Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

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Prepared in cooperation with the Skagit County Public Works Department, Washington State Department of Ecology, and Skagit County Public Utility District No. 1

Scientific Investigations Report 2009–5208

U.S. Department of the Interior U.S. Geological Survey Cover: Photograph of the Skagit River delat area, Skagit County, Washington. Image ©2009 Digital Globe ©2007Google™. Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

By Mark E. Savoca, Kenneth H. Johnson, and Elisabeth T. Fasser

Prepared in cooperation with the Skagit County Public Works Department, Washington State Department of Ecology, and Skagit County Public Utility District No. 1

Scientific Investigations Report 2009–5208

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation: Savoca, M.E., Johnson, K.H., and Fasser, E.T., 2009, Shallow groundwater movement in the Skagit River Delta Area, Skagit County, Washington: U.S. Geological Survey Scientific Investigations Report 2009–5208, 22 p. iii

Contents

Abstract...... 1 Introduction...... 1 Purpose and Scope ...... 2 Description of Study Area...... 2 Geologic Setting...... 2 Well-Numbering System...... 4 Methods of Investigation...... 4 Well Inventory and Water-Level Measurements...... 4 River Stage...... 6 Groundwater-Level and Flow-Direction Maps...... 7 Ocean Tides...... 8 Hydrogeology...... 8 Groundwater Movement...... 10 Groundwater-Flow Directions...... 10 Groundwater-Level Fluctuations...... 17 Summary and Conclusions...... 20 Acknowledgments...... 20 Selected References...... 21

Figures

Figure 1. Map showing location of the Skagit River Delta study area, Skagit County, Washington… ……………………………………………………………………… 3 Figure 2. Diagram showing well numbering system used in Washington… ………………… 4 Figure 3. Map showing location of wells and U.S. Geological Survey streamflow-gaging station, Skagit River Delta area, Washington… …………………………………… 5 Figure 4. Simplified surficial geologic map of the Skagit River Delta area, Washington… …… 9 Figure 5. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, August 2007… ………………………………………………………… 11 Figure 6. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, November 2007………………………………………………………… 12 Figure 7. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, February 2008… ……………………………………………………… 13 Figure 8. Map showing water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, May 2008… …………………………………………………………… 14 Figure 9. Map showing regression analysis derived annual water-level altitude and calculated groundwater-flow direction in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, 2007 and 2008… …………………… 16 Figure 10. Hydrographs of water levels in well 34N/03E-28M02 and precipitation at Anacortes, Washington, October 2007 through May 2008…………………………… 17 iv

Figures—Continued

Figure 11. Map showing wells that likely are influenced by stage on the Skagit River, Washington… ……………………………………………………………………… 18 Figure 12. Hydrographs of water levels in well 34N/03E-23B01 and stream stage at U.S. Geological Survey streamflow-gaging station 12200500, Skagit River near Mount Vernon, Washington, October 2007 through May 2008… ……………… 19 Figure 13. Hydrographs of water levels in well 34N/03E-19F01 and nearby tidal fluctuations near La Conner, Washington, October 2007… ………………………… 19

Tables

Table 1. Hydrogeologic characteristics of simplified geologic units used in this study and correlation with previous unit designations… ………………………………… 10 Table 2. Fitted regression coefficients and calculated hydrogeologic parameters for wells in the Skagit River Delta area, Washington, 2007 and 2008… ………………… 15

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain Length foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area acre 0.4047 hectare (ha) acre 0.004047 square kilometer (km2) section (640 acres or 1 mi2) 259.0 square hectometer (hm2) square foot (ft2) 0.0929 square meter (m2) square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Hydraulic gradient foot per foot (ft/ft) 1 meter per meter (m/m) foot per mile (ft/mi) 0.1984 meter per kilometer (m/km)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8

Datums Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88), referred to in this report as “sea level.” Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above or below sea level. Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

By Mark E. Savoca, Kenneth H. Johnson, and Elisabeth T. Fasser

Abstract April, and did not seem to begin to decline until the end of May in response to declining river stage. Groundwater levels in a well equipped with a continuous water-level recorder Shallow groundwater movement in an area between exhibited periodic fluctuations that are characteristic of ocean the lower Skagit River and Puget Sound was characterized tides. This well is less than 1 mile east of the tidally influenced by the U.S. Geological Survey to assist Skagit County Swinomish Channel, and exhibited water-level fluctuations and the Washington State Department of Ecology with the that correspond closely to predicted tidal extremes obtained identification of areas where water withdrawals from existing from a tide gage near La Conner, Washington. and new wells could adversely affect streamflow in the Skagit River. The shallow groundwater system consists of alluvial, lahar runout, and recessional outwash deposits composed of sand, gravel, and cobbles, with minor lenses of silt and Introduction clay. Upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine In Washington State, the availability of water for out- depositional environments. Bedrock exposures are limited to a of-stream uses must be determined before water can be few upland outcrops in the southwestern part of the study area, appropriated. This determination is most often made as part and consist of metamorphic, sedimentary, and igneous rocks. of an application for a water right; however, certain uses Water levels were measured in 47 wells on a quarterly are exempted from the water rights permitting system. To basis (August 2007, November 2007, February 2008, and prevent water withdrawals from affecting other out-of-stream May 2008). Measurements from 34 wells completed in and instream uses, Washington State may reserve a specific the shallow groundwater system were used to construct quantity of water in a stream basin for out-of-stream uses as groundwater-level and flow-direction maps and perform part of the regulation establishing minimum instream flows a linear-regression analysis to estimate the overall, time (the Instream-Flow Rule). The reservation allows for new averaged shallow groundwater-flow direction and gradient. groundwater withdrawals in basins where all available water Groundwater flow in the shallow groundwater system is appropriate. Once the total of new withdrawals equals the generally moves in a southwestward direction away from the quantity specified in the reservation, subsequent new uses Skagit River and toward the Swinomish Channel and Skagit would have to find an alternative source of water, obtain an Bay. Local groundwater flow towards the river was inferred existing water right, or provide compensating mitigation for during February 2008 in areas west and southwest of Mount affected streamflow. Vernon. Water-level altitudes varied seasonally, however, Recent population growth along the Interstate 5 corridor and generally ranged from less than 3 feet (August 2007) in near Mount Vernon, Washington, has led to increased water the west to about 15 feet (May 2008) in the east. The time- use, with many new domestic wells serving residents in the averaged, shallow groundwater-flow direction derived from lower part of the Skagit River basin in areas not served by a regression analysis, 8.5° south of west, was similar to flow regional public water system. Planning for future development directions depicted on the quarterly water-level maps. in the lower basin, including the reservation of water for Seasonal changes in groundwater levels in most wells new domestic wells, requires identification of areas where in the Skagit River Delta follow a typical pattern for shallow withdrawals from existing and new wells could adversely wells in western Washington. Water levels rise from October affect streamflow in the Skagit River or its tributaries. through March, when precipitation is high, and decline Skagit County, as the land use authority for unincorporated from April through September, when precipitation is lower. areas without access to public water systems, requires a Groundwater levels in wells along the eastern margin of the scientifically credible basis for implementing land use study area also are likely influenced by stage on the Skagit restrictions to protect instream resources. River. Water levels in these wells remained elevated through 2 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

In June 2006, the U.S. Geological Survey (USGS) are moderated by the Pacific Ocean and Puget Sound, and in cooperation with Skagit County, the Washington State these bodies of water provide an abundant supply of moisture Department of Ecology, and Skagit County Public Utility for storms that typically approach the area from the west. District No. 1, began a project to characterize shallow Mean annual (1971–2000) precipitation at Mount Vernon is groundwater movement in an area between the lower Skagit 32.7 in. (National Oceanic and Atmospheric Administration, River and Puget Sound, referred to as the “Skagit River Delta” 2007). Summers (June–August) typically are dry with a mean in this report (fig. 1), and to investigate the possible presence precipitation of 4.5 in. at Mount Vernon. Winters (December– of a groundwater divide to assist in the identification of areas February) are wetter than summers with a mean precipitation where wells could have an adverse affect on flow in the Skagit of 11.0 in. at Mount Vernon. The mean monthly temperature River, and therefore would be subject to regulation under the at Mount Vernon ranges from about 40°F in January to Skagit River Instream-Flow Rule. about 63°F in August (National Oceanic and Atmospheric Administration, 2007). Purpose and Scope Geologic Setting This report presents information used to characterize shallow groundwater movement in the part of the Skagit River A brief summary of major geologic events in the study Delta north of the North Fork distributary. The report includes area is described below, and is based on the work of Hansen seasonal water-level maps, a linear-regression analysis of and Mackin (1949), Easterbrook (1969), Marcus (1981), water levels, and descriptions of groundwater-flow direction, Johnson (1982), Tabor (1994), Booth (1994), Dragovich and gradient, and seasonal fluctuations. Groundwater level and Grisamer (1998), Dragovich and others (2002), and Dragovich Skagit River stage data were collected by the USGS quarterly and DeOme (2006). The geology of the study area records a from August 2007 through May 2008. complex history of accretion along the continental margin, mountain building, deposition of terrestrial and marine sediments, igneous intrusion, and the repeated advance and Description of Study Area retreat of continental glaciers. Bedrock in the study area consists of low-grade metamorphic rocks, formed during Late The study area covers about 42 mi2 along the lower Jurassic or Early Cretaceous continental margin subduction, Skagit River before it enters Skagit Bay in southwest Skagit and sedimentary units deposited in alluvial fan, braided County, Washington (fig. 1). The area is bounded by the stream, and near shore shallow marine settings. Skagit River to the east and south; an approximate watershed Continental glaciers advanced into Skagit County divide between the Samish and Skagit Rivers to the north; and several times during the Pleistocene Epoch. This ice is part Padilla Bay, Swinomish Channel, and Skagit Bay (all parts of of the Cordilleran ice sheet, and is known as the Puget Lobe. Puget Sound) to the west. The Skagit River occupies a large, The most recent period of glaciation, the Vashon Stade of relatively flat alluvial valley that primarily is underlain by the Fraser glaciation, began about 17,000 years ago when fluvial and deltaic sand and gravel deposits associated with the continental ice sheet in Canada expanded and the Puget the present and ancient Skagit River, and locally preserved Lobe advanced southward into western Skagit County, and lahar-runout deposits originating from Glacier Peak (located eventually covered the entire Puget Sound Basin before outside the study area to the southeast). Local upland areas halting and retreating. During the Everson Interstade, are underlain by glacial till and outwash deposits that show beginning about 13,500 years ago, the climate warmed and evidence of terrestrial and shallow marine depositional the lobe wasted back allowing marine waters to enter the environments. Bedrock (consisting of a complex assemblage Puget Sound Basin, which had been depressed due to glacial of metamorphic rocks, sedimentary units, and igneous rocks) isostatic loading. Marine inundation buoyed the retreating ice underlies the alluvial valley and upland areas. Surficial and produced marine and estuarine conditions in the study exposures of bedrock are limited to a few upland outcrops in area. Postglacial filling of the Skagit River valley, which had the southwestern part of the study area. been excavated by subglacial meltwater, was accomplished Land-surface altitude in the study area ranges from near through Holocene fluvial, estuarine, and deltaic deposition and sea level adjacent to the Swinomish Channel to about 130 ft in volcanic-lahar deposits originating from Glacier Peak. upland areas. The study area has a temperate marine climate with warm, dry summers and cool, wet winters. Temperatures Introduction 3

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay Burlington

SR 20

ver Ri

536

538

S w T.

o 34

i N. s

25'

Skagit River Delta study area t i g

C a k h S

a Mount

Vernon e

La Conner

k r

ork o

F F

h rt

h t

u o S T.

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 MILES Skagit River EXPLANATION 0 1 2 3 KILOMETERS Delta WASHINGTON Boundary of Skagit River Delta study area Figure location

Figure 1. Location of the Skagit River Delta study area, Skagit County, Washington.

tac09-9722-0386_SkagitDelta_Fig 01 4 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

Well-Numbering System

Wells in the study area were Study area assigned a local number that identifies each well within the Public Land Survey rectangular grid system for Washington State (fig. 2). For example, W A S H I N G T O N 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- WillametteMeridian quarter tract within the section. The Willamette Base Line number following the quarter-quarter tract (01) is a sequence number used to TOWNSHIP distinguish individual wells in the same 123456 quarter-quarter tract. A “D” following 7 12 the sequence number indicates a well T. ABCD that has been deepened. 18 13 34N/03E-09L01 34 EFGH 19 24 JKLM 30 25 N. Methods of Investigation NPQR 31 32 33 34 35 36 SECTION 09 Methods used to compile and R. 3 E. analyze information to characterize shallow groundwater movement in the Figure 2. Well numbering system used in Washington. study area include measurement of water levels in wells, estimates of Skagit River stage, construction of water-level maps, and determination of ocean tides.

Well Inventory and Water-Level Measurements the availability of the drillers’ log. The goal of the inventory was to obtain an even distribution of wells throughout the Characterization of the groundwater-flow system relied study area. However, this was not possible for the entire on the analysis of spatially distributed information about study area because of a lack of wells in less populated areas. groundwater levels, and the physical and hydraulic properties During the field inventory in August 2007, permission was of the geologic units detected during well construction. This obtained to measure water levels in 43 wells (fig. 3) on a information was obtained through the measurement of water quarterly basis (August 2007, November 2007, February 2008, levels in wells, and the evaluation of lithologic descriptions and May 2008). Four additional wells were added to the from well drillers’ logs. Lithologic information was used to quarterly monitoring network as they became available for identify wells completed in the shallow groundwater system. measurement. One of these wells (34N/03E‑35N01) was Well records were compiled from USGS and Washington State installed for this study using a rotary drill rig and constructed Department of Ecology databases to identify potential wells to of 6 in. steel casing and 0.01 in. stainless steel screen. Four be used in this study. Candidate wells were selected for field quarterly monitoring wells were instrumented with continuous inventory based on the location and depth of the well, and water-level recorders (fig. 3).

tac09-9722-0386_SkagitDelta_Fig 02 Methods of Investigation 5

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay Burlington 05B02 05B01 SR 20

05R01 01R01

10B01 10A01 SR 09E01 12A01 20

iver 09L01 R 12200500 09L02

08R01

16G01 536

14K01 538

16J01 S w T.

i 15R01 n 16P01

o 34

m 21A01 23D01

i 23B01 N. s

h 22F01

19F01

23G01 22H01

20M01 24L01 20J01 22F02 24K01 19F02

21K01

21M01 25'

t i 26D01 g

C a k h S

a Mount

n 26K01

Vernon e

28M01

28M02

26Q01

25N01

35D01

27Q01

La Conner 35G01

32K01 35R01

35R02 04C01 35N01

05H02

03J01

k r

ork o

F F

h rt

h t

u o S T.

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) EXPLANATION 0 1 2 3 MILES Skagit River Delta study area 0 1 2 3 KILOMETERS 32K01 Water-level observation well 05H02 Observation well with continuous water-level recorder 12200500 USGS streamflow-gaging station Well centroid

Figure 3. Location of wells and U.S. Geological Survey streamflow-gaging station, Skagit River Delta area, Washington.

tac09-9722-0386_SkagitDelta_Fig 03 6 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

The depth to groundwater below land surface was river based on available coverages of hydrography, aerial measured using a calibrated electric tape or graduated steel photography, USGS quadrangle topographic maps, and tape, both with accuracy to 0.01 ft. Latitude and longitude LiDAR imagery. These data were cross checked, and adjusted locations were determined for each well using a Global using ArcMapTM, version 9.3, by ESRI Inc., to produce a Positioning System receiver with a horizontal accuracy of consistent rendering of river hydrography. The points for the one-tenth of a second (about 10 ft). Altitudes of the land centerline were converted to x and y coordinates (NAD 83 surface at each well were interpolated from USGS 1:24,000– State Plane Washington North) in the software, and exported scale topographic maps, with an accuracy ranging from to Microsoft Excel®. The distances between centerline points ±5 ft (low-relief areas) to ±10 ft (high-relief areas). Light were computed and summed to obtain a total distance from Detection and Ranging (LiDAR) derived values for the the mouth of the Skagit River. River channel distance was altitude of land surface became available after completion of measured along the North Fork of the Skagit River, and the field inventory, and were used to determine the altitude extended out of the study area to open water in Skagit Bay, of land surface at each well for the computation of water- where water levels were expected to average to mean sea level altitudes. LiDAR data were collected and processed level. The total river mile distance from open water to the by a private vendor under contract with the USGS. Vertical USGS gage was computed to be 17.5 mi. GIS points were then accuracy was evaluated for internal consistency (repeatability), interpolated at one-half-river-mile intervals, and each point and conformance with independent ground-control points. was assigned a river-stage value by proportionally adjusting All water-level measurements were made by USGS the stage value at the USGS gage (for example, by a factor of personnel according to standardized techniques of the USGS (river mile)/17.5). (Drost, 2005). Well information collected during the field These estimates of river stage were considered during inventory and the quarterly monitoring-network measurement construction of groundwater-level maps and were compared periods were entered into the USGS National Water to nearby groundwater levels measured in the delta to help Information System (NWIS) database and published in Fasser infer the nature of the hydraulic connection between the and Julich (2009). Skagit River and adjacent groundwater system. In areas where estimates of river stage were higher than nearby groundwater levels, the potential for groundwater recharge from the river River Stage exists, and the orientation of groundwater-level contours were drawn to reflect surface-water recharge. Estimates of Skagit River stage (the altitude of the water The linear interpolation described above might differ surface in the river) were used to constrain groundwater-level from the actual stream-stage values due to unaccounted for contours along the eastern margin of the study area, and to variations in river hydraulics such as changes in cross section, infer groundwater recharge from or groundwater discharge gradient, and the effects of distributary channels (such as the to the river. Daily values of river stage were obtained for South Fork of the Skagit River), and tidal influence. To check USGS steamflow-gaging station 12200500 (Skagit River the procedure, LiDAR altitudes were obtained with GIS at near Mount Vernon, Wash.; fig. 3) and a mean gage height the points used for estimating river miles. Despite the fact was computed for each of the quarterly monitoring-network that the laser is reflected and/or absorbed rather than directed measurement periods. The initial measurement period (August back to the detector, it seems that sufficient LiDAR elevation 2007) required locating suitable wells and obtaining owner points are picked up immediately along the banks of the river permission and took 8 days to complete. All subsequent to give a good estimate of river stage along the river. Linear quarterly measurements (November 2007, February 2008, regression of the LiDAR data indicated a linear relation and May 2008) were accomplished in 2 days. Values of river between river mile and LIDAR derived river stage of 0.99 ft/ stage reported in vertical datum NGVD 29 were converted to mi, corresponding to a river stage value of approximately the datum used for the monitoring network, NAVD 88, using 16.5 ft at the USGS gaging station. The computed mean the on-line program VERTCON (http://www.ngs.noaa.gov/ gage height at the USGS gaging station for the quarterly cgi-bin/VERTCON/vert_con.prl) provided by the National measurement periods ranged from 16.46 to 25.63 ft. The Geodetic Survey. regression analysis of LiDAR data gave a good fit to the linear River stage at the USGS steamflow-gaging station interpolation of river stage with a root-mean-square error of was extrapolated along the length of the Skagit River 1.75 ft for the data points available, and supports the use of within the study area using several data coverages and linear interpolation to estimate river stage. Geographic Information System (GIS) procedures. Spatial data points were developed to represent the centerline of the Methods of Investigation 7

Groundwater-Level and Flow-Direction Maps regression coefficients were combined to obtain an overall (annual) flow regime description. The groundwater altitudes Of the 47 wells inventoried (fig. 3), measurements from (z) were input as the dependent variable and the two columns 34 wells completed in alluvial deposits (shallow groundwater of x and y coordinates of the wells were entered as the system) were used in the construction of groundwater-level independent variables. In this case, linear regression was used and flow-direction maps, and linear-regression analysis. to estimate coefficients 0a , a1, and a2 that describe a planar Groundwater level (altitude) was calculated by subtracting surface for the potentiometric surface as: depth to water from the LiDAR derived ground-surface altitude at each well. Groundwater-level altitude contours and zest =a 0 +a 1 *x+a 2 *y , (1) flow lines were generated manually for each of the quarterly measurement periods. Wells completed in glacial deposits where or below confining clay layers within the alluvium were not zest is estimated elevation of the potentiometric included in the construction of groundwater-level and flow- surface at location (x,y); direction maps or linear-regression analysis because water a0 is intercept constant; levels from these wells are likely influenced by hydrogeologic a1 is slope of the surface in the x direction; and conditions that are distinctly different from those controlling a is slope of the surface in the y direction. water levels in the shallow groundwater system. Furthermore, 2 upland areas consisting of relatively low permeability glacial deposits and bedrock were interpreted as probable shallow These coefficients were then used to estimate the groundwater-flow barriers. common hydrogeologic parameters of the groundwater regime Water levels in well 22F02 were influenced by active for that quarterly measurement: pumping during all quarterly measurements, and water levels 2 2 in well 26Q01 were significantly lower than surrounding Gradient = SQRT (a1 + a2 ) is the wells leading to uncertainties regarding the accuracy of well- slope of the potentiometric surface. construction and lithologic information or the possibility of nearby pumping. Therefore, water-level data from these two Direction = ATAN2 (a , a ) is the wells were not used in the construction of groundwater-level 1 2 upstream/downstream direction of the surface. and flow-direction maps or linear-regression analysis. Water levels in wells 08R01 and 20J01 were influenced by pumping Then, the relative water level was calculated as the elevation during the August 2007 quarterly measurement and were of the plane at the geographic center of all well locations not used for the construction of maps or regression analysis. (centroid; fig. 3): Some wells only had partial water-level records (water levels were not available for all quarterly measurements) and were Water level at the only used in the construction of maps and regression analysis centroid = z = a + a * x + a * y for quarterly measurements in which data were available. cent 0 1 cent 2 cent Hydrographs of groundwater levels were generated for all After measurements for all four sets of quarterly inventoried wells and published in Fasser and Julich (2009). measurements were processed through the regression A linear-regression analysis of groundwater altitudes in method, an estimate of direction and gradient of the annual the Skagit Delta area was done to estimate the overall, time- groundwater system was calculated by averaging the averaged shallow groundwater-flow direction and gradient. regression coefficients from each of the quarterly The data used in the regression analysis were similar to those measurements, for example: used for construction of the groundwater-altitude maps and included: (1) x and y coordinates for the wells according to the a = (a + a + a + a ) / 4 NAD 83 State Plane projection in feet for Washington (North) 0, annual 0, August 0, November 0, February 0, May and (2) quarterly groundwater altitudes from the 34 wells completed in alluvial deposits. After averaging to get the annual coefficients, the annual The regression analysis was calculated using Microsoft gradient, direction, and water level at the centroid were Excel® Data Analysis tool. Each set of quarterly water-level calculated from the annual regression coefficients. measurements was analyzed separately, and then the seasonal 8 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

Ocean Tides The till confining unit (Qgt) is present within local upland areas and shows evidence of terrestrial and shallow Predicted values of tidal extremes were used in an marine depositional environments. This low-permeability analysis of possible correlation between groundwater-level unit is composed of glacial diamicton and consists of various fluctuation at monitoring well 19F01 fig.( 3) and ocean tides. proportions of clay, silt, sand, gravel, cobbles, and boulders, The tide data were obtained from the closest National Oceanic with locally occurring sand and gravel lenses capable of and Atmospheric Administration tide gage (National Oceanic providing water for domestic use. and Atmospheric Administration, 2009) that had calibrated Exposures of advance outwash aquifer (Qga) deposits are tidal coefficients (tide gage 9448558 “La Conner, Swinomish limited to a few areas along the base of the glacial upland, and Slough, Wash.”). The resulting analysis (discussed later in the presence of this unit is inferred, at least in places, beneath this report) is only approximate, because of several factors: the till confining unit within upland areas. The unit consists (1) the groundwater-level data from well 19F01 also were primarily of sand and gravel with minor amounts of silt and influenced by recharge or other factors; (2) the tidal record scattered layers of pebble-cobble gravel and local silt and clay used in the analysis is only of predicted astronomical highs interbeds. Groundwater in this aquifer is potentially confined and lows, not actual measured tidal levels, that may have been by the overlying till confining unit, however, unconfined affected by wind and barometric pressure; (3) the location for conditions may occur locally where it is not fully saturated or the predicted tide is at La Conner, more than 2 mi to the south, exposed at land surface. not at the point in Swinomish Channel closest to the well, and Generally, glacial deposits are believed to be largely different tidal components may travel differently along the absent beneath low lying areas of the Skagit River Delta to narrow and shallow channel; and (4) high tides seem to have a depth of approximately 300 ft below sea level, likely due less influence on the well than do low tides (estimated from to removal by southward flowing subglacial meltwater, prior the correlation coefficients), although this may be caused by to subaerial exposure of the glacier bed during ice recession tidal attenuation along the Swinomish Channel. (Booth, 1994; Dragovich and others, 1994). This interpretation is supported by the absence of glacial deposits in wells from low lying areas of the delta, and similar interpretations from previous investigations (Dragovich and Grisamer, 1998; Hydrogeology GeoEngineers, 2003). Glacial deposits in upland areas within the delta likely represent erosion remnants of a previously A simplified surficial geologic mapfig. ( 4) for the more continuous distribution of glacial units. Skagit River Delta area was constructed based on previous The sedimentary bedrock aquifer (OEc) crops out within mapping by Schuster (2000). Geologic units were grouped and adjacent to uplands in the southern part of the study area; into one of five new unit designations table( 1) based on a lack of outcrop and well-log information makes it difficult similarities in lithologic and hydrogeologic characteristics. to determine the extent of this unit in the subsurface. The The hydrogeologic characteristics attributed to these units unit consists primarily of pebble and cobble conglomerate are similar to those defined for similar units by other and medium- to coarse-grained sandstone, with fine-grained investigations in areas adjacent to this study (Thomas and intervals of mudstone, siltstone, coal, and shale. Groundwater others, 1997; Dragovich and Grisamer, 1998; GeoEngineers, in the bedrock aquifer is unconfined where it crops out; 2003). Descriptions of the units used in this study are given however, confined conditions are likely where it is fully below. saturated and overlain by glacial till and glaciolacustrine units. The alluvial and recessional outwash aquifer (Qago) is Fine-grained intervals within the sedimentary bedrock aquifer present throughout the low lying areas of the Skagit River also may produce locally confined conditions. Delta and represents: (1) active or abandoned channel and The igneous and metamorphic confining unit (KJ) crops overbank deposits associated with the present and ancient out within and adjacent to uplands in the southwestern part Skagit River, (2) lahar-runout deposits, originating from of the study area; a lack of outcrop and well-log information Glacier Peak, and (3) terrestrial and marine recessional and makes it difficult to determine the extent of this unit in the deltaic glacial-outwash deposits. This unit comprises the subsurface. This low-permeability unit is composed of igneous shallow groundwater system in the study area, and consists and metamorphic rocks and consists of a complex assemblage of sand, gravel, and cobbles, with minor lenses of silt and of volcaniclastic deposits, and low-grade metasediments, and clay. Groundwater in this aquifer is unconfined where it is not metavolcanics. This unit is considered to be nonwater bearing fully saturated or exposed at land surface, however, confined except in localized areas of fracturing. conditions are likely where it is fully saturated and overlain by confining layers of clay. Hydrogeology 9

122°30' 25' 122°20' 48°29' T. Qago 35 Qgt N. Padilla Bay Burlington

Qgt

Qgt Qgt Qgt Qago

Qago

ver Ri

Qago

S w T.

o 34

m i N.

s Qgt

Qga h

Qago

Qga

25'

t i g

C a k h S

a Mount

Vernon e

Qgt

La Conner Qga

Qago Qago

KJ KJ KJ Qago KJ Qgt KJ

Qgt Qago KJ Qgt KJ Qago

KJ Qgt KJ k

OEc r

ork o

Qago F F

h KJ rt

Qgt No

Qgt OEc

OEc Qgt OEc OEc OEc h OEc t

u o S T.

Qago 33

Qgt

Skagit Bay

48°21' OEc

Base from U.S. Geological Survey digital data, 1983, 1:100,000 R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1988 (NAD 88) EXPLANATION 0 1 2 3 MILES SIMPLIFIED GEOLOGIC UNITS 0 1 2 3 KILOMETERS Qago Alluvial and Recessional Outwash (Aquifer) Qgt Till (Confining Unit) Qga Advance Outwash (Aquifer) OEc Sedimentary Bedrock (Aquifer) KJ Igneous and Metamorphic Rocks (Confining Unit)

Water

Figure 4. Skagit River Delta area, Washington.

tac09-9722-0386_SkagitDelta_Fig 04 10 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

Table 1. Hydrogeologic characteristics of simplified geologic units used in this study and correlation with previous unit designations.

[Geologic units used in this report are simplified and are from Schuster (2000).Symbol: –, not applicable]

Hydrogeologic Simplified Period Epoch Geologic units characteristic of units geologic units Holocene to Alluvial and Recessional Qago Qac, Qas, Qf, Qvlk, Pleistocene Outwash Aquifer Qgom

Till Confining Unit Qgt Qp, Qls, Qgdme, Qgt Quaternary Advance Outwash Aquifer Qga Qga, Qgas Pleistocene

Tertiary Oligocen to Eocene Sedimentary Bedrock Aquifer OEc OEcb, Ecc

Cretaceous – Igneous and Metamorphic KJ KJvcf, KJmsg, KJmvg to Confining Unit Jurassic

Groundwater Movement contours near this well indicate a westward groundwater- flow direction away from the river. Water-level altitudes varied seasonally, however, generally ranged from 1 to 2 ft The direction of horizontal groundwater movement (August 2007, fig. 7) in the west to about 15 ft (May 2008, can be inferred from maps of water-level altitude contours. fig. 8) in the east. Concave and convex bends in the water- Groundwater flow generally is from areas of recharge to level altitude contours indicate the presence of convergent and areas of discharge, in the direction of decreasing water- divergent groundwater-flow patterns on a local scale. These level altitudes and perpendicular to the water-level altitude flow patterns likely reflect variations in hydraulic conditions contours. Water-level altitude maps and regression analysis within the aquifer resulting from spatial variations in aquifer were used to determine shallow groundwater-flow directions material properties (clay, silt, and sand). Local groundwater and gradients in the Skagit River Delta area. withdrawals and agricultural drainage systems also may influence the groundwater-flow patterns. Groundwater-Flow Directions The fitted regression coefficients and calculated groundwater gradient, flow direction, and water level at the Groundwater flow in the alluvial and recessional outwash centroid are presented in table 2. An estimate of goodness of aquifer generally moves in a southwestward direction away fit (statistical parameter 2R ) for the regression also is given from the Skagit River and towards the Swinomish Channel for each seasonal estimate. The high R2 values indicate that and Skagit Bay (figs. 5–8). Local groundwater flow toward the planar representations of the groundwater potentiometric the river was inferred during February 2008 in an area west surfaces are in good agreement with the quarterly water-level of Mount Vernon near well 24K01, and in an area southwest measurements. The large negative a0 (intercept) coefficients of Mount Vernon near wells 35R01 and 35R02. Water-level are due to the origin of the State Plane System being far to the altitude contours in these areas indicate a south to south southwest and well away from the project area. eastward groundwater-flow direction (fig. 7). Contours in Contours of equal values of z (time-averaged these areas do not indicate groundwater flow to the river groundwater altitude for the four quarterly measurements) during the other quarterly measurement periods, and flow were calculated algebraically from the annual a0, a1, and a2 to the river in these areas likely is seasonal or transient in coefficients fig.( 9). Estimates of the overall (time averaged) nature. Water-level altitudes in well 35R02 also were greater groundwater-flow direction (8.5° south of west) and gradient than nearby estimates of river stage during November 2007 (2.67 ft/mi) also were computed (table 2). and May 2008 (figs. 6 and 8; however, water-level altitude Groundwater Movement 11

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay Burlington 05B01 05B02 9.28 8.14 SR 20

05R01 01R01 15.28 17.18 10A01 09E01 10B01 8.31 12A01 -5.13 14.96 SR -3.93 20

09L01

r ive 09L02 R

11 10 15.52 9 15.05 8

7 15.99 08R01 16.46 -11.42

6 14.58 5 3 1 4 2

16G01

2.81 SR SR 14K01 14.11

536 538 16J01 11.15

S 4.69 13.64 w 15R01 T. i 5.65 13.17 n

o 34

m 23B01 16P01 12.7

i 21A01 10.66 N. s 3.35 4.97 h 22F01 23D01 19F01 5.9 8.63

2.43 11 23G01 22H01

20M01 8.64 9.59

19F02 1.47 21M01 22F02 24K01

0.66 2.62 21K01 4.73 10 10.30 24L01 12.23 20J01 3.37 10.82 25' -4.53

t i 26D01 g a C 11.76 5.55 9 k 11.29 h S

a 10.82 Mount

n 7 6 5 n 3 4 2 26K01 Vernon e 1

l 28M01 8.99

1.00

28M02 10.35

1.12 25N01 27Q01

35D01 9.47

7.04 26Q01

5.88 9.88

35G01

La Conner 7.25

32K01 35R01 10.04 35R02 7.82 9.41 7.95 35N01 8.94 04C01 8.47 13.42

05H02 7.99 5.10

7.52

03J01 k

0.65 r

ork o

F F

h rt 7.05

6.58

3 6.11

h 3.29 t

u o 5.64 S T. 4.23 4.7

3.76 5.17 33

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 0 1 2 3 MILES R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 KILOMETERS EXPLANATION Well Characteristic August 2007 Inferred groundwater- 35R02 Well with short local number Wells not used to construct All altitude measurements flow direction 7.95 and water-level altitude water-level map are in feet above NAVD 88 Well completed in glacial deposits 5.64 Estimated river stage Well used to construct 3 Water-level altitude and altitude water-level map Well completed below confining clay Shallow groundwater system wells Well influenced by pumping

Figure 5. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, August 2007.

tac09-9722-0386_SkagitDelta_Fig 05 12 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay 05B01 10.64 Burlington 05B02 10.31 SR 20

05R01 01R01 15.96

10A01 12A01 10B01 8.98 09E01 9.30 SR 5.60 20

ver 09L01 Ri 09L02 5.53 2.04 19.21 18.62 19.79 08R01 20.37 3.32 4 SR 18.04

5 6 536 12 3

7 8

16G01 9 10 11 3.46 SR 17.46

16J01 538

S 5.28 14K01 16.88 2 w 15R01 11.42 T. i 6.00 16.30 n 16P01 o 4.04 21A01 23B01 34 m 5.49 23D01 11.27 15.71

i 22F01 8.97 N. s

h 6.55

19F01

2.97 23G01

22H01 20M01 9.90 20J01 8.78 24L01 24K01 19F02 2.47

3.41 21K01 12.83 12.12 1.44 22F02 15.13 4.07 21M01

25' 3.48

t i 26D01 g

C a 13.97 6.23 k 14.55 h S 13.39 a Mount

n 26K01

Vernon e 3 9.22 l

28M01

12 12.8

26Q01 11 28M02 25N01

2.55 5.89 9.71

9 27Q01 35D01 12.22

2 7.82

35G01 La Conner

8 8.39

32K01 6 35R02 35R01 5 10.09 11.38 10.34 11.64 35N01 11.06 04C01 10.48 13.03

05H02 9.89 4.99 03J01

-9.03 9.31

k r

ork o

F F

h rt 8.73

8.15

7.57 4.07 h t

u 5.82 6.98 o 5.24 S T.

4.66

6.4 33

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 0 1 2 3 MILES R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 KILOMETERS EXPLANATION Well Characteristic November 2007 Inferred groundwater- 35R02 Well with short local number Wells not used to construct All altitude measurements flow direction 7.95 and water-level altitude water-level map are in feet above NAVD 88 Well completed in glacial deposits 6.98 Estimated river stage Well used to construct 3 Water-level altitude and altitude water-level map Well completed below confining clay Shallow groundwater system wells Well influenced by pumping

Figure 6. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, November 2007.

tac09-9722-0386_SkagitDelta_Fig 06 Groundwater Movement 13

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay 05B01 Burlington 11.25 05B02 11.96 SR 20

01R01 05R01 17.32 10A01 10B01 10.22 12A01 SR 09E01 10.36 20 8.99

09L01 er 09L02 8.19 14 iv R 2.82

12 16.63 11 16.13 17.14

08R01 17.64 10

4.21 9 6 5 15.62 7 8 SR 3

4 536

16G01 15.12 538 4.76 16J01

S 6.84 14K01 14.62 w 13.57 T. i 14.11 n 16P01 15R01

o 5.29 7.96 34

m 23B01 13.61 i N. s 21A01 22F01 13.03 h 14 19F01 6.69 8.31 23D01

11.01 3.44 22H01

11.22 23G01 24L01 20M01 13 24K01 19F02 12.10 13.30 3.04 22F02 12.02 1.44 21K01

13.10 5.42 20J01 21M01 25'

4.24 4.42

t i g

C a 12.10 26D01 k 12.60 h S 11.59 a 6.45 Mount

3 n

Vernon e 12 4

28M01 26K01

3.40 11.88 11.09

6 27Q01

28M02

7.58 3.94 35D01 26Q01

10.50 8.13 25N01

7 10.58 10.66

35G01 La Conner 10 9

35R01 32K01 10.26 9.31 10.08 35R02 9.44 9.58 04C01 35N01 9.07 13.42 6.21 05H02 8.57 5.24

03J01 8.06

3.75 k r

ork o F F

h rt 7.56

7.06

6.55

h 3.53 t

u 5.04 o 4.54 6.05 S T.

4.03 5.54 33

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 0 1 2 3 MILES R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 KILOMETERS EXPLANATION Well Characteristic February 2008 Inferred groundwater- 35R02 Well with short local number Wells not used to construct All altitude measurements flow direction 9.44 and water-level altitude water-level map are in feet above NAVD 88 Well completed in glacial deposits 6.05 Estimated river stage Well used to construct 3 Water-level altitude and altitude water-level map Well completed below confining clay Shallow groundwater system wells Well influenced by pumping

Figure 7. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, February 2008.

tac09-9722-0386_SkagitDelta_Fig 07 14 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay 05B01 Burlington 05B02 11.63 11.68 SR 20

01R01 05R01 18.04 10A01 12A01 09E01 10B01 9.76 SR 9.79 6.38 20

r 09L01 ive 09L02 8.65 R 15 2.48

24.17 14 23.43 24.90

13 2 08R01 3.75 3 25.63 12 11 4 22.70 10

6 SR 9 16G01 7 8 536

4.36 SR 21.97

16J01 538

6.48 S 15R01 14K01 21.24 w 14.33 7.78 T. i 20.5 n 16P01 21A01 23B01

o 34

4.86 13.53 m 6.43

22F01 19.77 i N.

s 8.07 h 23D01

19F01 11.27 2.82 22F02 23G01 15

21K01 12.16 24K01 19F02 20M01 5.01 22H01 14 20J01 15.15 11.04 1.12 3.84 13 24L01 19.04 21M01 12 14.24 25' 4.03

11 t i g

26D01 10 17.57 C a k 18.31

7.1 9 h S 16.84 a 8 Mount

n 7

n 26K01 Vernon e

11.47 l

28M01

2.83

28M02 16.11

26Q01

4 3.2 27Q01

7.73

3 25N01 6.6

11.83 35D01

9.88

15.38

35G01 La Conner

35R01

35R02 12.45 32K01 14.17 14.65 10.71 35N01 6.67 13.91 04C01 13.18 13.51

05H02 12.45 5.51 03J01 6.29

11.72

k r

ork o F F

rt 10.98

10.25

9.52

h 5.13 t

u 8.79 o 6.59 7.32 S T.

5.86 8.06 33

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 0 1 2 3 MILES R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 KILOMETERS EXPLANATION Well Characteristic May 2008 Inferred groundwater- 35R02 Well with short local number Wells not used to construct All altitude measurements flow direction 14.17 and water-level altitude water-level map are in feet above NAVD 88 Estimated river stage Well used to construct Well completed in glacial deposits 3 Water-level altitude 8.79 and altitude water-level map Well completed below confining clay Shallow groundwater system wells Well influenced by pumping

Figure 8. Water-level altitude and inferred groundwater-flow directions in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, May 2008.

tac09-9722-0386_SkagitDelta_Fig 08 Groundwater Movement 15

Table 2. Fitted regression coefficients and calculated hydrogeologic parameters for wells in the Skagit River Delta area, Washington, 2007 and 2008.

[Abbreviations: ft/ft, foot per foot; ft/mi, foot per mile; NAVD 88, North American Vertical Datum of 1988]

Measurement August 2007 November 2007 February 2008 May 2008 Annual Fitted coefficients, from Microsoft Excel®

a0 (Intercept) -628.10 -589.49 -659.44 -768.90 -661.48 a1 (x variable 1) 0.000456 0.000462 0.000488 0.000597 0.000501

a2 (x variable 2) 0.000116 0.000030 0.000104 0.000051 0.000075 Calculated hydrogeologic parameters Gradient (ft/ft) 0.00047 0.00046 0.00050 0.00060 0.00051 Gradient (ft/mi) 2.49 2.44 2.63 3.16 2.67 Direction W 14.3° S W 3.7° S W 12.1° S W 4.9° S W 8.5° S Water level at centroid (feet 5.71 6.40 7.52 7.54 6.79 above NAVD 88) Goodness of fit, from Microsoft Excel® R2 0.896 0.879 0.848 0.874

The overall groundwater-flow direction derived from the quarterly monitoring well locations used in the construction of regression analysis is similar to those depicted on the water- the water-level maps. Ground surface slopes were determined level maps; shallow groundwater flows slightly south of west to be slightly steeper (3.82 ft/mi) and slightly more to the from the mainstem of the Skagit River out to the Swinomish southwest (W 24.0° S) than the groundwater potentiometric Channel and Skagit Bay. Regression analysis of the four surface. quarterly measurements generally agrees with the annual The potential for vertical groundwater movement within analysis, with only slight variations in gradient or direction the alluvial and recessional outwash aquifer was evaluated (table 2). in areas where a comparison of water-level altitudes could Time-averaged water levels in individual wells are be made in closely spaced wells completed above and below always higher or lower than the fitted planar surface fig.( 9). clay layers within the aquifer (figs. 5–8). Wells completed Where measured water levels in a subarea are higher on below clay layers are widely spaced and along the margins of average than the fitted plane, the potentiometric surface is the study area, however, comparisons were possible at three diverging from the uniform planar flow direction; conversely locations (19F01/19F02, 09L01/09L02, and 10A01/10B01). where water levels in a subarea are lower than the plane the Water-level altitude differences at the first two well-pair flow is converging. Differences in measured water levels and locations indicate the potential for year-round downward flow, the plane indicate a pattern in which groundwater converges with higher altitudes in wells completed above clay layers from the river meander, diverges into two preferential channel for all quarterly measurements. A more complex relation areas, converges to flow between the till outcrop uplands, and was observed at the last well-pair location in which higher finally diverges to flow separately toward Skagit and Padilla altitudes in wells completed above clay layers were observed Bays. Taking into account this final divergence, the sea level during August 2007 and May 2008 measurements indicating (z = 0) contour would coincide fairly well with sea level in the the potential for downward flow, and lower altitudes in wells bays and the Swinomish Channel. completed above clay layers during the November 2007 and To compare the overall slope of the ground surface in the February 2008 measurements indicating the potential for area with that of the groundwater surface, a similar regression upward groundwater flow. calculation was performed on ground-surface elevation at the 16 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay 05B01 10.7 Burlington 05B02 10.45 SR 20

05R01 01R01 17.11 17.18

10A01 12A01 10B01 09E01 9.31 14.96 SR 9.83 20 4.55

r 09L01 ive 09L02 R 7.46 2.44 16.5 08R01 16 18.88 3.76 18.31 17 19.45 17.5

15.5 20.03

17.74

SR 14

16G01 536

3.85 15 SR 17.16

16J01 14.5 538 14K01 16.59 S 5.82 12.62 w 16P01 15R01 14 T. i 6.85 16.02 n 4.38

o 23D01 13.5 34

m 21A01 23B01 9.97 15.45

i 5.89 22F01 12.12 N. s

h 19F01 7.21

2.92

23G01 24L01 20M01 22H01 24K01 10.94 2.33 20J01 9.92 12.32 19F02 22F02 12.4

3.83 13 1.17 21K01

14.88 12

21M01 4.46 25'

3.64 t W8.5°S i

26D01 g a C 12 2.67 feet per mile 6.33 k h S 13.73 12.5

a 14.30 10 11.5 Mount

n 13.16 n 26K01 8 Vernon e

l 10.39 28M01

2.41 4 11

26Q01

12.59

6.90 25N01

27Q01 35D01 10.41

28M02

2.71 7.09 8.81

10.5

35G01 12.02 La Conner

7.82

32K01 10.36 35R01 10 9.98 11.44 35N01 35R02 6.43 9.5 04C01 10.74 10.87 13.34 9 10.30 8.5 05H02 9.73 5.21 03J01 8

5.02 9.15

k r

ork o

F F

h rt

No 7.5

8.58

8.01 6.5

h 3.5 7.44 t 4.01 u o 5 S T. 4.5 6

5.72 6.87 4 5.15 5.5 33

4.58 6.29 I

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E.E.R. 0 1 2 3 MILES R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) EXPLANATION 0 1 2 3 KILOMETERS 4 Derived water-level altitude, 10 Estimated Skagit River stage Well Characteristic 35R02 11.44 in feet above NAVD 88 showing river mile and annual 10.74 Shallow groundwater system wells Well completed in glacial deposits average water-level altitude, in used in regression analysis Calculated groundwater- Well completed below confining clay feet above NAVD88 Wells with partial record Well influenced by pumping flow direction Wells with full record

Figure 9. Regression analysis derived annual water-level altitude and calculated groundwater-flow direction in the alluvial and recessional outwash aquifer, Skagit River Delta area, Washington, 2007 and 2008.

tac09-9722-0386_SkagitDelta_Fig 09 Groundwater Movement 17

Groundwater-Level Fluctuations pattern for shallow wells in western Washington. Regression analysis also indicates the groundwater gradient was steepest Groundwater levels fluctuate over time, both seasonally in May, likely due to elevated groundwater levels associated and long term, in response to changing rates of recharge to with Skagit River stage, and that by August the water levels and discharge from the groundwater system. When recharge along the river declined in response to declining river stage, exceeds discharge, the amount of water stored in an aquifer and the gradient was reduced (table 2). The groundwater increases and water levels rise; when discharge exceeds gradient increases through the winter probably in response to recharge, groundwater storage decreases and water levels groundwater recharge from precipitation. decline. Groundwater levels also may respond to changes Groundwater levels also may respond to changes in in nearby stream stage. When stream stage exceeds nearby nearby ocean tides. Cyclical water-level fluctuations in a well groundwater levels, streamflow may recharge the aquifer, equipped with a continuous water level recorder exhibited a causing a rise in groundwater levels; when groundwater levels periodicity that is characteristic of ocean tides (for example, exceed nearby stream stage, discharge from the aquifer to the component wave lengths of about 24 hours, and 12 hours stream may occur, resulting in a decline in groundwater levels. and 25 minutes). The well (34N/03E-19F01) is north of the Seasonal changes in groundwater levels in many of the town of La Conner and less than a mile east of the Swinomish wells in the Skagit River Delta follow a typical pattern for Channel, a tidally influenced surface-water body that connects shallow wells in western Washington. Water levels rise in Skagit Bay to the south to Padilla Bay to the north (fig. 3). autumn and winter, when precipitation is high, and decline Water-level fluctuations in the well correspond closely to during spring and summer, when precipitation is low (fig. 10). predicted tidal extremes obtained from the National Oceanic Groundwater levels in wells along the eastern margin of the and Atmospheric Administration tide gage near La Conner study area also are likely influenced by stage of the Skagit (fig. 13). Through a trial-and-error correlation analysis, it was River (fig. 11). Water levels in these wells remained elevated noted that water levels in the well showed similar relative through April, likely due to groundwater recharge from the high and low points, with a delay of approximately 28 hours river, and did not seem to begin to decline until the end of after the tidal high or low, and an attenuation factor of about May in response to declining river stage (fig. 12). 3 percent. The peak-to-peak fluctuation in the well was about Results of the regression analysis of quarterly 0.3 ft compared to the tidal fluctuation of about 10 ft at the groundwater levels, indicated by the calculated water level at La Conner gage. the centroid (table 2), are in general agreement with the typical

6.0 4.0 Well 34N/03E-28M02 Anacortes Weather Station 3.5 5.0

3.0

4.0 2.5

3.0 2.0

1.5 2.0

1.0 PRECIPITATION, IN INCHES

1.0 0.5 GROUNDWATER LEVEL, FEET IN NAVD 88 ABOVE 0 0 OCT NOV DEC JAN FEB MAR APR MAY 2007 2008

Figure 10. Water levels in well 34N/03E-28M02 and precipitation at Anacortes, Washington, October 2007 through May 2008.

tac09-9722-0386_SkagitDelta_Fig 10 18 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

122°30' 25' 122°20' 48°29' T.

I 35 5 N. Padilla Bay Burlington

SR 20

ver Ri

536

14K01 SR

538

S w T.

o 34

m 23D01

i 23B01 N. s

23G01

24K01

25'

t i g 26D01 C a k h S

a Mount

Vernon e

25N01

La Conner

35R01

35N01 35R02

k r

ork o F F

h rt

h t

u o S T.

Skagit Bay

48°21'

Base from U.S. Geological Survey digital data, 1983, 1:100,000 0 1 2 3 MILES R. 02 E. Universal Transverse Mercator projection, Zone 10 R. 03 E. R. 04 E. Horizontal Datum: North American Datum of 1927 (NAD 27) 0 1 2 3 KILOMETERS 14K01 EXPLANATION Well with short local number Well outside the area of likely influenced by stage likely influence on Skagit River Shallow groundwater system wells Well completed in glacial deposits Well completed below confining clay Well influenced by pumping

Figure 11. Wells that likely are influenced by stage on the Skagit River, Washington.

tac09-9722-0386_SkagitDelta_Fig 11 Groundwater Movement 19

14.0 30 Well 34N/03E-23B01 13.6 Gaging Station No 12200500 28

13.2 26

12.8 24

12.4 22

12.0 20

11.6 18

11.2 16

10.8 14

10.4 12 GROUNDWATER LEVEL, FEET IN NAVD 88 ABOVE STREAM HEIGHT, GAGE FEET IN NAVD 88 ABOVE 10.0 10 OCT NOV DEC JAN FEB MAR APR MAY 2007 2008

Figure 12. Water levels in well 34N/03E-23B01 and stream stage at U.S. Geological Survey streamflow- gaging station 12200500, Skagit River near Mount Vernon, Washington, October 2007 through May 2008.

6 1.8

4 1.7

2 1.6

0 1.5

-2 1.4

-4 1.3 (MEAN SEA LEVEL)

IN FEET ABOVE NAVD 88 IN FEET ABOVE NAVD 1.2 -6 Tide Prediction -8 Well 34N/03E-19F01 1.1 TRANSDUCER RECORD, IN FEET PREDICTED HIGH AND LOW TIDAL RANGE, -10 1.0 21 22 23 24 25 26 27 OCTOBER 2007

Figure 13. Water levels in well 34N/03E-19F01 and nearby tidal fluctuations near La Conner, Washington, October 2007.

tac09-9722-0386_SkagitDelta_Fig 12

tac09-9722-0328 Skagit Delta_Fig 13 20 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

Summary and Conclusions generally ranged from less than 3 ft (August 2007) in the west to about 15 ft (May 2008) in the east. Concave and convex bends in the shape of water-level altitude contours indicate Recent population growth along the Interstate 5 corridor the presence of convergent and divergent groundwater flow near Mount Vernon, Washington, has led to increased patterns on a local scale, and likely reflect variations in domestic water use, with many new wells serving residents in hydraulic conditions within the aquifer resulting from spatial the lower part of the Skagit River basin. Planning for future variations in aquifer material properties (clay, silt, and sand). development in the lower basin, including the reservation The time-averaged shallow groundwater-flow direction of water for new domestic wells requires identification derived from regression analysis (8.5° south of west) was of areas where withdrawals from existing and new wells similar to flow directions depicted on the quarterly water-level could adversely impact streamflow in the Skagit River or maps, with a gradient of 2.67 ft/mi, its tributaries. A study to characterize shallow groundwater Seasonal changes in groundwater levels in most of the movement in an area between the lower Skagit River and wells in the Skagit River Delta follow a typical pattern for Puget Sound was conducted by the U.S. Geological Survey shallow wells in western Washington. Water levels rise in to assist Skagit County and the Washington Department of the fall and winter, when precipitation is high, and decline Ecology with the identification of areas where withdrawals during the spring and summer, when precipitation is lower. from existing and new wells could have adverse impact on Groundwater levels in wells along the eastern margin of the streamflow in the Skagit River. study area also are likely influenced by stage on the Skagit Land-surface altitude in the study area ranges from near River. Water levels in these wells remained elevated through sea level adjacent to the Swinomish Channel to about 130 ft April, and did not begin to decline until the end of May, in in upland areas. The shallow groundwater system consists response to declining river stage. Groundwater levels in a well of alluvial, lahar runout, and recessional outwash deposits equipped with a continuous water-level recorder exhibited composed of sand, gravel, and cobbles, with minor lenses periodic fluctuations that are characteristic of ocean tides of silt and clay. The aquifer is unconfined where it is not (for example, component wave lengths of about 24 hours, fully saturated or exposed at land surface, however, confined and 12 hours and 25 minutes). The well (34N/03E-19F01) conditions are likely where it is fully saturated and overlain by is less than a mile east of the Swinomish Channel (tidally confining layers of clay. Upland areas are underlain by glacial influenced), and exhibited water-level fluctuations that till and outwash deposits that show evidence of terrestrial and correspond closely to predicted tidal extremes obtained from a shallow marine depositional environments. Bedrock exposures tide gage near La Conner. are limited to a few upland outcrops in the southwestern part of the study area, and consist of metamorphic, sedimentary, and igneous rocks. Water levels were measured in 47 wells on a quarterly Acknowledgments basis (August 2007, November 2007, February 2008, and May 2008), and measurements from 34 wells completed in The authors thank the many well owners, farmers, and the shallow groundwater system were used in the construction irrigators for their cooperation in providing access to wells. of groundwater-level maps and linear-regression analysis. The authors also thank Skagit County and the Washington Estimates of Skagit River stage were used to constrain State Department of Ecology for providing assistance in groundwater-level contours along the eastern margin of the the compilation of well information, and Aquatech Well study area. Groundwater flow in the shallow groundwater Drilling & Pumps, Inc., for constructing a monitoring well at system generally moves in a southwestward direction away no cost to the project. from the Skagit River and towards the Swinomish Channel and Skagit Bay. Local groundwater flow towards the river was inferred during February 2008 in areas west and southwest of Mount Vernon. Water levels varied seasonally, however, Selected References 21

Selected References GeoEngineers, 2003, Lower and upper Skagit watershed plan Samish River sub-basin, ground water hydrology evaluation: File no. 7291‑001‑00‑1180, 175 p. Booth, D.B., 1994, Glaciofluvial infilling and scour of the Puget Lowland, Washington, during ice-sheet glaciation: Hansen, H.P., and Mackin, J.H., 1949, A pre-Wisconsin forest Geology, v. 22, no. 8, p. 695-698. succession in the Puget Lowland, Washington: American Journal of Science, v. 247, no. 12, p. 833-855. Dragovich, J.D., and DeOme, A.J., 2006, Geologic map of the McMurray 7.5‑minute quadrangle, Skagit and Snohomish Johnson, S.Y., 1982, Stratigraphy, sedimentology, and tectonic Counties, Washington: Washington Division of Geology setting of the Eocene Chuckanut Formation, northwest and Earth Resources Geologic Map GM-61, 18 p., 1 sheet, Washington: Seattle, University of Washington, Ph.D. scale 1:24,000. thesis, 221 p., 4 plates. Dragovich, J.D., Gilbertson, L.A., Norman, D.K., Anderson, Marcus, K.L., 1981, The rocks of Bulson Creek: Eocene- Garth, and Petro, G.T., 2002, Geologic map of the Utsalady Oligocene sedimentation and tectonics in the Lake and Conway 7.5‑minute quadrangles, Skagit, Snohomish, McMurray area, Washington: Bellington, Western and Island Counties, Washington: Washington Division of Washington University, M.S. thesis, 84 p., 1 plate. Geology and Earth Resources Open-File Report 2002‑5, National Oceanic and Atmospheric Administration, 2007, 34 p., 2 sheets, scale 1:24,000. Climatological data, annual summary, Washington, 2007: Dragovich, J.D., and Grisamer, C.L., 1998, Quaternary Asheville, North Carolina, National Climatic Data Center, stratigraphy, cross sections, and general geohydrologic v. 111, no. 13, 30 p. potential of the Bow and Alger 7.5-minute quadrangles, National Oceanic and Atmospheric Administration, western Skagit County, Washington: Washington Division 2009, NOAA tides and currents: National Oceanic and of Geology and Earth Resources Open File Report 98‑8, Atmospheric Administration database, accessed May 21, 30 p., 6 plates. 2009, at http://tidesandcurrents.noaa.gov/tides08/tab2wc1b. Dragovich, J.D., Pringle, P.T., and Walsh, T.J., 1994, Extent html and geometry of the Mid-Holocene Osceola mudflow in the Schuster, J.E., 2000, Digital 1:100,000-scale geology of Puget Lowland—Implications for Holocene sedimentation Washington State: Washington Division of Geology and and paleogeography: Washington State Department of Earth Resources Open File Report 2005-3, digital coverage, Natural Resources, Washington Geology, v. 22, no. 3, accessed August 26, 2009, at http://www.dnr.wa.gov/ p. 3-26. ResearchScience/Topics/GeologyPublicationsLibrary/ Drost, B.W., 2005, Quality-assurance plan for ground-water Pages/pub_ofr05-3.aspx activities: U.S. Geological Survey Open-File Report Tabor, R.W., 1994, Late Mesozoic and possible Early Tertiary 2005‑1126, 27 p., available at http://pubs.usgs.gov/ accretion in western Washington State–The Helena- of/2005/1126/ Haystack mélange and the Darrington-Devils Mountain Easterbrook, D.J., 1969, Pleistocene chronology of the Puget fault zone: Geological Society of America Bulletin v. 106, Lowland and San Juan Islands, Washington: Geological no. 2, p. 217-232, 1 plate. Society of America Bulletin, v. 80, no. 11, p. 2273-2286. Thomas, B.E., Wilkinson, J.M., and Embry, S.S., 1997, The Fasser, E.T., and Julich, R.J., 2009, Hydrographs showing ground-water system and ground-water quality in western ground-water level changes for selected wells in the lower Snohomish County, Washington: U.S. Geological Survey Skagit River basin, Washington: U.S. Geological Survey Water-Resources Investigations Report 96 4312, 218 p., Data Series 441, available at http://pubs.usgs.gov/ds/441/ 9 plates. 22 Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington

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For more information concerning the research in this report, contact the Director, Washington Water Science Center U.S. Geological Survey, 934 Broadway — Suite 300 Tacoma, Washington 98402 http://wa.water.usgs.gov Savoca and others— Shallow Groundwater Movement in the Skagit River Delta Area, Skagit County, Washington—Scientific Investigations Report 2009–5208