Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Abstract

Hydrographs for selected wells in the Lower Skagit River basin, Washington, are presented in an interactive web-based map to illustrate monthly and seasonal changes in ground-water levels in the study area. Ground-water level data and well information were collected by the U.S. Geological Survey using standard techniques and were stored in the USGS National Water Information System (NWIS), Ground-Water Site-Inventory (GWSI) System.

Introduction

This report is part of a cooperative study by the U.S. Geological Survey (USGS), Skagit County, Washington, and the Washington State Department of Ecology to evaluate the effects of potential ground-water withdrawals and consumptive use on streamflow in four tributary subbasins to the Lower Skagit River. The study also is investigating the possible presence of a ground-water divide between the Lower Skagit River and Puget Sound to identify areas where wells would be subject to regulation under the Skagit River Instream-flow Rule. The measurement of ground-water levels was an important component to develop a better understanding of the ground-water flow system in the Lower Skagit River basin, Washington (fig. 1) and its relation to surface-water resources.

Purpose and Scope

This report presents ground-water hydrographs for 118 wells from two monitoring networks established by the USGS in the Lower Skagit River basin. The first network of 71 wells was established in four tributary subbasins to the Lower Skagit River (fig. 1). The purpose of this tributary network was to provide information on the ground-water flow system and its relation to surface water resources in the study area. Ground-water levels in the tributary network were measured monthly from October 2006 through September 2008. A second network of 47 wells was established in the Skagit River Delta area (fig. 1). The purpose of this network was to investigate the possible presence of a ground-water divide between the Lower Skagit River and Puget Sound. Ground-water levels in the Skagit River Delta network were measured quarterly from August 2007 through May 2008. All ground-water level measurements in both networks were made by the USGS Washington Water Science Center using standard techniques described in Drost (2005), and the data are stored in the USGS National Water Information System (NWIS), Ground-Water Site-Inventory (GWSI) system.

Well-Numbering System

Wells in the study area were assigned a unique well local number that identifies each well within the Public Land Survey rectangular grid system for Washington State (fig. 2). For example, for well number 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 (01) is 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.

Well Locations and Hydrographs

Well locations are plotted on an interactive image of the study area http://wa.water.usgs.gov/projects/skagit/hydrographs.htm. The map image can be controlled using the buttons in the upper-left corner of the image or by clicking with the mouse to pan and double-clicking to zoom. Holding the cursor over a plotted well site opens a message box that displays the local well number. Double-clicking on the well location will display well information and hydrograph. A link is also provided to view the data stored in the GWSI database. A listing of all wells shown on the map is presented on table 1. Two drop-down lists of wells are displayed above the map image representing each well located in the specific network. Clicking on a specific well will also display the well information and hydrograph.

Each well location on the base map is hyperlinked to a hydrograph that illustrates ground-water level changes in the well during the study period. Each hydrograph is divided into a text area containing local well number, land surface altitude, and well depth, and a hydrograph showing the ground-water level. The horizontal axis shows the date water levels were measured or observed and a vertical axis shows the water level in feet below land surface. Water levels that are above land surface are preceded by a minus sign.

Ground-Water Level Changes

Recharge, ground-water withdrawals, and hydrogeologic system characteristics are a few of the many factors influencing ground-water levels. Periods of wet or dry conditions may result in ground-water level rise or decline, respectively. Water levels in a well also may decline in response to water being withdrawn from that well or from a nearby well. The time required for water levels to respond to these and other conditions may vary significantly in different parts of the aquifer system. An improved and more detailed understanding of natural and human factors controlling these water level changes is a primary objective for the larger study of the aquifer system in the Lower Skagit River basin.

Acknowledgments

The authors wish to thank the many well owners, farmers, and irrigators for their cooperation in providing access to wells. The authors also wish to thank Skagit County, Washington, and the Washington State Department of Ecology for providing assistance in the compilation of well information.

Reference Cited

Drost, B.W., 2005, Quality-assurance plan for ground-water activities: U.S. Geological Survey Open-File Report 2005-1126, 27 p.

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