Influences of Hardwood Riparian Vegetation on Stream Channel Geometry in Eastern Forested Environments

Abstract:

Riparian vegetation has been recognized as a controlling factor of stream channel morphology, but specific influences on bed topography and planform geometry are yet to be fully clarified. In temperate environments, hardwood trees serve as prominent bank stabilizers and help create diverse habitats for a variety of aquatic organisms in alluvial channels. This project explores the influence of riparian vegetation on channel geometry in alluvial streams of different sizes. Exposed rootwads increase bank stability and slow channel migration rates, but also cause pool scour that affects thalweg and bedform locations downstream, implying that woody riparian vegetation influences flow conditions and two-dimensional bed geometry in alluvial streams. Field data suggest that the presence of hardwood vegetation modulates channel width, bed topography and planform geometry in low-order streams. In larger channels, rootwads have less influence on planform curvature, but create patchy variations in bed topography that establish thalweg locations and amplify relief of curvature-dominated bedforms. Flume experiments illustrate the genesis of rootwad-induced pool scour and its effect on downstream pool and bar formation. Experimental rootwad pools reflect the relative size and shape of those observed in natural channels. Introduction of riparian obstructions to planar beds also influences thalweg location several channel widths downstream, further supporting the idea of riparian influence on bedform modulation and regulation.