Doctoral Dissertation Research: Multi-Scalar Geomorphic and Vegetative Feedbacks in the Colorado River Basin

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New research using geomorphological models has recently shown that riparian vegetation plays a significant role in the formation and maintenance of single-thread channels. Knowing what geomorphic, hydraulic and hydrologic conditions promote invasive riparian plants is critical to the development of these models. Doctoral student Rebecca Manners, under the guidance of Professor John Schmidt in watershed sciences at Utah State University will examine the interactions between riparian vegetation and geomorphic processes at the field scale. The study design takes advantage of the unique geography and geomorphology of the Colorado River basin and its abundance of published studies and long-term hydrologic and physical datasets. The tractable scale of understanding geomorphic-vegetation interactions is the small scale, where the hydraulic and depositional environments can be described and quantified. To account for the expression of these processes at the larger-scale, this study takes a multi-scalar approach. Four reaches of the Colorado River basin have been chosen to capture the gradient of hydrologic, climatologic, and sedimentologic controls, as well as the difference in timing of tamarisk invasion and hydrologic alteration in the watershed. These reaches are used to describe the interactions between channel form and riparian vegetation to determine the magnitude of channel change caused by changes in physical drivers (hydrology and sediment supply) and riparian vegetation (invasion of tamarisk). Floodplain stratigraphy and 2-D flow modeling of representative geomorphic surfaces within each of the four reaches will be used to describe and quantify how the invasion of non-native riparian plants induces a positive feedback that causes accelerated rates of channel narrowing. The accuracy of flow modeling as a tool in examining vegetation/geomorphic feedbacks will depend on spatially precise characterization of the hydraulic roughness of various plant communities at different life stages. Roughness parameterization will be examined over the life-history of a single species (tamarisk). Our ability to make predictions in geomorphology about channel response to perturbations remains error prone with much uncertainty because of the inability to adequately describe what determines channel shape and size. This is a critical issue in the Colorado River Basin during this period of rapid climate change because this region is sensitive to snow accumulation processes that are the primary source of the basins high water demand. Recognizing that channel changes impact aquatic and riparian species as well as alter the recreational value of the river setting, the National Park Service (NPS) has expressed an interest in identifying where and how channels will change provided altered environmental conditions. Results from this research will not only further geographic science by helping to isolate and identify which environmental variables have contributed to channel changes, but also help inform managers regarding management of the Colorado River basin. Specifically, this research will work towards answering questions regarding which areas of a river system are most sensitive to hydrologic changes and invasion by non-native plant species and when in the life-history of an invasive plant species does it become effective in altering the shape of the channel.