First Advisor

Andrés Holz

Term of Graduation

Spring 2023

Date of Publication

7-24-2023

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geography

Department

Geography

Language

English

Subjects

Groundwater flow -- Oregon -- McKenzie River Watershed, Douglas fir -- Growth -- Climatic factors -- Oregon -- McKenzie River Watershed

DOI

10.15760/etd.3619

Physical Description

1 online resource (viii, 102 pages)

Abstract

Widespread mountain snowpack declines have been occurring across the western United States over the last century, and are increasing in duration and severity, with several record-breaking low-snowpack years in the last decade. In the Pacific Northwest, summer streamflow is largely a result of melting snow and groundwater discharge, as summers are typically dry and warm in this Mediterranean bioclimate, thus making spring-fed rivers critical for regional water resources. However, current trends and predictions in hydroclimate modeling indicate spring-fed rivers will experience reduced summer discharge driven by the increasingly larger impact of anthropogenic climate change (warming) on (decreased) mountain snowpack. A prominent spring-fed tributary of the Willamette River (25% of its late summer baseflow), the McKenzie River, serves as an important water source for Oregon's most densely populated and water-use intensive areas. Considering the recent record-breaking snow drought (i.e., 2015) and several low-snow years (2014, 2016, 2018, 2020, 2021) in the last decade, water managers have identified the need for estimates of groundwater transit time for this prominent spring-fed river.

Presented here is a novel approach using newly sampled and developed annual (total) and sub-annual (early- and late-wood) Douglas-fir (Pseudotsuga menziesii) tree-ring chronologies to disentangle multi-century groundwater dynamics, including statistical estimates of transit times (i.e., referred here as the time from when water enters an aquifer to when it exits at springs) at the headwaters of the McKenzie River at Clear Lake, Oregon. In addition, climate-growth relationships were assessed by pairing six Douglas-fir tree-ring chronologies with hydroclimate data in order to examine the influence of a changing climate on forest ecosystems. Model results suggest a groundwater transit time of 5- to 15-years for the McKenzie River, Oregon. Moreover, the Douglas-fir stand's growth patterns are governed by summer temperatures, with negative correlations found with summer temperatures prior to and during the growing season, suggesting low soil moisture hinders growth. Summer radial (latewood) growth is also influenced by Clear Lake's levels throughout the year, with the highest correlations found from February to July, which coincide with warmer spring temperatures and accessibility to snowmelt water. The newly discovered groundwater transit times can aid in improving resource managers' understanding of groundwater dynamics at the headwaters of the McKenzie River and how spring-fed systems can sustain periods of low-flow under a warming climate.

Rights

© 2023 Taylor Nicole Salazar

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Persistent Identifier

https://archives.pdx.edu/ds/psu/40851

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