstad et al (2006) suggest, without recognising landscape dynamics, wetland restoration efforts are unlikely to realise the full ecosystem performance that is sought.…”