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Accueil du site → Master → Etats Unis → 2012 → Sediment Dynamics in a Dryland River : Grain-Size Variations, Erosion Rates, Sediment Mixing and Dam Effects

University of Montana (2012)

Sediment Dynamics in a Dryland River : Grain-Size Variations, Erosion Rates, Sediment Mixing and Dam Effects

Dekker, Franklin Jacob

Titre : Sediment Dynamics in a Dryland River : Grain-Size Variations, Erosion Rates, Sediment Mixing and Dam Effects

Auteur : Dekker, Franklin Jacob

Université de soutenance : University of Montana

Grade : Master of Science 2012

Résumé
Dryland rivers are fundamentally different from humid rivers because large infrequent floods often set channel form and sediment grain size patterns for long periods of time. The large natural variability in dryland river stream flow causes susceptibility to geomorphic alteration when they are dammed. Cosmogenic nuclide analysis, a tool for determining catchment-wide erosion rates and addressing associated geomorphic questions, has seen few applications to large dryland rivers and dammed rivers. Here, three questions are addressed concerning dam effects on sediment supply and grain size : (1) What is the downstream extent of dam effects on sediment supply, as a function of both supply reductions from the upstream watershed and supply from downstream tributaries ? (2) what is the pattern of grain size in a dryland river, downstream of a dam, and (3) how do valley confinement and tributaries control that pattern ? The Bill Williams River (BWR) in western Arizona is dammed by Alamo Dam and is well suited to addressing these questions. Twenty cosmogenic nuclide samples were collected in the mainstem of the BWR upstream and downstream of the Alamo Dam and from tributaries downstream of the dam. Grain size data were also collected upstream and downstream of Alamo Dam with 70 bulk samples, 18 pebble counts and 411 grain size photos. Cosmogenic nuclide analysis revealed that erosion rates vary considerably in the BWR watershed between the upper catchment (136 t/km^2/yr^1) and tributaries downstream of Alamo Dam (45.5 t/km^2/yr^1). Mixing equations showed that sediments from large dryland catchments were well mixed and nuclide concentrations could determine dam effects. However, nuclide concentrations were small in the mainstem of the BWR, and additional data are needed to adequately determine the downstream extent of dam effects. Grain size was coarsest immediately downstream of Alamo Dam (D50= 41 mm) but fined exponentially downstream. Tributary confluence effects had no statistically significant effect on mainstem grain sizes ; it is possible their ephemeral flow does not yield enough sediment to have grain size effects. Wide floodplain reaches appeared to have a fining effect on grain size but were not a significant predictor of grain size.

Mots clés : •tributary confluence •valley confinement •cosmogenic nuclide analysis •beryllium-10 •Alamo Dam •Bill Williams River •environmental flows

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Page publiée le 1er décembre 2013, mise à jour le 12 juillet 2018