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Modeling disturbance and channel evolution in mountain streams Davidson, Sarah

Abstract

Researchers and managers have sought for centuries to model the dynamics of river systems for hazard protection, water management, and ecological restoration. Models of channel dynamics generally assume that rivers adopt a constant geometry in response to a set of relatively static governing parameters. In this research, we develop two stochastic biogeomorphic models which we use to simulate the range of channel conditions associated with fluctuating governing conditions, including wood loading and discharge. We begin by developing a version of the Reach Scale Channel Simulator (RSCS) that models the impact of riparian disturbance on channel morphology at a range of channel scales, in a reach subject to an annual flood event of constant magnitude and duration. The simulations show that small- to intermediate-sized channels are the most morphologically sensitive to fluctuations in wood loading. We then develop a STochastic CHannel Adjustment SIMulator (STOCHASIM) that simulates the competition between bank erosion and vegetation colonization in a reach subject to variable annual floods. The model produces a dynamic channel geometry that adjusts in response to individual floods. The results challenge a major underlying assumption of most regime models by demonstrating that the return period of the formative flow varies with watershed hydrology. Introducing variable floods and lateral migration has important implications for wood loading, as bank erosion increases wood recruitment and changes piece characteristics. In the final chapters, we use data from a series of flume experiments to investigate the effects of piece characteristics on wood stability and transport. Rootwads -- which are more common on wood pieces recruited through bank erosion than via toppling -- increase piece stability while reducing travel distance. We use this research to further modify the RSCS model to account for wood inputs through bank erosion, as well as temporal changes in channel geometry and flood magnitude. When lateral mobility is considered, bank erosion inputs dominate wood loading while piece stability and morphologic impact decreases. As these stochastic models produce a range of channel conditions they are more likely to encompass the range of variability observed in natural systems than deterministic models of channel dynamics.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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