| Popis: |
The processes and interactions that determine the width of a river channel remain a fundamental area of investigation in geomorphology. An increasing appreciation of the capacity of riparian vegetation to alter fluvial processes, and thus influence channel form, has highlighted the need to include vegetation in these analyses. However, a disconnect exists between the small spatial and temporal scales over which the linkages among flow patterns, sediment, and plants are evaluated and the larger spatial and temporal scales in which river systems operate. In this dissertation, I strove to identify some of the key mechanisms by which vegetation affects channel width. I worked to reconcile the issue of scale by developing a novel tool that resolves patch-scale (sub-meter) patterns of hydraulic roughness over the reach scale. While the approach can be generalized to evaluate any vegetated floodplain, the multi-scalar model was specifically applied to stands dominated by the non-native riparian shrub, tamarisk, that invaded the riparian corridor of southwestern US rivers during the past century. I focused my analyses on the lower Yampa River in western Colorado. Tamarisk colonized the Yampa in the absence of other environmental perturbations. As a result, adjustments to channel form may be linked to an altered vegetation community. From a careful geomorphic and vegetation reconstruction of the Yampa, I determined that tamarisk was the driving force in channel narrowing. Application of the multi-scalar model of vegetation resistance to the Yampa enabled me to reconstruct the changing hydraulic conditions as tamarisk established and the channel narrowed over time. This hydraulic reconstruction furthered our understanding of the interactions among vegetation recruitment patterns, the increased hydraulic resistance, and the changing flow and sediment transport field. Positive feedbacks between vegetation and geomorphic change created additional areas within the channel where tamarisk could establish, and thus accelerated the rate of channel narrowing. However, these feedbacks also changed the importance of common and large floods for vegetation establishment and sediment transport. Application of this process-based understanding to future flow regimes will help managers anticipate locations along the channel that are susceptible to vegetation encroachment and changes to channel width. |
