Earth and Planetary Sciences ETDs

Publication Date

7-1-2016

Abstract

Beaver dams effectively trap sediment in stream channels, leading to the hypothesis that persistent beaver damming on millennial timescales causes valley floor aggradation. The available field data, however, are inconclusive. I investigated modern and Holocene beaver-related deposition to understand beaver contributions to fluvial dynamics on multiple timescales in one stream system. Field investigations were conducted on Red Rock and Odell Creeks at Red Rock Lakes National Wildlife Refuge, Centennial Valley, Montana, documenting patterns of sediment storage at 4 stages of beaver damming in the modern channel (1) active; (2) transition from active to breached; (3) breached within the last decade; and (4) undammed. Ground surveys, airborne Lidar, stratigraphic analysis, soil surveys and 64 carbon-14 (14C) ages, were used to investigate beaver impacts from the Early Holocene to present. Upstream of active dams, fine (≤ 2 mm) sediment storage volumes ranged from 48 – 182 m3 with additional storage on the floodplain from dam-induced overbank flows. In-channel persistence of dam-induced sedimentation is limited by frequent breaching (<1 – 5 years). Dam breaching and subsequent downstream transport of willow cuttings from dams and beaver herbivory, however, extend beaver impacts beyond active dam sites, aiding colonization of willow, and adding roughness that promotes additional sedimentation. Major quantities of willow cuttings from beaver herbivory were observed on three streams in southwest Montana. Accumulations of beaver cuttings are also common in Holocene floodplain sediments on Odell Creek, with the majority of beaver-related deposits consistent with beaver-generated willow cutting accumulations on upper point bars and frequent dam breaching. Beaver-pond deposits exist, but rarely. Beaver-related deposition exists through most of the late Holocene when channel activity was dominated by lateral migration. Only ± 2 m of aggradation and incision occurred. The ages of beaver-related deposits overlap the severe droughts of the Medieval Climatic Anomaly, implying persistence of perennial flows in the large, north-facing basins of the studied drainages. Collectively, the modern and Holocene data show that basin attributes play important roles in how beaver influence fluvial systems, and that beaver contribute to both lateral and vertical deposition in the context of larger scale fluvial processes.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Stone, Mark

Second Committee Member

McFadden, Leslie

Third Committee Member

Fawcett, Peter

Project Sponsors

The National Science Foundation, The Earth and Planetary Sciences Department at The University of New Mexico, The Geological Society of America, Red Rock Lakes National Wildlife Refuge, The University of Montana Western and The AMS lab and staff at The University of Arizona.

Language

English

Keywords

beaver, Holocene, meandering river, riparian, sediment storage, willow

Document Type

Dissertation

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