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Date of Award

Spring 2025

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Mitchell, Robert J. (Geologist)

Second Advisor

Riedel, Jon L. (Geologist)

Third Advisor

Clark, Douglas H., 1961-

Abstract

The Skagit River, its tributaries, and groundwater are important water resources for salmon habitat, agriculture, municipalities, and industries in the lower Skagit River basin. Unfortunately, these resources are threatened by receding glaciers and reduced meltwater due to a warming climate, compounded by increasing groundwater withdrawals driven by development and agricultural practices in the lowlands. Floodplain aquifers have been identified as a source of groundwater for the Skagit River and research has demonstrated that groundwater extracted from the floodplain reduces baseflow to the river which is problematic considering the instream flow rules established in 2001 for the Skagit River. Other potential sources for groundwater in the lower Skagit region are glacial outwash and glacial upland deposits north of the floodplain identified by recent geologic mapping. For water resources management purposes, it is important to characterize the aquifers in the glacial terraces and uplands and determine their connection to the floodplain and the Skagit River.

To characterize the hydrogeologic framework in the lower Skagit Valley, I synthesized well log data, gravel pit and natural stream exposures, recent geomorphic mapping and hydrogeologic studies, borehole data, and LiDAR data and developed two-dimensional (2D) cross sections and 3D conceptual models between Sedro-Woolley and Birdsview, WA. My results reveal a complex geology that differentiates six different hydrogeologic units. Glacial deposits dominate most of the upland aquifers, with lahars and alluvium composing most of the floodplain, with sedimentary and metamorphic rocks at higher elevations. My hydrogeologic framework indicates that glacial outwash deposits in the eastern portion of the study area are connected to and recharging Skagit floodplain aquifers and the Skagit River. The aquifer continuity in the east is also validated by groundwater monitoring data and seepage runs results. There are likely low rates of groundwater flow from the low-conductive units in the uplands to the floodplain deposits in the western half of the study area, meaning that there may be no truly isolated aquifers in the upland area that would serve as an alternative water supply. The hydrogeologic framework serves as a basis for the region, however, more detailed studies will be necessary to inform water resource management decisions going forward in the lower Skagit River Valley, i.e., more extensive well monitoring, long-term pump tests, and groundwater modeling to constrain recharge rates and groundwater flow directions and rates in the study area.

Type

Text

Keywords

Hydrogeology, hydrogeologic framework, conceptual model, glacial deposits, lower Skagit River valley

Publisher

Western Washington University

OCLC Number

1522756304

Subject – LCSH

Hydrogeology--Skagit River Valley (B.C. and Wash.); Hydrogeological modeling--Skagit River Valley (B.C. and Wash.); Glaciology--Skagit River Valley (B.C. and Wash.); Aquifers--Skagit River Valley (B.C. and Wash.); Groundwater--Skagit River Valley (B.C. and Wash.)

Geographic Coverage

Skagit River Valley (B.C. and Wash.)

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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