Date of Award:

12-2022

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Watershed Sciences

Committee Chair(s)

John C. Schmidt

Committee

John C. Schmidt

Committee

Peter R. Wilcock

Committee

Colin B. Phillips

Committee

Joseph M. Wheaton

Committee

Tammy M. Rittenour

Abstract

Effective management of in-channel and floodplain habitat requires an ability to forecast river response to changes in water and sediment supply. These changes may result from dam construction/decommissioning, changes in reservoir operations, or changes in grazing or forestry practices. If a change in water and sediment supply causes sediment to be delivered faster than the channel’s capacity to transport it, sediment will accumulate in the reach, leading to changes in channel form and increasing the potential for flooding. A decrease in sediment supply relative to transport capacity can lead to channel incision. The extent and timing of sediment accumulation or evacuation can be moderated, even reversed, by poorly understood changes in the river bed grain size. This dissertation explores the joint response of river bed texture and sediment storage in order to better predict the magnitude of channel change in response to upstream changes in infrastructure or land use in sand bed rivers. We use a combination of field measurements of sediment transport, analysis of channel change from repeat aerial images, and numerical modeling to explore the interaction among sediment supply rate and grain size with bed grain size and morphologic adjustment. We find that sand sizes are sorted throughout the alluvial channel and floodplain such that the in-channel response may be different than floodplain adjustments. At our field site, certain sand sizes evacuate as the bed coarsened while other sand sizes accumulated in the floodplains. These findings indicate that conditions of sediment accumulation or evacuation cannot reveal important changes in the river and size fractional sediment budgets are needed to evaluate channel change. Our numerical modeling indicates that the supply grain size has a strong effect on the grain size of sand on the bed, making the river more or less effective at transporting sediment. Under conditions when the sediment supply increases, such as after wildfires, dam removals, or changes in land use or forestry practices, small changes in the grain size of sand on the channel bed can cause sediment to accumulate, evacuate, or there may be no morphologic adjustment depending on the length of the disturbance and how the supply grain size changes. This has important implications for forecasting downstream impacts. Management concerns may be delayed or even eliminated depending on the bed grain size response. These findings demonstrate the importance of considering the supply grain size and behavior and destination of the fractional sizes in transport when predicting a sand-bed river response to a watershed disturbance.

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