Doctoral Dissertation Research: A Hierarchical Modeling Approach to Simulating the Geomorphic Response of River Systems to Environmental Change

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This doctoral dissertation research investigates how future environmental change may affect the fluvial geomorphology, or physical form, of rivers. Environmental change, including climate variability, is acknowledged as a factor influencing river flow, particularly in mountainous watersheds, in which snowmelt makes a large contribution to annual discharge. Potential environmental changes in the hydrology of such basins have been simulated by hydrologic models. Most notably, these impacts include higher temperatures leading to an increase in the proportion of winter precipitation falling as rain rather than snow, with consequent increased winter discharge, a lower spring snowmelt peak, and decreased summer discharge. Few of these modeling studies, however, have projected how hydrologic changes associated with climate change may affect geomorphic characteristics such as the width and depth of river channels, the shape of the river course, and the transport of sediment. This is a gap in understanding of climate-change impacts on river systems, because river morphology responds dynamically to hydrology. This research will contribute to better understanding of the geomorphic response of river systems to climate change through development of a hierarchical series of linked models to investigate how climate change influences hydrology, which in turn influences fluvial geomorphology. This modeling framework will be applied to three snowmelt-dominated watersheds in the interior Pacific Northwest, with the following objectives: 1) development of downscaled basin-scale climate change scenarios, which are projections of future changes in climate variables such as temperature and precipitation that are locally specific to the study basins; 2) application of a watershed-scale hydrologic model to project how the study basins' hydrology, including the magnitude and timing of river flow, may change in response to the downscaled climate change scenarios; 3) examination of the impact of the modeled hydrologic changes on the study rivers' morphology, channel form, river shape, and sediment transport, using a reach-scale geomorphic model that can simulate an individual river segment in greater detail than a watershed model; and 4) quantification of past rates of change in river shape using historic aerial photos to compare with the geomorphic modeling results. This hierarchical modeling process is an innovative approach to linking physical processes that occur across multiple scales, from global and regional climate to watershed hydrology to local geomorphology.The results of this research will have implications for the management of river systems. Geomorphic processes significantly affect the value of rivers, including their suitability for threatened and endangered species and for human uses of water. Knowledge of how climate change may affect these processes will allow water resource managers to make more informed decisions about how to respond to future changes. The results will be disseminated at national scientific conferences and in peer-reviewed journals. As a Doctoral Dissertation Research Improvement award, this project will provide support to enable a promising student to establish a strong independent research career.