Scale associated coupling between channel morphology and riparian vegetation in a bedrock-controlled stream

Abstract

Most fluvial systems exhibit systematic, continuous upstream to downstream variations in channel morphology and related ecological and hydrological parameters (emphasized by conceptual frameworks such as downstream hydraulic geometry and the river continuum concept), and discontinuous, shorter range variations (emphasized by hierarchical patch dynamics). This study investigates the relative importance of broader-scale upstream to downstream variation and local variation at the hydraulic unit scale in a bedrock-controlled stream in central Kentucky, USA. A nested ANOVA analytical approach was used to determine the relative importance of three nested spatial scales in explaining variations in channel morphology and riparian trees. Results show that channel morphology is largely controlled by local-scale variation explaining about 92% of slope, 46% of bankfull width, 99% of average depth, 54% of width-depth ratio, 86% of channel cross-sectional area, and 100% of the hydraulic radius of the channel. Different categories of substrate characteristics, however, represent anomalies with respect to variance explained at different levels. Furthermore, local-scale controls explain 60% of variations in species richness, 59% of variations in the total number of individual trees, 68% of variation in the proportion of Platanus occidentalis basal area and 43% of variation in the total number of biogeomorphic impacts. These results are consistent with the idea of tight coupling between channel morphology and riparian vegetation, although they do not, by themselves, prove such interactions. The morphological variation of the channel at the local scale is primarily attributable to the geological controls (e.g., faults, bedding planes, joints and fractures) and incision status associated with the study area. The local scale variation in vegetation pattern can be explained by the highly local edaphic differences along the riparian corridor, which is likely to be related to the local scale fluvial process-form variations, and biogeomorphic impacts and feedbacks. These patterns may therefore be common in bedrock rivers strongly influenced by geological controls.