Tasnuba Jerin

Importance of Local Scale in Fluvial Biogeomorphic Landscape Evolution:

Examples from a Bedrock Fluvial System

Fluvial biogeomorphic patterns and processes are interlinked and typically scale dependent. Thus, selection of appropriate spatial scale has substantial impacts on result interpretation. This presentation will illustrate three example studies from a limestone bedrock-controlled fluvial system, Shawnee Run, located in Central Kentucky USA, and examine the importance of local scale in fluvial (bio-) geomorphic landscape evolution.

(i) The first study is exemplified by the case of Icicle Bend on Shawnee Run; this bend represents abandonment of a steeper, straighter, more efficient channel at the reach scale in favor of a longer, currently less steep and less efficient flow path. This apparently occurred owing to capture of Shawnee Run flow by a subsurface karst flow path that was subsequently exhumed. Thus, the development of Icicle Bend illustrates the “local” nature of efficiency in geomorphic evolution.

(ii) The second study explores the concept of “biogeomorphic keystone species” by examining the locally developed pools linked to biogeomorphic impacts in Shawnee Run. This research suggests that American sycamore (Platanus occidentalis) plays a keystone role by promoting development of ~42% of pools of the study area. While geomorphic pools are formed by fluvial process-form linkages, these biogeomorphic pools are developed by sycamore root induced channel bed bioweathering. These local scale alterations by American sycamore, thus, result in fundamentally different stream morphology, and manifest the importance of process-form linkage at local scale in fluvial biogeomorphology.

(iii) The third study investigates the relative importance of broader scale up-to downstream variation and local variation at the hydraulic unit scale in Shawnee Run. Results show that channel morphology and vegetation patterns are largely controlled by local-scale, primarily attributable to the geological controls (e.g. faults, bedding planes, joints and fractures) and incision status associated with the study area. Thus, this research depicts divergence from the classical downstream hydraulic geometry theory and the linked river continuum concepts, and underscores the importance of fine-scale functional units in fluvial system evolution.

These examples thus suggest that fluvial (bio-)geomorphic systems are characterized by complexity, caused by multi-causality and variable process-form linkages at local scales. Thus, accounting for local scale variations is substantial for fluvial landscape interpretation.

KEYWORDS: Biogeomorphic Impacts, Bedrock Streams, Local Scale