Year of Award

2019

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Dr. Andrew Wilcox

Commitee Members

Dr. Rebecca Bendick, Dr. Robert Hall

Publisher

University of Montana

Subject Categories

Geomorphology

Abstract

Sediment storage by instream wood in forested mountain streams mediates sediment movement from hillslopes through the channel network and can alter channel morphology at multiple spatial scales. Mixed bedrock-alluvial channels are prevalent in mountain stream networks, yet the distribution and geomorphic impact of large wood within these streams are poorly understood. To estimate how the distribution of large wood in a mixed bedrock-alluvial stream relates to sediment storage, we measured and characterized large wood, and surveyed the volume of associated sediment within a stream in the Bitterroot Mountains, Montana. The upstream portion of the study reach is predominantly alluvial and the downstream portion has significant bedrock exposure along the channel bed and banks. Wood volume and sediment storage in the mixed bedrock-alluvial subreach are 50% and 15%, respectively, of those measured in the alluvial subreach. Most wood is organized into jams, and two channel-spanning jams within the upstream subreach account for 52% and 76% of the reach-averaged wood and sediment volume, respectively. The volume of sediment stored by wood in the full reach is the same order of magnitude as the estimated annual bedload export. Even as wood may significantly alter local hydraulics and transport dynamics, the geomorphic impact and influence of wood on sediment storage may vary substantially by channel type. As previous studies have indicated, the formation and persistence of channel-spanning jams, which store a disproportionate amount of wood and sediment in channel networks, may drive observed differences in wood, sediment storage, and channel morphology.

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© Copyright 2019 Robin Welling