Geomorphic mapping and taxonomy of fluvial landforms

- Despite more topography of rivers available than ever, we see less interpretation and geomorphic mapping of fluvial landforms
- To make consistent and useful geomorphic maps, the taxonomy behind the legend needs to be clear
- We present a taxonomy of geomorphic units, margins and structural elements as a pre-cursor to morphometric analyses

Fluvial geomorphologists use close to a 100 different terms to describe the landforms that make up riverscapes. We identified 68 of these existing terms that describe truly distinctive landforms, in which form is maintained under characteristic conditions and fluvial processes. Clear topographic definitions for these landforms to consistently identify and map them are lacking. With the explosion of continuous, high-resolution topography and digital elevation models, we have plenty of new basemaps in which these landforms are clearly visible, but very few examples of manual or automated classification of fluvial landforms. Fluvial landforms are the building blocks of a river and are variously referred to as geomorphic units, morphological units, habitat units, and channel units. We present a tiered framework for describing geomorphic units, with tier 1 differentiating units on the basis of their stage, tier 2 separating shape (e.g., concave, convex, or planar), tier 3 using particular key attributes to narrow in on the likely specific geomorphic unit type, and tier 4 differentiating those types on the basis of vegetative or roughness modifiers. Information on the assemblage and configuration of geomorphic units can be used to inform process-based interpretations of the range of river behavior. The accuracy and transferability of such analyses is fundamentally tied to the taxonomy we assign to these discrete building blocks. In this paper we clarify the terminology and definitions relating to the identification and delineation of geomorphic units, margins, and structural elements. We establish a set of procedures that can be used to manually map and identify these features. The proposed framework provides a rigorous and repeatable approach to identification of topographically defined features of riverscapes. We demonstrate the application of these systematic yet flexible procedures with a series of maps from rivers in differing valley settings.

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