Award Date

5-1-2021

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geoscience

First Committee Member

Bethany Coulthard

Second Committee Member

Matthew Lachniet

Third Committee Member

David Kreamer

Fourth Committee Member

Scott Abella

Number of Pages

77

Abstract

Spring freshets and summer droughts have recently worsened in the Fraser River Basin, British Columbia, Canada, with significant impacts to the keystone Pacific salmon populations, the food and economic sovereignty of over eighty First Nations, and the western Canadian economy. These extreme events present a potential risk since, unlike many large and less hydroclimatically-complex and/or empounded watersheds, the Fraser River Basin is susceptible to a combination of unregulated spring freshet and summer drought events even within the same year. A major limitation for understanding past and future extreme event risk in the Fraser River Basin is that observational streamflow datasets are both short in duration and potentially forced by anthropogenic warming. They therefore provide a potentially incomplete record of natural hydrological variability and inaccurate benchmarks of long-term natural runoff extremes. While longer-term, highly-resolved (annual), tree ring (TR) based paleohydrological reconstructions are increasingly being used worldwide by water managers and stakeholders to extend short observational streamflow records, this approach is difficult in complex temperate watersheds like the Fraser. For this study I developed the first multi-century, sub-annual resolution (seasonal), paired freshet and drought reconstructions within a single watershed. I targeted the Upper Fraser Basin since it represents the headwaters and primary runoff source of the greater Fraser Basin. By focusing on sub-annual streamflow seasons, I was able to both independently reconstruct seasonal extreme flow events, and also overcome methodological limitations that precluded prior attempts to reconstruct total water-year runoff in this watershed. Newly developed and existing TR chronologies from multiple species were used as proxies for seasonal temperature and cool-season precipitation which are, in turn, drivers of streamflow in each reconstruction season. I analyzed the magnitudes, durations, and statistical probabilities of high freshets and droughts over the past 140 years. My results suggest that the instrumental records do not accurately reflect the full variability of high freshet events or drought events as both duration and magnitudes of past events are higher than any during the observed time period. There is also a change in the frequency of high freshet events towards more frequent occurrences since 1950 AD. The new extreme event reconstructions presented here provide paleoenvironmental benchmarks that can be used by water managers and stakeholders to significantly change and improve water management-relevant statistical analyses such as frequency analysis and return periods calculation, and adapt to future freshets and droughts on the Fraser River under climate change.

Keywords

British Columbia; Dendrohydrology; Drought; Freshet; Streamflow; Tree Rings

Disciplines

Environmental Sciences | Hydrology

File Format

pdf

File Size

1630 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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