River response to disturbance: Geomorphology and nutrient dynamics following dam removal and urbanization

Martin Wayne Doyle, Purdue University

Abstract

Rivers are dynamic in responding to environmental changes, and predicting responses is important for understanding of fluvial geomorphology and applied river management. Channel adjustments following dam removal and urbanization were examined using field studies and modeling approaches, as were their impacts on local ecology. Channels adjusting to dam removal eroded fine sediment, resulting in deposition up to 3–5 km downstream, although the rates of channel changes were strongly controlled by the size and consolidation of reservoir sediment. Fluvial changes were summarized in a conceptual channel evolution model that highlighted (1) similarities between adjustments associated with dam removal and other events that lower channel base-level, and (2) the role of reservoir sediment characteristics (particle size, cohesion) in controlling the rates and mechanisms of sediment movement and channel adjustment. In addition to the field based studies, two modeling approaches were developed to examine channel response to base-level lowering. Analytical modeling showed that the equilibrium time-scales for sand-bed and gravel-bed channels are comparable, if not the same. Results of simulations run with a new model showed that profile adjustments are more important than channel form adjustments in controlling equilibrium time-scales. Dam removal caused mortality of downstream macroinvertebrates, although these communities recovered within a year of removal. Changes in channel morphology following dam removal also decreased the ability of the reservoir reach to retain phosphorus. Field and modeling results suggested that biotic uptake processes were as important as channel morphology in controlling phosphorus retention in streams. In addition, the following actions were suggested for implementation by agencies faced with dam removal: (1) the development and adoption of a prioritization scheme for what constitutes an important dam removal, and (2) the establishment of minimum levels of analysis prior to decision-making about a dam removal. To study the impact of urbanization, two qualitative and seven quantitative stability measures were tested for distinguishing between stable and degrading sites along urban channels in Indianapolis, Indiana; quantitative measures were generally more successful than qualitative measures. Urbanization was not a distinguishable causal factor in channel instability, although some measures of channel instability were associated with higher levels of urbanization.

Degree

Ph.D.

Advisors

Harbor, Purdue University.

Subject Area

Hydrology|Environmental science|Geography|Biogeochemistry

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