Introduction

In Washington State, the availability of water for out-of-stream uses must be determined before water can be appropriated. This determination is most often made as part of an application for a water right; however, certain uses are exempted from the water rights permitting system. To prevent water withdrawals from affecting other out-of-stream and instream uses, Washington State may reserve a specific quantity of water in a stream basin for out-of-stream uses as part of the regulation establishing minimum instream flows (the Instream-Flow Rule). The reservation allows for new groundwater withdrawals in basins where all available water is appropriate. Once the total of new withdrawals equals the quantity specified in the reservation, subsequent new uses would have to find an alternative source of water, obtain an existing water right, or provide compensating mitigation for affected streamflow.

Recent population growth along the Interstate 5 corridor near Mount Vernon, Washington, has led to increased water use, with many new domestic wells serving residents in the lower part of the Skagit River basin in areas not served by a regional public water system. Planning for future development in the lower basin, including the reservation of water for new domestic wells, requires identification of areas where withdrawals from existing and new wells could adversely affect streamflow in the Skagit River or its tributaries. Skagit County, as the land use authority for unincorporated areas without access to public water systems, requires a scientifically credible basis for implementing land use restrictions to protect instream resources.

In June 2006, the U.S. Geological Survey (USGS) in cooperation with Skagit County, the Washington State Department of Ecology, and Skagit County Public Utility District No. 1, began a project to characterize shallow groundwater movement in an area between the lower Skagit River and Puget Sound, referred to as the “Skagit River Delta” in this report (fig. 1), and to investigate the possible presence of a groundwater divide to assist in the identification of areas where wells could have an adverse affect on flow in the Skagit River, and therefore would be subject to regulation under the Skagit River Instream-Flow Rule.

Purpose and Scope

This report presents information used to characterize shallow groundwater movement in the part of the Skagit River Delta north of the North Fork distributary. The report includes seasonal water-level maps, a linear-regression analysis of water levels, and descriptions of groundwater-flow direction, gradient, and seasonal fluctuations. Groundwater level and Skagit River stage data were collected by the USGS quarterly from August 2007 through May 2008.

Description of Study Area

The study area covers about 42 mi² along the lower Skagit River before it enters Skagit Bay in southwest Skagit County, Washington (fig. 1). The area is bounded by the Skagit River to the east and south; an approximate watershed divide between the Samish and Skagit Rivers to the north; and Padilla Bay, Swinomish Channel, and Skagit Bay (all parts of Puget Sound) to the west. The Skagit River occupies a large, relatively flat alluvial valley that primarily is underlain by fluvial and deltaic sand and gravel deposits associated with the present and ancient Skagit River, and locally preserved lahar-runout deposits originating from Glacier Peak (located outside the study area to the southeast). Local upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine depositional environments. Bedrock (consisting of a complex assemblage of metamorphic rocks, sedimentary units, and igneous rocks) underlies the alluvial valley and upland areas. Surficial exposures of bedrock are limited to a few upland outcrops in the southwestern part of the study area.

Land-surface altitude in the study area ranges from near sea level adjacent to the Swinomish Channel to about 130 ft in upland areas. The study area has a temperate marine climate with warm, dry summers and cool, wet winters. Temperatures are moderated by the Pacific Ocean and Puget Sound, and these bodies of water provide an abundant supply of moisture for storms that typically approach the area from the west. Mean annual (1971–2000) precipitation at Mount Vernon is 32.7 in. (National Oceanic and Atmospheric Administration, 2007). Summers (June–August) typically are dry with a mean precipitation of 4.5 in. at Mount Vernon. Winters (December–February) are wetter than summers with a mean precipitation of 11.0 in. at Mount Vernon. The mean monthly temperature at Mount Vernon ranges from about 40°F in January to about 63°F in August (National Oceanic and Atmospheric Administration, 2007).

Geologic Setting

A brief summary of major geologic events in the study area is described below, and is based on the work of Hansen and Mackin (1949), Easterbrook (1969), Marcus (1981), Johnson (1982), Tabor (1994), Booth (1994), Dragovich and Grisamer (1998), Dragovich and others (2002), and Dragovich and DeOme (2006). The geology of the study area records a complex history of accretion along the continental margin, mountain building, deposition of terrestrial and marine sediments, igneous intrusion, and the repeated advance and retreat of continental glaciers. Bedrock in the study area consists of low-grade metamorphic rocks, formed during Late Jurassic or Early Cretaceous continental margin subduction, and sedimentary units deposited in alluvial fan, braided stream, and near shore shallow marine settings.

Continental glaciers advanced into Skagit County several times during the Pleistocene Epoch. This ice is part of the Cordilleran ice sheet, and is known as the Puget Lobe. The most recent period of glaciation, the Vashon Stade of the Fraser glaciation, began about 17,000 years ago when the continental ice sheet in Canada expanded and the Puget Lobe advanced southward into western Skagit County, and eventually covered the entire Puget Sound Basin before halting and retreating. During the Everson Interstade, beginning about 13,500 years ago, the climate warmed and the lobe wasted back allowing marine waters to enter the Puget Sound Basin, which had been depressed due to glacial isostatic loading. Marine inundation buoyed the retreating ice and produced marine and estuarine conditions in the study area. Postglacial filling of the Skagit River valley, which had been excavated by subglacial meltwater, was accomplished through Holocene fluvial, estuarine, and deltaic deposition and volcanic-lahar deposits originating from Glacier Peak.

Well-Numbering System

Wells in the study area were assigned a local number that identifies each well within the Public Land Survey rectangular grid system for Washington State (fig. 2). For example, for well 34N/03E‑09L01, the number and letter preceding the slash (34N) indicates the township north of the Willamette Base Line. The number and letter between the slash and the hyphen (03E) indicates the range east of the Willamette Meridian. The number following the hyphen (09) indicates the section number within the township. The letter following the section number (L) indicates the 40-acre quarter-quarter tract within the section. The number following the quarter-quarter tract (01) is a sequence number used to distinguish individual wells in the same quarter-quarter tract. A “D” following the sequence number indicates a well that has been deepened.