Year of Award

2011

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Andrew Wilcox

Commitee Members

Johnnie Moore, Scott Woods

Keywords

Clark Fork River, fine sediment infiltration, flow dynamics, sediment

Abstract

Pulses of fine sediment in gravel-bedded rivers can cause extensive fine sediment infiltration (FSI), potentially altering river morphodynamics and aquatic ecosystems. FSI occurs when sand and silt are deposited into void spaces between larger grains at the riverbed. Flume and theoretical modeling provide a background for a conceptual model of FSI in natural systems. In this model, FSI will occur to a limited depth as a function of the relative grain size of bed sediment to infiltrating sediment. At a larger scale, fine sediment supply, feed rate and local flow dynamics also dictate the extent of FSI. In 2008, the Milltown Dam near Missoula, MT was removed as part of a Superfund remediation action and released contaminated sediment downstream. I used bulk sampling, freeze cores, infiltration bags and a suspended sediment water bottle to collect samples for metal analysis and comparison with USGS data. The analysis of these metal concentrations indicates that the sediment associated with the dam removal, identified by the highest metal concentrations, is not found in the bed of the field site 14 km downstream. Pore space through the reach was either full when the dam removal sediment pulse fluxed through the field site or the substrate has been reworked. Fine sediment fractions from bulk samples are lowest in riffles, have intermediate values in main channel and complex flow areas and the highest fractions in backwater areas. At depth and across depositional settings, fine sediment has multi-year residence times. My work suggests that because the timing and spatial variability of substrate reworking strongly influence fine sediment content in river beds, understanding of such factors is essential to remediation efforts concerned with fine sediment infiltration.

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© Copyright 2011 Elena Gronli Evans