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Date of Award

Fall 2023

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Pfeiffer, Allison

Second Advisor

Amos, Colin B.

Third Advisor

Finnegan, Noah J.

Abstract

Debris flows commonly occur following wildfire in steep landscapes, introducing large volumes of sediment to downstream fluvial systems. Fire-related sediment supply perturbations impact channel morphology, and importantly, fragile aquatic and riparian ecosystems downstream of disturbance. The Big Creek watershed drains 57 km2 of steep chaparral and coast redwood forest along California’s Central Coast. Streams in the Big Creek watershed typically exhibit step-pool/cascade morphology and serve as vital spawning habitat for anadromous Steelhead Trout (Oncorhynchus mykiss). In 2020, 97% of the Big Creek watershed burned in the Dolan Wildfire. In January 2021, an atmospheric river event triggered a series of post-fire debris flows and floods in Big Creek which drastically altered channel morphology.

Here, I characterize morphologic change following post-fire disturbance in Big Creek using pre- and post-fire structure from motion (SfM) and airborne light detection and ranging (lidar) datasets. I use topographic differencing to quantify post-fire topographic change within channels in the Big Creek watershed. I quantify grain size change using pre- and post-fire SfM datasets and couple these data with hydraulic and hydrologic modeling to estimate changes in sediment transport capacity and channel stability following post-fire disturbance in Big Creek. I also perform these analyses for Devil’s Creek: a tributary of Big Creek which burned but did not experience post-fire debris flows within the main channel.

Post-fire debris flows initiated in Cathedral Creek and Brunette Creek, two steep tributaries of Big Creek, which delivered roughly 30,000 m3 of material to Upper Big Creek. By October 2022, approximately 72% of debris flow material exported from Cathedral and Brunette Creek had been evacuated from the channel. Over 90% of the material remaining in Big Creek by 2022 was stored upstream of a valley-spanning log jam emplaced near the confluence of Upper Big Creek and Brunette Creek during post-fire debris flows. Upper Big Creek’s initial response to post-fire disturbance is characterized by substantial fining of the D16 and D50, followed by re-coarsening 2-years post-fire. Conversely, Devil’s Creek exhibits marginal fining 1-year post-fire, followed by little change 2-years post-fire. Changes in grain size distribution are responsible for considerable change in sediment transport capacity and channel stability. In Big Creek, sediment transport capacity increased 1-year post-fire, while channel stability decreased; 2-years post-fire, both sediment transport capacity and channel stability exhibit signs of recovery towards pre-fire conditions. In Devil’s Creek, sediment transport capacity exhibited a substantial increase 1-year post-fire, followed by little change 2-years post-fire; changes in channel stability mirror trends in sediment transport capacity.

My findings indicate that post-fire geomorphic recovery occurs remarkably quickly in Upper Big Creek. Topographic differencing, sediment transport capacity, and channel stability analyses suggest that debris flows play an integral role in reconfiguring the vertical structure of steep, step-pool channels. Post-fire morphologic change has several potential implications for aquatic habitat; the quantitative data presented in this thesis may be valuable for biologists monitoring fish population dynamics in the Big Creek watershed.

Type

Text

Keywords

wildfire, debris flow, fluvial geomorphology, grain size, sediment transport, channel stability, salmonid habitat

Publisher

Western Washington University

OCLC Number

1410859284

Subject – LCSH

Fluvial geomorphology--California--Big Sur Region; Debris avalanches--California--Big Sur Region; Sediment transport--California--Big Sur Region; Watersheds--Effect of fires on--California--Big Sur Region; Watersheds--Effect of sediments on--California--Big Sur Region; Aquatic habitats--Effect of sediments on--California--Big Sur Region; Aquatic habitats--Effect of fires on--California--Big Sur Region; Rainbow trout--Habitat--Effect of sediments on--California--Big Sur Region; Fishes--Habitat--Effect of sediments on--California--Big Sur Region; Salmonidae--Habitat--Effect of sediments on--California--Big Sur Region; Particle size determination; Big Sur Region (Calif.)

Geographic Coverage

Big Sur Region (Calif.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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