U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5013

Prepared in cooperation with the U.S. Fish and Wildlife Service and the Joint Water Commission

Evaluation of Restoration Alternatives Using Water-Budget 

Tools for the Wapato Lake National Wildlife Refuge, 

Northwestern Oregon
Cover: Pumphouse structures at northern end of Wapato Lake (background, 

right) where water is discharged to Wapato Creek (foreground), northwestern 

Oregon. Photograph by Stewart Rounds, U.S. Geological Survey, March 17, 2016.
Evaluation of Restoration Alternatives 

Using Water-Budget Tools for the 

Wapato Lake National Wildlife Refuge, 

Northwestern Oregon

By Stewart A. Rounds, T. Zach Freed, Daniel T. Snyder, Cassandra D. Smith, 

Micelis C. Doyle, Erin Holmes, Curt Mykut, Tim Mayer, Erin Stockenberg, and 

Stephen L. Pilson

Prepared in cooperation with the U.S. Fish and Wildlife Service and the Joint 

Water Commission

Scientific Investigations Report 2020–5013

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Rounds, S.A., Freed, T.Z., Snyder, D.T., Smith, C.D., Doyle, M.C., Holmes, E., Mykut, C., Mayer, T., Stockenberg, E., 

and Pilson, S.L., 2020, Evaluation of restoration alternatives using water-budget tools for the Wapato Lake National 

Wildlife Refuge, northwestern Oregon: U.S. Geological Survey Scientific Investigations Report 2020–5013, 26 p., 

https://doi.org/10.3133/sir20205013.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Study Site ...............................................................................................................................................2

Restoration Scenarios .........................................................................................................................5

Methods...........................................................................................................................................................5

Shoreline Management Tool and Topographic Data ......................................................................6

Water-Budget Construction ................................................................................................................6

Stage/Area and Stage/Volume Tables .....................................................................................6

Precipitation..................................................................................................................................6

Evaporation from Open Water ...................................................................................................8

Evapotranspiration from Land Surface ....................................................................................8

Pumping .........................................................................................................................................8

Changes in Storage .....................................................................................................................8

Seepage Through Levees, Groundwater Seepage, and Leakage Through 

Pumphouse ......................................................................................................................9

Water Management Scenario Tool ....................................................................................................9

Tributary Inflows ........................................................................................................................10

Weir Losses ................................................................................................................................10

Water Depths and Potential Habitat .......................................................................................11

Results—Water Budget and Water Management Scenarios .............................................................11

Water Budget ......................................................................................................................................11

Water-Management Scenarios ........................................................................................................13

Scenario 1—Reroute Three Tributaries and Install Variable-Elevation Weir ..................13

Scenario 2—Reroute Three Tributaries and Use Weir and Pumps ..................................18

Implications for Restoration and Water Management ..........................................................................22

Supplementary Material .............................................................................................................................24

Acknowledgments.......................................................................................................................................24

References Cited..........................................................................................................................................24

Figures

1. Photograph showing Wapato Lake when the lake was refilling with rainwater, 

northwestern Oregon, winter 2015 ............................................................................................3

2. Map showing Tualatin River Basin, including Wapato Lake in southwestern part 

of the basin, near Gaston, and Joint Water Commission water treatment plant 

downstream of Wapato Lake, northwestern Oregon .............................................................3

3. Map showing Wapato lakebed and study area for the water budget inside the 

levees of the lake, colored by land-surface elevation, southeast of Gaston, 

northwestern Oregon ...................................................................................................................4

4. Diagram showing potential input and loss components considered in the water 

budget of Wapato Lake, northwestern Oregon .......................................................................7

5. Diagram showing processes moving water into and through the unsaturated 

zone and included in the water budget of Wapato Lake, northwestern Oregon ...............7
iv

6. Graph showing area and volume as a function of water-surface elevation of 

Wapato Lake near Gaston, northwestern Oregon ................................................................12

7. Pie diagrams showing percentages of total water inputs to, and losses from 

the Wapato lakebed near Gaston, northwestern Oregon, as computed from the 

daily lake water budget for October 2011 through March 2014 ..........................................13

8. Graphs of monthly aggregated results of the daily water budget showing 

estimated inflows, estimated outflows, measured changes in lake storage 

and estimated changes in water storage in the saturated and unsaturated 

zones, and monthly residuals of the water-budget analysis, for Wapato Lake 

near Gaston, northwestern Oregon, for the calibration period of October 2011 

through March 2014 ...................................................................................................................14

9. Graph showing daily mean water level at Wapato Lake as measured in 

the canal near the pumphouse (USGS station 14202630), near Gaston, 

northwestern Oregon .................................................................................................................15

10. Graph showing water levels predicted from scenario 1 using median flow and 

meteorological conditions from water years 1992–2014 at Wapato Lake near 

Gaston, northwestern Oregon ..................................................................................................17

11. Pie diagrams showing percentages of total water inputs and losses as 

computed from the daily lake water budget for scenario 1 and using median 

flow and meteorological conditions from water years 1992–2014 at Wapato 

Lake near Gaston, northwestern Oregon ...............................................................................17

12. Graph showing water-depth and potential habitat areas associated with three 

water-depth ranges, as predicted from scenario 1 using median flow and 

meteorological conditions from water years 1992–2014 at Wapato Lake near 

Gaston, northwestern Oregon ..................................................................................................18

13. Maps showing specific locations that would meet certain water-depth criteria, 

using minimum and maximum depth targets for scenario 1, at Wapato Lake 

National Wildlife Refuge near Gaston, northwestern Oregon ............................................19

14. Graph showing water levels predicted from scenario 2 using median flow and 

meteorological conditions from water years 1992–2014 at Wapato Lake near 

Gaston, northwestern Oregon ..................................................................................................20

15. Pie diagrams showing percentages of total water inputs and losses as 

computed from the daily lake water budget for scenario 2 and using median 

flow and meteorological conditions from water years 1992–2014 at Wapato 

Lake near Gaston, northwestern Oregon ...............................................................................20

16. Graph showing water levels predicted from a modified scenario 2 using 

median flow and meteorological conditions from water years 1992–2014 and 

allowing the large pump to be used year-round at Wapato Lake near Gaston, 

northwestern Oregon .................................................................................................................21

17. Graph showing water-depth and potential habitat areas associated with three 

water-depth ranges, as predicted from scenario 2 using median flow and 

meteorological conditions from water years 1992–2014 at Wapato Lake near 

Gaston, northwestern Oregon ..................................................................................................21

18. Graph showing water-depth and potential habitat areas associated with 

three water-depth ranges, as predicted from a modified scenario 2 using 

median flow and meteorological conditions from water years 1992–2014 and 

allowing the large pump to be used year-round at Wapato Lake near Gaston, 

northwestern Oregon .................................................................................................................22

Figures—Continued
v

Tables

1. Drainage areas upstream of Wapato Lake tributary sites near Gaston, 

northwestern Oregon .................................................................................................................11

2. Hypothetical scenarios tested with the Water Management Scenario Tool for 

Wapato Lake near Gaston, northwestern Oregon ................................................................16

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

International System of Units to U.S. customary units

Multiply By To obtain

 Length

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre

hectare (ha) 2.471 acre

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft)

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d)

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F = (1.8 × °C) + 32.
vi

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above the vertical datum.

Abbreviations

AET actual evapotranspiration

DEM digital elevation model

FOGO Forest Grove AgriMet station

GIS geographic information system

JWC Joint Water Commission

lidar light detection and ranging

NWR National Wildlife Refuge

PET potential evapotranspiration

RK river kilometer

RM river mile

SMT Shoreline Management Tool

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

WID Wapato Improvement District

WMST Water Management Scenario Tool

WTP water treatment plant
Evaluation of Restoration Alternatives Using 

Water-Budget Tools for the Wapato Lake National Wildlife 

Refuge, Northwestern Oregon

By Stewart A. Rounds1, T. Zach Freed2, Daniel T. Snyder1, Cassandra D. Smith1, Micelis C. Doyle1, 

Erin Holmes3, Curt Mykut3, Tim Mayer3, Erin Stockenberg3, and Stephen L. Pilson3

1U.S. Geological Survey

2The Nature Conservancy

3U.S. Fish and Wildlife Service

Abstract

The lakebed in Wapato Lake National Wildlife Refuge 

(NWR) in northwestern Oregon was farmed for decades 

prior to the establishment of the refuge in 2013. Planning for 

restoration of these lands required extensive data collection 

and construction of a water budget and tools to design and 

evaluate potential restoration strategies. The U.S. Geological 

Survey (USGS) and U.S. Fish and Wildlife Service worked 

together to monitor streamflow and water levels in and around 

Wapato Lake NWR, apply the USGS Shoreline Management 

Tool (SMT), then construct and apply a water-budget-based 

Water Management Scenario Tool (WMST). The SMT was 

used to determine the spatial availability of different water 

depths (as potential habitat for different species) as a function 

of water level and other factors, based on topographic data. 

The WMST uses a water-budget approach to predict daily 

water levels, inflows, outflows, and areas of specific categories 

of water depth in the refuge over the course of a water year in 

response to a range of hydrologic and meteorological conditions and potential water-management strategies. In this study, 

two hypothetical water-management strategies were simulated 

to predict their effect on water levels and areas with specific 

water depths as an indicator of potential habitat. In the first 

scenario, several tributaries that had been diverted around the 

lakebed since the 1930s were reconnected to the lake, and an 

outflow weir was used to control lake level and to create a 

lake and seasonal wetlands of specific depths. In the second 

scenario, an outflow weir was combined with pumps to help 

meet target lake levels. Results showed that reconnecting the 

largest three tributaries to Wapato Lake would provide sufficient water to create a range of aquatic conditions in most 

years. For a median water year, rainfall and tributary flows in 

these scenarios provided 99 percent of total inputs to the lake, 

whereas pumping, weir outflows, and open-water evaporation 

accounted for 95–97 percent of losses. Management of lake 

levels could be accomplished with a variable-elevation outflow 

weir or a combination of a weir and pumps. The lake would 

take longer to fill to a higher seasonal target level during a 

dry year. Without an outflow weir or other means of allowing 

water to flow out of the lake, the largest of two existing pumps 

would need to be used during late spring or early summer to 

attain a lower seasonal target water level in summer. Highwater conditions downstream of Wapato Lake may prevent the 

use of a simple outflow weir, as historical downstream water 

levels in winter and spring sometimes were higher than the 

target water levels used in these scenarios. Water-budget-based 

methods applied in this study have proven to be valuable for 

the design and evaluation of potential restoration strategies at 

Wapato Lake NWR.

Introduction

The Wapato Lake National Wildlife Refuge (NWR) 

is located in the upper Tualatin River Basin near the city 

of Gaston in northwestern Oregon and was established in 

December 2013 as the 562nd refuge in the National Wildlife 

Refuge System operated by the U.S. Fish and Wildlife 

Service (USFWS). Wapato Lake NWR contains important 

wetland and riparian areas that USFWS plans to enhance 

through restoration and water management. Restored habitats may support various fish species, including salmonids 

listed as threatened or endangered under the Endangered 

Species Act of 1973 (Public Law 93–205, 87 Stat. 884, as 

amended) as well as wildlife and migratory birds including 

breeding landbirds, wading birds, shorebirds, and wintering 

waterfowl. Historically, Wapato Lake was one of the most 

important waterfowl sites in northwestern Oregon (U.S. Fish 

and Wildlife Service, 2007). The motivation for establishing Wapato Lake as a national wildlife refuge was the area’s 

high potential for restoration of stream, wetland, and riparian systems.
2 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

Prior to western settlement, Wapato Lake was influenced 

by precipitation, seasonal floodwaters from the Tualatin River, 

and water flowing from several tributary streams. Alterations 

to Wapato Lake began in the late 19th century when settlers 

changed the surrounding topography and hydrology of the 

lake to enable farming of the lakebed during the summer dry 

season. In the 1930s, levees were built around the perimeter of 

the lakebed, with canals on the outside of the levees to capture 

and divert tributary streams that originally fed the lake. Cut off

from the surrounding rivers, a lake still formed in winter from 

seasonal rains, but the levees allowed the lake to be pumped 

out in spring to enable farming on the lakebed during summer. 

Conversion from a predominantly wet environment to dry 

farmland caused subsidence through aerobic decay of exposed 

peat soils; the land surface elevation of the lakebed has 

decreased substantially from its pre-settlement level (Christy, 

2015). Converting the lakebed to agricultural use removed 

the native plants, and areas not actively farmed became 

overgrown with invasive vegetation. Although changes to the 

lake disrupted its natural hydrologic processes and altered 

the associated habitat, the area likely can be restored to a 

mixture of riparian forest, seasonal scrub-shrub wetlands, and 

herbaceous wetlands with diverse land-cover types to support a wide range of fish and wildlife (U.S. Fish and Wildlife 

Service, 2007).

In 2011, USFWS needed to quantitatively assess the 

water resources available to Wapato Lake NWR to define the 

type of restoration scenarios that might be possible, and to 

begin to determine how water resources might be managed 

under those scenarios. Evaluation of restoration alternatives is 

a complex balance between the optimization of available habitat for fish, birds, and wildlife and the appropriate management 

of river and lake hydrology and water quality. USFWS and 

U.S. Geological Survey (USGS) staff monitored streamflows 

and water levels in the Wapato Lake area from October 2011 

to April 2013. Those data then were used to develop a daily 

water budget to determine water fluxes throughout the system. 

USFWS collected high-resolution light detection and ranging 

(lidar) data and used those data to create a digital elevation 

model (DEM) of the Wapato Lake area. Using that DEM, 

the USGS Shoreline Management Tool (SMT; Snyder and 

others, 2013) was applied to Wapato Lake to assess the relations between surface-water stage and water depth, inundated 

area, and water volume in the lake and surrounding area. 

Combining information from the water budget, the SMT, and 

the DEM, a spreadsheet-based Water Management Scenario 

Tool (WMST) was created to allow water-resource managers 

to design potential restoration and water-management strategies and to evaluate their effects on the hydrology and habitat 

characteristics of Wapato Lake under a range of conditions.

Purpose and Scope

The objectives of this study were to (1) construct a water 

budget for Wapato Lake, (2) develop and apply the WMST for 

Wapato Lake, (3) and describe and document how the WMST, 

in conjunction with the SMT, could be used to assist restoration and water management planning for Wapato Lake NWR. 

The water budget was developed using estimated and measured water fluxes for October 2011 through March 2014—the 

time period with the most available data. The WMST then 

was constructed using estimated and measured water fluxes 

for water years 1992–2014. Both the water budget and WMST 

focused on the lakebed area constrained by surrounding 

levees, which constitutes the spatial extent of modern Wapato 

Lake. This report describes the water budget and technical 

development of the WMST, then describes and discusses 

two hypothetical water-management scenarios in which the 

WMST was used to evaluate resulting lake levels and water 

depths that are critical for determining the quantity of potential 

habitat for target species.

Study Site

Wapato Lake NWR is located between the Coast Range 

and the Chehalem Mountains near river kilometer (RK) 100 

(river mile [RM] 62) of the Tualatin River near the city of 

Gaston in northwestern Oregon (figs. 1–2). The Tualatin River 

flows east out of the Coast Range, then turns north on the 

valley floor near Wapato Lake to skirt the northern edge of 

the Chehalem Mountains before continuing eastward toward 

its confluence with the Willamette River. Water discharged 

from the Wapato Lake area flows northward in Wapato Creek, 

exiting the low-lying areas of the lakebed at Gaston Road and 

joining the Tualatin River about 3 km (1.9 mi) downstream 

near the river’s confluence with Scoggins Creek (fig. 2). 

Summertime streamflow in the Tualatin River is augmented 

from two upstream reservoirs—Henry Hagg Lake on Scoggins 

Creek and Barney Reservoir in the adjacent Trask River Basin 

(through an interbasin diversion). Wapato Lake is located 

upstream of an important municipal water intake at RK 90.3 

(RM 56.1); the drinking-water treatment plant operated by 

Joint Water Commission serves more than 300,000 people in 

the basin.

Near-surface alluvium sediments of the Tualatin River 

Basin are underlain with Missoula Flood deposits of the 

Willamette Silt formation, composed of micaceous clay and 

sand materials in the area near Wapato Lake (Wilson, 1998). 

The bed of Wapato Lake consists of poorly drained soils of 

high organic content in the Labish soil series, described as 

0.3–0.6 m (1–2 ft) of black, mucky clay interspersed with 

lenses of organic material and underlain by thick peat deposits below about 1 m (3 ft) (Green, 1982). Peat deposits in the 

Labish and Semiahmoo soils of this area were reported in the 

1930s to be as thick as 3.5 m (11.5 ft) (Dachnowski-Stokes, 

1936; Christy, 2015).
Introduction 3

Figure 1. Wapato Lake when the lake was refilling with rainwater, northwestern Oregon, winter 2015. Photograph by Stewart Rounds, 

U.S. Geological Survey, January 27, 2015.

WILLAMETTE

TUALATIN RIVER

RIVER

RIVER

COLUMBIA

Hagg

Lake

Wapato

Lake

Barney

Reservoir

East

Fork

Dairy

Gales

Creek

Scoggins

Creek

McKay

Creek

Rock

Creek

Fanno

Creek

Beaverton Creek

Creek Chicken

0

5

10

35 25 15

45

20

50

85

55

90

65 95 60 70 80 75

40

30

125

130

120

100 110 105 115

205

5

26

5

205

84

217

5

WASHINGTON

COLUMBIA

TILLAMOOK

YAMHILL

CLACKAMAS

MULTNOMAH

TUALATIN MOUNTAINS

CHEHALEM MOUNTAINS

COAST RANGE

Lake

Oswego

Tualatin

Sherwood

Portland

Tigard

Beaverton

Cornelius Hillsboro

Forest

Grove

Cherry

Grove

Gaston

JWC

WTP

OREGON

Study

Area

50

EXPLANATION

Designated urban growth boundary (2009)

Basin boundary

Water treatment plant

River kilometer

JWC

WTP

0 10

10

20 MILES

0 20 KILOMETERS

Base map modified from U.S. Geological Survey and Metro digital data sets (1:24,000)

Projection: Oregon Lambert, North American Datum 1983

123°30’ 123°15' 123° 122°45' 122°30'

45°

45'

45°

37'

30"

45°

30'

45°

22'

30"

Figure 2. Tualatin River Basin, including Wapato Lake in southwestern part of the basin, near Gaston, and Joint 

Water Commission (JWC) water treatment plant (WTP) downstream of Wapato Lake, northwestern Oregon.

Wapato Lake was the only large natural lake in the 

Tualatin River Basin (Hart and Newcomb, 1965). In 1882, 

the Tualatin River was channelized between Gaston and the 

river’s confluence with Scoggins Creek, bypassing the section of river that flowed closer to the outlet of Wapato Lake 

(Washington County, 1883), thus weakening the strong winter 

high-water connection between the lake and river and making 

it feasible to drain the lake for farming. By 1895, Wapato Lake 

had been drained by the installation of canals (Cass and Miner, 

1993). The Wapato Improvement District (WID), a statesponsored irrigation and drainage district, was formed in the 

1930s to manage the lake. The WID installed 8.9 km 
4 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

56.08

53.04

49.38

EXPLANATION

Elevation, in meters above 

North American 

Vertical Datum of 1988

0 0.5

0.5

1 MILE

0 1 KILOMETERS

123°09'30" 123°09' 123°08'30" 123°08' 123°07'30" 123°07' 123°06'30" 123°06' 123°05'30"

45°

26'

30"

45°

26'

45°

25'

30"

45°

25'

45°

24'

30"

45°

24'

Base map coyright from Esri, 2019. Coordinate system: Universal Transverse 

Mercator, Zone 10 North. Horizontal datum is North American Datum of 1983.

Wapato 

lakebed

Figure 3. Wapato lakebed and study area for the water budget inside the levees of the lake, colored by land-surface elevation, 

southeast of Gaston, northwestern Oregon. Water flow direction is indicated by arrows along the creeks and rivers. Diversion 

canal from the Tualatin River to the canal outside the levees at the north end of the lake is indicated by a brown line. Rd, Road.

(5.5 mi) of levees and exterior canals around the lake, a lift 

pump station at the northern end of the lake, and interior 

ditches to facilitate drainage of the lake to the pumphouse 

(fig. 3). Creeks that historically flowed into the lake (Ayers, 

Goodin, Hill, Olsen, and Wapato Creeks) were diverted around 

the lake outside the levees to allow pumps to more easily 

empty the lake in spring so that the lakebed could be farmed 

in summer. Prior to channelization of the river and construction of levees, seasonal high water flowed into Wapato Lake 

from the Tualatin River, providing additional storage of flood 

waters and causing the lake to expand and contract in response 

to river conditions.
Methods 5

For many years, the canals outside the levees were used 

in summer to deliver water for irrigation. In 1975, the WID 

entered into agreement with the Tualatin Valley Irrigation 

District and the Bureau of Reclamation to allow the use of 

these canals for water delivery to customers on the periphery 

of the lake. In spring 2012 after USFWS had agreements to 

purchase most of the lakebed within the levees, the WID voted 

to disband and donate its assets to USFWS. A diversion canal 

from the Tualatin River at RK 99.6 (RM 61.9) to the Wapato 

canal system (fig. 3) was still used in 2014 to deliver water to 

irrigation district customers outside the lakebed as well as to 

any farmed areas within the lakebed.

Despite the construction of levees, channelization of the 

Tualatin River, and diversion of tributary streams, Wapato 

Lake forms and inundates the lakebed each winter through 

a combination of precipitation and subsurface seepage. 

USFWS has continued to manage the system by dewatering 

the lakebed in winter and spring and working with farmers 

to cultivate grain crops over most of the lakebed, an activity 

that minimizes the intrusion of invasive vegetation and provides additional forage for waterfowl. Drainage of the lake is 

accomplished with two pumps that lift water from the lakebed 

and discharge it to Wapato Creek near Gaston Road on the 

north end of the lake.

Restoration Scenarios

USFWS and its partners are working to restore Wapato 

Lake to create a year-round shallow lake surrounded by a 

seasonally inundated herbaceous wetland (U.S. Fish and 

Wildlife Service, 2019). USFWS and USGS evaluated a 

combination of potential actions, including reconnection of 

tributary streams to Wapato Lake, and using pumps and an 

adjustable-height outlet weir to manage lake levels. A scenario 

that includes an open lake outlet connecting Wapato Lake with 

Wapato Creek and the Tualatin River downstream was documented elsewhere by Rounds and others (2020). The intent 

of these scenarios was to maximize year-round shallow-water 

habitat for aquatic plants and animals as well as surrounding seasonal wetland areas to provide additional diversity of 

habitat for uses such as waterbird nesting. USFWS and USGS 

used the WMST to quantify the effects of these restoration 

scenarios on a daily basis throughout the water year, focusing 

on the extent and duration of lake inundation, depth/area/volume of the lake, and associated areas of specific water depths 

that relate to potential habitat for species such as waterbirds.

Reconnection of several tributary streams to Wapato Lake 

for part of the year would provide substantial additional water 

(and additional management flexibility) to fill the lake and to 

create a range of water depths. Without tributary inflows, rainfall and subsurface seepage might not be sufficient to create 

and maintain specific (higher) water levels during a dry year. 

Continuous inflows and outflows caused by tributary reconnection also could be useful in avoiding poor water-quality 

conditions that might result from long residence times and 

stagnation of water in the lake. However, tributary reconnection without an active means of exporting water from the lake 

might create deep-water conditions that have less habitat value 

for many waterbirds, and could create a flooding threat to 

adjacent homes and agricultural fields.

Restored lake levels could be controlled in a number of 

ways—through (1) an open-outlet connection with Wapato 

Creek and the Tualatin River downstream, (2) continued use 

of levees and pumps, or (3) installation of a variable-height 

weir at the lake outlet, to list just a few. During conditions of 

high water in the Tualatin River, water might back up through 

Wapato Creek and enter the lake if not prevented from doing 

so by levees and sufficiently high outlet structures. Allowing 

floodwaters from the Tualatin River to inundate the lake could 

be beneficial in some ways, but may result in deeper water 

in winter than desired for target species; therefore, an open 

connection between Wapato Lake and downstream reaches of 

Wapato Creek requires critical evaluation, despite its seeming 

simplicity.

Evaluation of future restoration and management alternatives for Wapato Lake NWR must consider local habitat conditions as well as implications for the broader Tualatin River 

watershed. The Tualatin River provides aquatic and riparian 

habitat for many fish and wildlife species and serves as a 

water supply for extensive agricultural and municipal areas. 

Management of Wapato Lake NWR must avoid flooding of 

adjacent private farmlands when possible. Permanent inundation throughout the season might lead to increased mosquito 

activity and water-quality problems in the lake. Discharges 

from Wapato Lake also can affect water quality downstream, 

with important ramifications for municipal water treatment 

and recreation. For example, a levee failure at Wapato Lake in 

December of 2007 led to deeper-than-normal water in the lake 

in winter that could not be pumped out until June and July of 

2008; typically, the lake was pumped out by the end of April 

each year. An algal bloom in the lake and high concentrations 

of dissolved organic matter and phosphorus, in combination with pumping to evacuate the lake in June and July of 

2008, caused municipal water-treatment problems as well 

as instream water-quality problems about 70-100 km 

(44-62 mi) downstream in the Tualatin River (Bonn, 2008; 

Rounds and others, 2015). The Tualatin River has Total 

Maximum Daily Load programs in place for parameters such 

as phos-phorus and ammonia that could be affected by Wapato 

Lake discharges. Downstream effects on river hydrology and 

water quality from refuge water management must be 

considered in any restoration alternatives.

Methods

Three tools were used in this study to assess potential 

water-management strategies for Wapato Lake NWR and 

to evaluate water-depth areas created by those strategies 

over a range of hydrologic conditions. The USGS Shoreline 
6 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

Management Tool (SMT) was applied to Wapato Lake to 

assess the topography of the lakebed and to quantify the types 

and amounts of potential habitat for target species produced 

under a range of lake levels. A water budget for Wapato Lake 

was constructed to quantify the inputs and losses of water 

to and from the lakebed and to estimate the rates of several 

unmeasured seepage inputs. Using information from the SMT 

and the water budget, a Water Management Scenario Tool 

(WMST) was developed to predict daily lake levels throughout the year in response to different water-management strategies and climatic conditions, with ties to the amount and type 

of habitat associated with the predicted water levels.

Shoreline Management Tool and Topographic 

Data

The USGS SMT is a geographic information system 

(GIS) software program that runs in ArcMapTM and is designed 

to quantify the results of water-management strategies for 

areas subject to periodic inundation such as wetlands and 

seasonal lakes (Snyder and others, 2013). Using land-surface 

topographic data, the tool allows resource managers to calculate water quantity, water depth, area of inundation, and area 

of dry land based on surface-water levels. Such information 

can be useful to balance competing management priorities 

and needs, including water supply, water quality, aquatic and 

terrestrial habitat for plants and animals, and human use of 

water and land areas. In addition to quantifying water depths 

and the areas and volumes of inundation, the SMT allows 

the user to define habitat criteria such as water depth, landsurface slope and aspect, or other factors, and then determines 

the quantity and location of areas meeting those criteria. 

Documentation and the GIS program are available in Snyder 

and others (2013).

USFWS collected high-resolution lidar data for the 

Wapato Lake area to define the elevation of the lakebed and 

surrounding area with centimeter accuracy. These data were 

collected in May 2012 when the lake was empty and vegetation on levees and other areas of interest had not fully leafed 

out. After independent quality-assurance reviews were completed and the data were tied to multiple surveyed benchmark 

locations, the lidar data were converted to a DEM in GIS. This 

DEM became the basis for the analysis of potential habitat 

areas for the target species in the SMT, and were used to generate stage/area and stage/volume tables for Wapato Lake.

Water-Budget Construction

Evaluation of hypothetical restoration and watermanagement strategies for Wapato Lake NWR requires a 

knowledge of the lake water budget to predict the water levels 

and areas of water depths that might result. As part of the 

development of such a predictive tool, data were collected 

and a water budget was constructed for Wapato Lake for 

October 2011 through March 2014. Several streamflow and 

water-level (stage) gages were operated in and around the lake 

for the 18 months from October 2011 to March 2013. To augment flow and water-level measurements, data were obtained 

for meteorological parameters as well as for pumping duration 

and frequency. The DEM derived from lidar data was used to 

create relations between stage, area, and volume so that the 

surface area and volume of the lake could be estimated from 

its water level.

The water budget for the lake comprises inputs, losses, 

and storage. Water is added to the system as precipitation, 

seepage through levees, groundwater discharge, and leakage 

through the pumphouse (fig. 4). Water typically leaves the 

system through open-water evaporation, evapotranspiration, 

groundwater recharge, and pumping. No data were available 

to quantify any summertime irrigation-water inputs; such 

inputs were not included in the water budget because the lake 

was dry in summer for the period of interest and the resulting 

water table was at or below the lakebed. In the future, tributaries could be reconnected to the lake to provide additional 

inputs, and outflow weirs or other mechanisms could be added 

to control or manage outflows. Within the area bounded by 

levees, water is stored in the lake as well as in the subsurface 

soil and groundwater system (fig. 5). Combining all known 

and measured components of the water budget, the residuals of a comparison between modeled and measured data for 

October 2011 through March 2014 were used to estimate the 

unmeasured components of the water budget, such as groundwater discharge and seepage of water through the levees. The 

following sections describe each water-budget component in 

more detail.

Stage/Area and Stage/Volume Tables

Python scripts developed for use with the SMT (Snyder 

and others, 2013) were used to analyze the land-surface 

topography in the DEM and to create tables relating lake water 

level to lake area and lake volume—also known as stage/

area and stage/volume tables. The analysis is similar to that of 

filling a bathtub (or lakebed) and determining the surface area 

and volume of the resulting waterbody for each incremental 

increase in water-level elevation. Tables of surface area and 

lake volume that correspond to each water level then can be 

used to predict lake areas and volumes for use in the waterbudget calculations.

Precipitation

Precipitation data were obtained from the Forest Grove 

AgriMet station (FOGO) in Verboort, Oregon, about 

13 km north-northeast of Wapato Lake. The FOGO data were 

multiplied by 1.06 to account for the predicted difference 

between annual mean precipitation at FOGO and at Wapato 

Lake as estimated by the PRISM model (Daly and others, 

1994), which predicts spatial variations in precipitation based 

on measured rainfall and topographic features. When data 
Methods 7

Estimated 

Stage in 

Wapato Creek

Groundwater 

Seepage

Weir 

Outflow

Pumping

Precipitation

Evapotranspiration

Open-Water Evaporation

Levee 

Seepage

Pumphouse 

Leakage

Diverted Connected 

Tributaries

Soil

(Groundwater Storage)

Lake

(Surface-Water Storage)

Wapato Lakebed

Figure 4. Potential input and loss components considered in the water budget of Wapato Lake, northwestern Oregon. Blue arrows 

indicate inputs, red arrows indicate losses, and dashed lines indicate feedback processes in the calculations.

evapotranspiration precipitation

runoff to lake

infiltration

Saturated Zone

(Groundwater Storage)

Lake

(Surface-Water Storage)

Unsaturated 

Zone

(Soil Storage) water table

drainage

Figure 5. Processes moving water into and through the unsaturated zone and included 

in the water budget of Wapato Lake, northwestern Oregon. Blue arrows indicate inputs of 

water to the lake system and red arrow indicates a loss.
8 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

from FOGO were not available, uncorrected precipitation 

values from the nearby Dilley 1S National Weather Service 

cooperative precipitation station (site 352325) were used; no 

correction was needed because of the proximity of this site to 

Wapato Lake. Percentiles of daily precipitation (median, 10th, 

25th, 75th, 90th) were computed for later use by calculating 

the daily cumulative rainfall rate for each water year from 

1992 to 2014, then computing the percentile for each day.

Evaporation from Open Water

Evaporation can be estimated with various methods and 

typically depends on the vapor pressure of water, relative 

humidity, and wind speed, among other factors (Finch and 

Calver, 2008). In the absence of water-temperature data from 

the lake, however, an alternate approach was needed. Openwater evaporation from Wapato Lake was estimated using 

methods developed by Linacre (1993), who simplified the 

Penman evaporation equation to create a relation that relies 

only on air temperature, dew-point temperature, wind speed, 

solar radiation, and site elevation data:

 (1)

where

E is the estimated evaporation rate in 

millimeters per day,

T is the daily mean air temperature in 

degrees Celsius,

z is the site elevation (50.3 meters),

Rs is the solar irradiance in Watts per 

square meter,

u is wind speed in meters per second at 2 meters

height, and

Td is dew-point temperature in degrees Celsius.

Like the precipitation data, air temperature, solar 

radiation, wind speed, and dew-point temperature data 

were obtained from the nearby AgriMet station (FOGO) in 

Verboort, Oregon. The total loss of water from the lake due to 

open-water evaporation was computed by applying this rate 

over the calculated surface area of the lake on any particular 

day. Percentiles of the daily open-water evaporation rates 

(median, 10th, 25th, 75th, 90th) were computed for later use 

by calculating the daily cumulative open-water evaporation 

rate for each water year from 1992 to 2014, then computing 

the percentile for each day.

Evapotranspiration from Land Surface

Evapotranspiration is the combined loss of water to the 

atmosphere through evaporation from soil and land surfaces 

and through transpiration of water via the stomata of vegetation. Potential evapotranspiration (PET) is the quantity of 

water that could be lost through evapotranspiration, typically calculated for a particular crop type and assuming that 

adequate water is available in the soil. For this study, daily 

PET data were obtained from the FOGO AgriMet station. 

When adequate supplies of water are available to soils, the 

actual evapotranspiration (AET) is assumed to equal the 

PET; otherwise, it is common to adjust the AET estimate by 

multiplying PET by some factor to account for the scarcity of 

moisture. For the region near Wapato Lake, a factor of 0.7 was 

derived from information provided by Thompson and others 

(2012). When the daily precipitation rate exceeded the daily 

PET, AET was set to PET; otherwise, AET was estimated by 

applying the 0.7 factor to PET. These loss rates were applied 

to the area of the Wapato lakebed that was not occupied by 

open water each day.

Pumping

Two pumps were located at the pumphouse at Wapato 

Lake, simply referred to as the “large pump” and the “small 

pump.” For downstream water-quality management reasons, 

the large pump typically is not used during May–October, 

whereas the small pump can be used anytime. Data were available during 2011–14 indicating when each pump was turned 

on or off, but pump discharges were not directly measured 

and no documentation was available to characterize pumping 

rates as a function of lift (the water-level difference between 

the pump intake and the discharge point on the other side of 

the levee). Pumping rates were estimated for the water budget 

by creating relations between lake-stage elevation, streamflow 

measured in Wapato Creek downstream of the pumphouse, 

and the duration and timing of pumping episodes. The pumping rate for the small pump originally was estimated to be 

0.17 m3/s (6.0 ft3/s) and unaffected by variations in the 

required lift; subsequent calibration of the water budget 

increased that rate to 0.20 m3/s (6.9 ft3/s). For the large pump, 

the minimum pumping rate was estimated to be 0.453 m3/s 

(16.0 ft3/s), and the maximum pumping rate was originally 

estimated to be 0.651 m3/s (23.0 ft3/s); subsequent calibration 

of the water budget increased the maximum rate to 0.883 m3/s 

(31.2 ft3/s). To approximate a dependence on the required lift, 

the pumping rate of the large pump was linearly interpolated 

between the minimum rate at and below a lake-stage elevation 

of 49.40 m (162.07 ft) and the maximum pumping rate at and 

above an elevation of 49.50 m (162.40 ft). In the water-budget 

calcula-tions (2011–14), pumping losses accounted for the 

fraction of the day that each pump was operating. In the 

WMST calcula-tions, pumping losses were applied as if each 

pump was either on or off for the entire day.

Changes in Storage

Water on/in the lakebed can be stored either in the lake 

or in the groundwater and soil system (figs. 4–5). The volume 

of water in the lake was computed from the stage/volume 

, E T

 

  

  





( . .

. . . .

)

[ ( )

0 015 0 00042 10

0 8 40 25 10 87 10

6

5

z

R z s



 u   T T d ]
Methods 9

table. For the soil and groundwater system, it was necessary to 

account for the fact that water can only fill open pore spaces. 

In the groundwater zone (below the water table), pore spaces 

are entirely filled with water, whereas in the unsaturated zone 

(above the water table), pore spaces typically are partially 

filled with water. Based on values from the Natural Resources 

Conservation Service SNOTEL station at Miller Woods, 

Oregon, about 23 km (14 mi) south-southwest of the study 

area (the closest SNOTEL station with geologic characteristics similar to the study area), the effective porosity (fraction 

of open pore space) of the soil is about 39 percent and the 

unsaturated soil zone typically has a moisture content of 25 

percent. These values were used to determine the volume of 

water stored in the soil and groundwater systems for the initial 

lake water budget, and the moisture content of the unsaturated 

zone was assumed to be constant. Inputs and losses to the 

lake water budget were used to compute a change in overall storage, and that daily change in storage was distributed 

among the lake, groundwater, and unsaturated zones using the 

assumption that the water table in the lakebed was at the same 

elevation as the lake level.

Measurements of water level in the internal lakebed canal 

near the pumphouse (USGS station 14202630) generally were 

assumed to be representative of the water-surface elevation of 

the lake, which in turn was assumed to be the elevation of the 

water table in the non-inundated parts of the lakebed. When 

pumps were active, however, drawdown in the canal near the 

pumphouse caused the measured water level at that station to 

underestimate the actual lake level. To remedy that problem 

in the water-budget calculations, a correction was applied to 

the measured water-level data whenever pumps were activated 

and a sharp decline in water level occurred due to drawdown. 

The corrected water levels then were used as an indication of 

the lake level.

Although a constant moisture content in the unsaturated 

zone was assumed for the initial water budget for the lake, it 

was determined that a more rigorous treatment of the unsaturated zone (fig. 5) was needed for scenario evaluations. For 

that purpose, the moisture content of the unsaturated zone 

was allowed to vary from 5 to 30 percent, with an initial value 

of 10 percent. The effective porosity also was decreased to 

30 percent. Using a variable moisture content in the unsaturated zone allowed that zone to “soak up” water from the first 

storms of autumn; otherwise, the water-budget calculations 

would assume those water inputs to infiltrate directly to the 

water table and cause the lake to form too quickly from a dry 

initial lakebed. Runoff from the soil surface directly to the 

lake was computed from a classic runoff equation (National 

Resources Conservation Service, 2004) using curve number 88, and the remaining rainfall was assumed to be absorbed 

into the unsaturated zone. Once pores in the unsaturated zone 

were filled, excess water entering the unsaturated zone would 

push water through to the saturated zone or run off to the 

lake, thus increasing the elevation of the water table and the 

lake level.

Seepage Through Levees, Groundwater 

Seepage, and Leakage Through Pumphouse

Groundwater seepage and levee seepage components of 

the water budget (fig. 4) were estimated using porous-media 

flow equations with estimated values of water levels and leakance. Seepage is proportional to the difference in water level 

between two points (head difference) and the ability of the 

sediments between the points to transmit water (hydraulic conductivity), and inversely proportional to the thickness of the 

sediments. For the water budget, leakance was used, which is 

the ratio of the hydraulic conductivity of the soil to the thickness through which the head is transmitted, with units of sec-1. 

The water gained or lost was computed as a head or waterlevel difference through an effective area times the leakance. 

The leakance values for each of these flow processes were 

calibrated based on the residuals of the 2011–14 water budget. 

Specific values were set by minimizing residuals while staying 

within a range of expected values for each leakance calculation. For groundwater seepage, the water level in sediments 

outside the lakebed was assumed to be at the same elevation as 

stage in Wapato Creek at Gaston Road and the water level in 

sediments in non-inundated areas of the lakebed was assumed 

to be identical to the lake level. If the Wapato Creek stage and 

water level in sediments outside the lake ever were lower than 

the lake level, then water would seep from the lake through the 

levees into surrounding canals, from the lake into the underlying sediments, and through the pumphouse to the exterior 

canals. Such flows would be losses instead of inputs.

Water Management Scenario Tool

The Water Management Scenario Tool (WMST) is a userinteractive spreadsheet that applies a water-budget approach 

to predict the effects of water-management strategies on lake 

level and habitat tied to water depths in Wapato Lake. The 

WMST draws on results from the 2011–14 Wapato Lake water 

budget, and allows users to:

• Impose a range of hydrologic and meteorological 

conditions (dry, wet, or normal years), drawing on data 

from water years 1992–2014 or a statistical percentile 

of those historical conditions;

• Specify an initial lake level and starting date (the 

default is to simulate an entire water year starting on 

October 1);

• Route some or all flows of selected tributary streams 

into the lakebed;

• Add an optional outlet weir with a time-variable crest 

elevation to control maximum lake levels;

• Schedule the use of pumps based on time of year and 

lake level; and

• Select a set of habitat criteria based on water depth, 

duration of water depth, and time of year.
10 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

The WMST calculates daily water budgets over a full water 

year (October 1 through September 30), including all daily 

inputs and losses, daily water levels in the lake, and daily 

measures of habitat quantity.

Use of the WMST can be general—to understand the 

feasibility of potential management strategies by simply 

assessing whether enough water is available to attain certain 

goals—or more detailed. The user can select from among six 

tributaries that may be routed into the lakebed, the percentage of the tributary flows that are routed into the lakebed, and 

the dates of two seasons (irrigation/non-irrigation, wet/dry, 

hunting/non-hunting, etc.) in which those percentages might 

be different. Tributary flows not routed to the lake are assumed 

to be diverted around the lake. Lake levels can be actively 

or passively managed in the WMST through the addition of 

a variable-height weir and (or) target time periods and water 

levels that activate the pumps. The WMST required some 

data and information beyond what was needed to construct a 

current water budget for Wapato Lake, including streamflow 

in all lake tributaries and meteorological data for water years 

1992–2014, algorithms to compute outflows over a weir, and 

criteria to compute habitat quantities related to water depth 

and lakebed topography. The following sections describe these 

data and features used by the WMST.

Tributary Inflows

Six tributaries drained into Wapato Lake before the 

levees were constructed (fig. 3; one tributary is small and 

unnamed). To allow WMST users to route one or more of 

these streams into the lake, flows in all these streams were 

required for water years 1992–2014. Such data did not exist, 

but daily mean streamflow in each tributary could be estimated 

using drainage-area ratio techniques and regressions against 

measured streamflow at other sites using methods outlined by 

Hirsch (1979). Streamflow in Ayers Creek, one of the Wapato 

Lake tributaries, was measured continuously by USGS at NE 

North Valley Road (station 14202550) from September 15, 

2011 to April 11, 2013. To extend the length of that record, 

measured streamflow data from nearby sites with similar characteristics were examined to determine the viability of regression models. Several sites and models were tested, and the best 

model relied on streamflow data from East Fork Dairy Creek 

near Meacham Corner (USGS station 14205400) and Fanno 

Creek at Durham (USGS station 14206950). Streamflow in 

Ayers Creek responded relatively quickly to rainfall; therefore, including a flashy urban stream such as Fanno Creek 

in the regression model was important for capturing that fast 

response. Forcing the model intercept through zero to prevent the prediction of negative flows, the model produced a 

relatively small mean absolute error of 0.05 m3/s (1.7 ft3/s) 

and an adjusted coefficient of determination (R2) of 0.85. The 

resulting model is:

QA = 0.02585 QEFD + 0.04315 QF, (2)

where

QA is the estimated daily mean discharge in Ayers 

Creek at USGS station 14202550,

QEFD is the daily mean discharge in East Fork Dairy 

Creek at USGS station 14205400, and

QF is the daily mean discharge in Fanno Creek at 

USGS station 14206950.

Discharge data from the East Fork Dairy and Fanno 

Creek sites had some gaps for water years 1992–2014. 

Datasets were extended using comparisons with nearby, and 

hydrologically similar, gaged streams. East Fork Dairy Creek 

data were extended using a regression with streamflow data 

from Sain Creek above Henry Hagg Lake (Oregon Water 

Resources Department station 14202920). Fanno Creek at 

Durham data were extended using a regression with measured 

streamflows from Fanno Creek at 56th Avenue (USGS station 

14206900). Using these measured and constructed datasets, 

equation 2 was used to generate a daily streamflow dataset for 

the Ayers Creek at NE North Valley Road site for water years 

1992–2014.

A drainage-area ratio method was used to estimate daily 

mean streamflow in each of the Wapato Lake tributaries from 

the Ayers Creek flow data:

Q1 = 

_

A1

AA

 QA, (3)

where

Q1 is the estimated daily mean discharge at the 

stream site of interest,

A1 is the drainage area upstream of the stream 

site of interest,

AA is the drainage area upstream of USGS station 

14202550 on Ayers Creek, and

QA is the daily mean discharge in Ayers Creek at 

USGS station 14202550.

Drainage areas upstream of each of the sites of interest 

(table 1) were estimated using StreamStats (U.S. Geological 

Survey, 2014). Percentiles of daily streamflow (median, 10th, 

25th, 75th, 90th) were computed for use in the WMST by 

calculating the daily cumulative streamflow for each water 

year from 1992 to 2014, then computing the percentile for 

each day.

Weir Losses

If an outflow weir is selected as part of a watermanagement scenario, the user can define as many as 24 crest 

elevations and the dates when those elevations are effective during the water year. The weir crest elevation sets a 

maximum lake level. If the computed water level for Wapato 
Results—Water Budget and Water Management Scenarios 11

Table 1. Drainage areas upstream of Wapato Lake tributary sites near Gaston, northwestern Oregon.

Site Drainage area (square kilometers)

Ayers Creek at NE North Valley Road 6.27

Ayers Creek near confluence with Wapato Creek 16.73

Wapato Creek at confluence with Ayers Creek 6.53

Hill Creek at confluence with canal at Wapato Lake levee 16.14

Goodin Creek at confluence with Wapato Creek 8.73

Unnamed creek between Goodin and Olsen Creeks 0.34

Olsen Creek at confluence with Wapato Creek 1.84

Lake exceeds the weir height on any day, all excess water is 

assumed to flow out of the lake and the lake level is set to the 

weir height. Although water levels downstream of the weir, 

outside the lake, are estimated by the WMST, flows over the 

weir are assumed to be in one direction—out of the lake. 

Backflows into the lake because of downstream high-water 

conditions are not allowed in the WMST (without specifying 

a downstream open outlet; see Rounds and others [2020] for 

more information on downstream open outlets), but the user is 

warned of the potential for such conditions.

Water Depths and Potential Habitat

The WMST can compute the presence, abundance, and 

duration of different areas of water-depth categories within 

Wapato Lake NWR, derived from the predicted water level 

combined with information derived from the SMT. To quantify 

the amount of specific types of potential habitat for target species for each day throughout a water year in a manner useful 

for restoration planning, one must first (1) define the target 

species and set some criteria to define the potential habitats of 

interest, and (2) track each parcel of the lakebed to determine 

when such criteria are met. Although many criteria might 

be used to define potential habitats for a target species, the 

WMST focuses on water depth as the most important criterion, as research has shown that water depth is a critical factor 

linking the abundance and foraging success of waterbirds 

(Bancroft and others, 2002; Bolduc and Afton, 2008; Lantz 

and others, 2011). Knowing the water level of the lake, it is 

straightforward to determine the lakebed area that falls into a 

particular depth/habitat category each day. The utility of this 

information may be limited, however, because an increase in 

lake water level from day to day or season to season might 

cause shallow-water conditions to move from one part of the 

lakebed to another. Such a move might be acceptable for some 

waterbirds, but not for rooted aquatic plants. To establish and 

maintain certain types of vegetation, it may be necessary to 

restrict the range of water depths on parcels of the lakebed 

during the summer growing season; therefore, the WMST 

tracks each parcel of land to compute the duration of inundation or the number of days that each parcel is inundated to 

certain depths.

To achieve such tracking, the SMT was used to identify 

contiguous parcels of the lakebed with nearly identical lakebed 

elevations. For Wapato Lake, 851 parcels were defined with 

a range of sizes (mean = 3,900 m2, or close to 1 acre). Those 

parcels then were numbered and programmed into the WMST 

along with their associated areas and average lakebed elevations. In this way, the WMST can calculate not only the area 

of the lakebed in each depth category every day, but also the 

duration of inundation in that depth category for every parcel 

and for a season of interest. At Wapato Lake, the water depth 

is likely to vary seasonally, with more water in the lake during 

winter than summer. In summer, a small lake may be surrounded by herbaceous wetland areas that become inundated 

in winter. Target water depths to produce desired habitats for 

waterbirds and aquatic plants likely are less than 1 m (3.28 

ft); greater depths may be acceptable during winter, but might 

not provide useful areas for target waterbird species and could 

prove problematic for certain vegetation species in summer.

Results—Water Budget and Water 

Management Scenarios

Water Budget

Analysis of topographic data for the lakebed indicated 

that as water is added to Wapato Lake, it spreads out to reach 

the constraining levees relatively quickly, gaining surface 

area rapidly at low lake levels, then accumulating volume 

in a near-linear fashion as the lake becomes deeper (fig. 6). 

Post-development lake levels tend to be at the low end of 

these curves, as the levees were built to keep the lake level 

low and to facilitate the process of emptying the lake in spring 

for farming in summer. For example, during October 2011 

through March 2014, the lake had a maximum water-surface 

elevation of about 51.1 m (167.7 ft), which corresponds to a 

surface area of about 276 hectares (683 acres) and a volume of 

about 2.75×106 m3 (2,230 acre-ft).
12 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

0

50

100

150

200

250

300

350

Lake surface area, in hectares

0

3

6

9

12

15

18

21

Lake volume, in 

millions of cubic meters

Lake water-surface elevation, in meters above NAVD 1988

Lake surface area

Lake volume

EXPLANATION

49.0 49.5 50.0 50.5 51.0 51.5 52.0 52.5 53.0 53.5 54.0 54.5 55.0 55.5 56.0 56.5

Figure 6. Area and volume as a function of water-surface elevation of Wapato Lake near 

Gaston, northwestern Oregon. One hectare = 0.01 square kilometers = 2.471 acres, and 1 

million cubic meters = 811 acre-feet. NAVD 88, North American Vertical Datum of 1988.

A detailed daily water budget for Wapato Lake was 

constructed from measurements of rainfall, pumping, and 

evapotranspiration; computed rates of open-water evaporation; 

storage in the lake and subsurface; and calibrated estimates 

of groundwater and levee seepage, and leakage through the 

pumphouse. The daily water budget for October 2011 through 

March 2014 was calibrated by adjusting several unknown 

parameters (leakance rates, for example) and comparing the 

sum of input (positive) and output (negative) water volumes 

to the total change in storage as computed by the measured 

change in lake volume and the estimated change in water 

volume in the saturated and unsaturated zones of the lakebed. 

The resulting water-volume errors can be expressed as inputs 

minus losses minus total change in storage, and as a percentage of the sum of the input and output volumes—in other 

words, the volume not properly accounted for in the change 

in storage as a function of the water estimated to be moving 

in and out of the lake. Aggregated monthly results for months 

when a lake was present showed that the mean absolute error 

(typical error) was about 34 percent, with a mean error (bias) 

of -7 percent, confirming that most of the water was accounted 

for, but that substantial uncertainties still exist.

Results from the water budget showed that precipitation was the dominant input to the lakebed (82 percent of 

all inputs) and pumping was the dominant loss (80 percent 

of all losses; figs. 7–8). During that time, all tributary flows 

were diverted around the lake and the lake was pumped dry 

in spring so that the lakebed could be farmed in summer. 

Although those conditions are not likely to mirror a restored 

condition of a lake surrounded by seasonal wetlands, such 

pre-restoration conditions were sufficient to provide insights 

into the lake’s hydrology and to estimate several of the waterbudget components that were not directly measured.

Seepage and leakage inputs in the water budget were 

minor relative to precipitation, but they still accounted for 

about 18 percent of all inputs, underscoring the importance of 

calibrating the water budget and measuring such inputs in the 

future. The calibrated leakance values were 8×10-8 sec-1 for 

seepage through the levees, 2×10-4 sec-1 for groundwater seepage, and 2.5×10-5 sec-1 for leakage through the pumphouse; an 

adjustment to the calibration after water year 2016 resulted in 

increased leakance values (4×10-7, 6×10-4, and 7.5×10-5 sec-1, 

respectively). Groundwater discharge (16.2 percent) was estimated to add more water to the lake than seepage through the 

levees (1.5 percent) and leakage through the pumphouse 

(0.4 percent), but more research and field measurements are 

needed to refine the rates of these inputs. More water was lost 

through evapotranspiration (14.9 percent) than open-water 

evaporation (5.5 percent), but the small quantity of open-water 

evaporation was caused mainly by the absence of a lake in 

summer. For the period of this analysis (October 2011 through 

March 2014), the lake was consistently inundated by 

December of each year, and typically was pumped dry by the 

end of March or April (fig. 9).

Despite the estimated nature of some inputs and losses, 

the water budget captured the major inputs and losses of 

Wapato Lake fairly well. Uncertainties in the predictions of 

the largest inputs (precipitation) and largest outputs (pumping) 

were small, although pumping rates were not definitively measured. The largest uncertainties among inputs and losses were 

associated with smaller inputs, which bodes well for garnering 

accurate insights from the results. For comparison, the total 

volume of water in diverted tributaries during October 2011 

through March 2014 (79.8×106 m3 or 64,723 acre-ft) was 

more than 8 times larger than all precipitation that fell to the 

lakebed (9.62×106 m3 or 7,802 acre-ft), and precipitation was 
Results—Water Budget and Water Management Scenarios 13

by far the largest input of water to the lake. Therefore, any 

routing of tributary flows into the lake in the future may constitute a primary and dominant input of water to Wapato Lake, 

further decreasing the significance of any errors in the seepage 

and leakage inputs.

The pre-restoration mixture of terrestrial and aquatic 

conditions in Wapato Lake varies seasonally. Appropriate 

conditions for aquatic animals and plants typically are either 

nonexistent or negligible from mid-April to the beginning of 

October because of farming activities and the absence of a 

lake. During October 2011 through March 2014, water depths 

never exceeded 1.7 m (5.6 ft). The period from October 2013 

to February 2014 had a relatively constant maximum water 

depth (0.56-0.86 m [1.8-2.8 ft], fig. 9) that provided shallowwater conditions and substantial quantities of terrestrial land 

cover around the margins of the lake.

Water-Management Scenarios

Two hypothetical water-management scenarios were 

tested with the WMST to assess potential changes to water 

levels and habitat in Wapato Lake NWR and to show the 

utility of the WMST (table 2). In these scenarios, several 

tributaries were routed into the lakebed rather than around it, 

and a combination of weirs and pumps were used to control 

the lake level. The goal was to create a range of shallow-water 

conditions in a mixture of lacustrine and seasonally inundated 

wetland environments. Optimal water depths will depend 

on the habitat requirements of the fish and wildlife species 

of most interest, but research has shown that shallow-water 

habitats (<0.25 m [<0.82 ft]) tend to favor many species of 

shorebirds, wading birds, and dabbling ducks, whereas deeper 

water is selected by species such as diving ducks and coots 

(Fredrickson, 1991; Murkin and others, 1997; Elphick and 

Oring, 1998; Colwell and Taft, 2000; Taft and others, 2002).

Scenario 1—Reroute Three Tributaries and 

Install Variable-Elevation Weir

In both scenarios, all flows in Ayers, Wapato, and Hill 

Creeks were rerouted into Wapato Lake rather than diverted 

around the lake, resulting in greatly increased inflows to the 

lake in a normal year. These three creeks are the largest of the 

Wapato Lake tributaries and probably are the easiest to route 

into the lake because they are upstream of other tributaries—

channels to route these tributaries into the lake could be cut 

directly through the levee without requiring infrastructure to 

accommodate the continued diversion of any upstream tributaries (fig. 3). The initial lake-stage elevation at the start of the 

simulated water year was set to 49.42 m (162.15 ft)—the level 

at which the lake is just barely dry. A range of hydrologic and 

meteorological conditions were imposed, drawn from conditions that occurred during water years 1992–2014. The meteorological conditions affect many components of the water 

budget, including open-water evaporation and evapotranspiration in addition to the more-obvious precipitation rate.

12.8% 14.9%

5.5%

66.8%

A. Water inputs

47.9%

34.1%

1.5%

16.2%

0.4%

B. Water losses

Precipitation on land

Precipitation on lake

Seepage through levees

Groundwater discharge

Leakage through 

pumphouse

EXPLANATION

Evapotranspiration

Open-water evaporation

Large pump

Small pump

EXPLANATION

Figure 7. Percentages of total water (A) inputs to, and (B) losses from the Wapato lakebed near Gaston, northwestern Oregon, 

as computed from the daily lake water budget for October 2011 through March 2014.
14 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

0

300

600

900

1,200

1,500

1,800

0

0.37

0.74

1.11

1.48

1.85

2.22

Volume, in acre-feet

Volume, in millions of cubic

meters

A. Monthly inflows

Volume, in millions of cubic meters

0

0.37

0.74

1.11

1.48

1.85

2.22

Volu

me, in acre-feet

0

300

600

900

1,200

1,500

1,800

B. Monthly outflows

-3.70

-2.47

-1.23

0

1.23

2.47

Volu

me, in millions of cubic

meters

-3,000

-2,000

-1,000

0

1,000

2,000

Volume, in acre-feet

C. Monthly changes in storage

-1,500

-1,000

-500

0

500

1,000

1,500

Volume, in acre-feet

-1.85

-1.23

-0.62

0.00

0.62

1.23

1.85

Volume, in millions of cubic meters

D. Monthly residuals

Seepage through levees

Leakage through pumphouse

Groundwater discharge

Precipitation on land

Precipitation on lake

EXPLANATION

Evapotranspiration

Open-water evaporation

Large pump

Small pump

EXPLANATION

EXPLANATION

Lake

Saturated zone

Unsaturated zone

EXPLANATION

Estimated inputs minus estimated outputs minus

measured and estimated change in storage

O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M

O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M

O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M

2011 2012 2013 2014

O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M

Figure 8. Monthly aggregated results of the daily water budget showing (A) estimated inflows, (B) estimated outflows, (C) measured 

changes in lake storage and estimated changes in water storage in the saturated and unsaturated zones, and (D) monthly residuals 

of the water-budget analysis, for Wapato Lake near Gaston, northwestern Oregon, for the calibration period of October 2011 through 

March 2014. One million cubic meters = 811 acre-feet.
Results—Water Budget and Water Management Scenarios 15

49.0

49.5

50.0

50.5

51.0

51.5

Daily mean water level, in meters above NAVD 88

0

0.5

1.0

1.5

2.0

Estimated maximum water depth in lake, in meters

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

WY 2012

WY 2013

WY 2014

WY 2015

EXPLANATION

Figure 9. Daily mean water level at Wapato Lake as measured in the canal near the pumphouse (USGS station 14202630), 

near Gaston, northwestern Oregon. Gray area indicates water levels below the lake bottom. Lake is essentially empty at 

water levels less than about 49.5 meters above North American Vertical Datum of 1988. Estimated maximum lake depth is 

noted on the right-hand axis. NAVD 88, North American Vertical Datum of 1988; WY, water year.
16 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

Table 2. Hypothetical scenarios tested with the Water Management Scenario Tool for Wapato Lake near Gaston, northwestern 

Oregon.

 [Symbol: –, option not used]

Input category Details Scenario 1 Scenario 2

Initial level Set the initial lake water-level elevation on October 1 to 49.42 meters 

(162.15 feet). X X

Hydrology Test a range of hydrologic and meteorological conditions, including 

the wettest and driest water years between 1992 and 2014. X X

Tributaries

Route 100 percent of the flow in Ayers, Wapato, and Hill Creeks to 

the lakebed during the entire year. Continue to divert other tributaries around the lake.X X

Pumps

Do not use pumps. Keep them only as a backup. X –

Use pumps to help regulate lake level. Target lake levels were between 50.3 m (165 feet) and 51.8 meters (170 feet), with minimum 

levels between mid-July and mid-October, and maximum levels 

between mid-February and mid-May. The large pump typically 

cannot be used during May-October.

– X

Weir

Use a variable-elevation weir to regulate lake levels between 50.3 

meters (165 feet) and 51.8 meters (170 feet), with minimum levels 

between mid-July and mid-October, and maximum levels between 

mid-February and mid-May.

X –

Use a fixed-elevation weir to limit maximum lake level at 51.8 meters 

(170 feet). – X

A variable-elevation weir was used in scenario 1 to 

control the lake level, and pumps were not used. The weir 

sets the maximum lake level, such that excess inflows are 

exported over the weir and out of the lake. To show how a 

variable-elevation weir might be used to control the lake 

level, and to indicate a transition between deeper water in 

winter (maximum 2.4 m [7.9 ft]) and shallower water in 

summer (maximum 0.9 m [2.9 ft]), the weir elevation in this 

scenario was set between 50.3 m (165 ft) and 51.8 m 

(170 ft), with the lower level from mid-July to mid-October, 

the higher level from mid-February to mid-May, and small 

step changes were applied to transition between those levels. 

These levels and time periods are for illustration purposes and 

may not represent future water-management strategies, but 

they do provide a variation in shallow and deeper-water 

condi-tions over the course of the year. During part of the 

year, the weir crest elevation was lower than historical water 

levels in Wapato Creek downstream of the lake outlet at 

Gaston Road (fig. 3); it is unclear whether water would be 

able to flow out of the lake and over a weir crest set at 50.3 m 

(165 ft). Wapato Creek likely would not rise as high if some 

of the tributaries were routed into the lake, but obstructions 

and beaver dams in Wapato Creek downstream of the lake 

might limit the use of a weir at less than about 51.2 m (168.1 

ft). The WMST provides warnings for potential downstream 

high-water conditions, but does not attempt to model 

backflow over the weir from the creek into the lake.

During a water year with 1992–2014 median values for 

the hydrologic and meteorological conditions, lake levels 

in scenario 1 followed the prescribed weir-crest elevations 

throughout most of the year, with tributary inflows allowing 

the lake to quickly rise to higher target levels and the weir 

allowing excess water to be exported downstream (fig. 10). 

This scenario started with a dry lake; if a higher initial lake 

level had been set for October 1st, the lake level would have 

followed the weir-crest elevations even more closely. The 

lake’s water budget (just the lake, not the entire lakebed) under 

this scenario was dominated by tributary flows (86 percent) 

and precipitation (13 percent) as inputs, and weir outflows 

(88 percent) and open-water evaporation (9 percent) as losses 

(fig. 11). The seepage and leakage parts of the water budget 

constituted only 3 percent of the losses under these conditions. 

Results indicate that inflows from tributaries and groundwater 

are sufficient to maintain the target lake level through the dry 

summer season during a normal year. During a dry year such 

as water year 2001, tributary inputs were much smaller and, as 

a result, lake levels would not quite reach their higher targets 

during most of the winter season. The lake water budget under 

dry 2001 conditions would reflect the decreased tributary 

inflow (76 percent) among the inputs, and decreased weir 

outflow (61 percent) among the losses.
Results—Water Budget and Water Management Scenarios 17

160.8

164.0

167.3

170.6

173.9

177.2

180.4

Elevation, in feet above NAVD 88

49

50

51

52

53

54

55

Elevation, in meters above NAVD 88

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

Lake bottom

Estimated Wapato Creek stage

Weir crest elevation

Lake level / water table

EXPLANATION

Figure 10. Water levels predicted from scenario 1 using median flow and meteorological conditions from water years 1992–2014 

at Wapato Lake near Gaston, northwestern Oregon. NAVD 88, North American Vertical Datum of 1988.

A. Water inputs

85.6%

13.0%

0.2% 1.2%

Tributary Inflow

Precipitation

Runoff to lake

Seepage/leakage

B. Water losses

88.0%

9.0% 3.0%

Open-water 

evaporation

Seepage/leakage

Weir outflow

EXPLANATION EXPLANATION

Figure 11. Percentages of total water (A) inputs and (B) losses as computed from the daily lake water budget for scenario 1 and 

using median flow and meteorological conditions from water years 1992–2014 at Wapato Lake near Gaston, northwestern Oregon. 

Total inputs were 22.2 million cubic meters (m3, 18,000 acre-feet). Total losses were 21.3 million m3 (17,300 acre-feet).
18 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

Managed water levels in this scenario produced water 

depths throughout most of the lakebed that were less than 

2 m (6.6 ft) during most of the year, and less than 1 m (3.3 ft) 

during summer (fig. 12). Because the lake bottom is relatively 

flat, depths across most of the lake are somewhat uniform, 

only becoming shallower near its edges. The SMT was used 

to map lakebed water depths associated with the minimum 

and maximum target water levels in scenario 1. SMT results 

in figure 13A show that at a water level of 51.8 m (170 ft), the 

vast majority of the lakebed is inundated to depths of 1–2 m 

(3.3–6.6 ft) with a few areas deeper, whereas shallow water 

less than 1 m (3.3 ft) depth is restricted to areas along about 

half of the lake perimeter, constituting only 37 hectares (about 

91 acres) and 13 percent of the lake area. In contrast, a water 

level of 50.3 m (165 ft; fig. 13B) results in a smaller inundated 

area and shallow water less than 1 m (3.3 ft) throughout the 

lake, covering roughly 208 hectares (about 513 acres). Target 

water depths for the Wapato Lake NWR will depend on the 

habitat requirements of prioritized species of fish and waterbirds as well as the requirements of desired aquatic vegetation.

Scenario 2—Reroute Three Tributaries and Use

Weir and Pumps

Scenario 2 was identical to scenario 1, except that the 

outflow weir was held constant at a maximum target lake 

level (51.8 m [170 ft]) and pumps were activated and used to 

attempt to meet a schedule of target water levels that was the 

same as the schedule of weir elevations in scenario 1 (table 2). 

A previous water-quality management plan for Wapato Lake 

(Wapato Improvement District, 2009) specified that the large 

pump could be used only during November–April, but the 

small pump could be used anytime; this scenario followed that 

recommendation.

Using 1992–2014 median values for the hydrologic 

and meteorological conditions, lake levels in this scenario 

increased in autumn and early winter to the maximum 

level specified by the weir by mid-January, but the lake 

was slightly deeper than target levels in January and early 

February because the pumps could not keep up with incoming tributary flows (fig. 14). Similarly, in late spring and early 

summer when target lake levels decreased, the capacity of 

the small pump was insufficient to export the large volume 

of water required to attain lower lake levels. Under these 

conditions, tributary flows (86 percent) and precipitation 

(13 percent) accounted for almost all inputs to the lake, 

whereas pumping (47 percent), flow over the weir (37 

percent), and open-water evaporation (12 percent) accounted 

for almost all losses (fig. 15). Under the dry conditions of 

water year 2001, tributary inflows were not large enough to 

attain the target lake levels from autumn through early spring, 

the maximum lake level was never reached, and pumping 

with just the small pump again was insufficient to bring the 

lake level down to target levels in summer. Because of lower 

water levels, weir outflows in a dry year did not occur (0 

percent) and open-water evaporation accounted for a larger 

fraction (43 percent) of total losses.

0

123.6

247.1

370.7

494.2

617.8

741.3

0

50

100

150

200

250

300

Area, in acres

Area, in hectares

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Habitat 1: 0 to 1 m depth

EXPLANATION

Habitat 2: 1 to 2 m depth

Habitat 3: 2 to 7 m depth

Water year

Figure 12. Water-depth and potential habitat areas associated with three water-depth ranges, as predicted from scenario 1 

using median flow and meteorological conditions from water years 1992–2014 at Wapato Lake near Gaston, northwestern Oregon. 

m, meter.
Results—Water Budget and Water Management Scenarios 19

EXPLANATION

Water depth, in meters 

Dry land

0 to 1

1 to 2

Greater than 2

123°08' 123°7'30" 123°07'

45°

26'

30"

45°

26'

45°

25'

30"

45°

25'

45°

24'

30"

0

0

0.5 MILE

0.5 KILOMETER

Base map coyright from Esri, 2019. Coordinate system: Universal Transverse 

Mercator, Zone 10 North. Horizontal datum is North American Datum of 1983.

123°08' 123°7'30" 123°07'

Figure 13. Specific locations that would meet certain water-depth criteria, using minimum and maximum depth targets for 

scenario 1, at Wapato Lake National Wildlife Refuge near Gaston, northwestern Oregon. Maps show depth categories for water 

levels of (A) 51.8 meters (m; 170 feet [ft]), and (B) 50.3 m (165 ft) above North American Vertical Datum of 1988.
20 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

160.8

164.0

167.3

170.6

173.9

177.2

180.4

Elevation, in feet above NAVD 88

49

50

51

52

53

54

55

Elevation, in meters above NAVD 88

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

Lake bottom

EXPLANATION

Estimated Wapato Creek stage

Weir crest elevation

Pump activation target

Lake level / water table

Figure 14. Water levels predicted from scenario 2 using median flow and meteorological conditions from water years 1992–2014 

at Wapato Lake near Gaston, northwestern Oregon. This scenario did not use the large pump during May–October. NAVD 88, 

North American Vertical Datum of 1988.

A. Water inputs

85.6%

13.3%

0.2% 0.8%

Tributary inflow

Precipitation

Runoff to lake

Seepage/leakage

EXPLANATION

B. Water losses

36.9%

11.7%

4.5%

46.9%

Open-water 

evaporation

EXPLANATION

Seepage/leakage

Pumping

Weir outflow

Figure 15. Percentages of total water inputs (A) and losses (B) as computed from the daily lake water budget for scenario 2 and 

using median flow and meteorological conditions from water years 1992–2014 at Wapato Lake near Gaston, northwestern Oregon. 

Total inputs were 22.2 million cubic meters (m3; 18,000 acre-feet). Total losses were 19.1 million m3 (15,500 acre-feet).

As a test to determine whether the existing pumps could 

be used to achieve target lake levels in summer, scenario 2 

was modified to allow the large pump to be used year-round. 

Decreasing the lake level from 51.8 m (170 ft) to 50.3 m 

(165 ft) requires the export of about 4.1×106 m3 (3,290 acreft) of water from the lake, plus additional water that would be 

released from groundwater storage as the water table declined. 

At a combined maximum pump capacity of 1.08 m3/s 

(38.1 ft3/s), evacuating that quantity of water by pumping 

alone would require at least 44 days, even if additional inputs 

did not exist. Using the small pump alone would require at 

least 237 days. Predictions from the WMST showed that using 

both pumps in summer would allow target water levels to be 

achieved, and with nearly the desired timing (fig. 16). Because 
Results—Water Budget and Water Management Scenarios 21

this pumped drawdown of the lake level occurred in early 

summer when streamflow and rainfall was much lower than 

in winter, drawdown results were much the same regardless of 

whether wet or dry conditions were imposed.

Target water levels in scenario 2 were identical to those 

in scenario 1, but scenario 2 was not as successful in attaining 

the targets in summer; therefore, results for water depth and 

potential habitat were different. In scenario 2 without using the 

large pump in summer, water levels were substantially deeper 

than in scenario 1, putting most of the lakebed area in the 

deeper category (1–2 m [3.3–6.6 ft]) with less than 20 percent 

of the lake, mainly around the margins, in the shallower depth 

category (0–1 m [0–3.3 ft]; fig. 17). Using the large pump in 

summer in a modified scenario 2, resulting lake levels were 

more similar to those in scenario 1 and almost all the aquatic 

habitat in summer was in the shallower category (fig. 18).

160.8

164.0

167.3

170.6

173.9

177.2

180.4

Elevation, in feet above NAVD 88

49

50

51

52

53

54

55

Elevation, in meters above NAVD 88

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

Lake bottom

EXPLANATION

Estimated Wapato Creek stage

Weir crest elevation

Pump activation target

Lake level / water table

Figure 16. Water levels predicted from a modified scenario 2 using median flow and meteorological conditions from water years 

1992–2014 and allowing the large pump to be used year-round at Wapato Lake near Gaston, northwestern Oregon. NAVD 88, 

North American Vertical Datum of 1988.

0`

123.6

247.1

370.7

494.2

617.8

741.3

0

50

100

150

200

250

300

Area, in acres

Area, in hectares

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

Habitat 1: 0 to 1 m depth

EXPLANATION

Habitat 2: 1 to 2 m depth

Habitat 3: 2 to 7 m depth

Figure 17. Water-depth and potential habitat areas associated with three water-depth ranges, as predicted from scenario 2 using 

median flow and meteorological conditions from water years 1992–2014 at Wapato Lake near Gaston, northwestern Oregon. This 

scenario did not use the large pump during May–October. m, meter.
22 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

0

50

100

150

200

250

300

Area, in hectares

0.0

123.6

247.1

370.7

494.2

617.8

741.3

Area, in acres

Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

Water year

Habitat 1: 0 to 1 m depth

EXPLANATION

Habitat 2: 1 to 2 m depth

Habitat 3: 2 to 7 m depth

Figure 18. Water-depth and potential habitat areas associated with three water-depth ranges, as predicted from a modified 

scenario 2 using median flow and meteorological conditions from water years 1992–2014 and allowing the large pump to be used 

year-round at Wapato Lake near Gaston, northwestern Oregon. m, meter.

Implications for Restoration and Water 

Management

Effective use of water-budget analyses of wetland 

environments historically has aided in restoration planning 

and evaluation of potential water-management strategies. At 

the Levy Prairie wetland in Florida, Kirk and others (2004)

successfully used a water-budget and mapping approach to 

compare potential restoration outcomes, with a goal of producing a wetland with water depths of 0.15–0.61 m (0.5–2.0 ft) 

for waterfowl habitat. Wilcox and others (2006) constructed 

a water budget for part of the Seney NWR in Michigan so 

that USFWS staff could more easily and reliably restore and 

manage water levels there. Without knowledge of the water 

fluxes moving through surface-water and groundwater systems 

at Seney NWR, managers would be less able to plan and 

implement a successful restoration. At Wapato Lake NWR, 

the issues are similar—refuge managers did not know what 

variations in water depth could be created in the lake through 

restoration strategies until a water budget was constructed to 

determine the quantity of water available to create aquatic 

conditions throughout the year.

Accordingly, water budgets and topographical analyses 

were used in the WMST and SMT to develop potential watermanagement strategies and to evaluate their effects on lake 

level and water depth at Wapato Lake NWR. The goal of the 

restoration and management strategies was to create a mixture 

of shallow-water conditions through the creation of lacustrine 

areas and seasonally inundated wetlands. Results showed that 

shallow-water targets could be achieved through the rerouting of tributaries into the lakebed and management of lake 

levels with weirs and pumps. Fewer tributary inputs would be 

needed if target water levels in the lake were lower than those 

applied in these scenarios, although some tributary inputs 

might be useful in a particularly dry year; the WMST could be 

used to determine how many and which tributary inputs would 

be sufficient to help meet certain water-level targets. The use 

of pumps could be minimized if outflows were controlled 

through the well-timed management of a variable-elevation 

outflow weir, assuming that downstream obstructions and 

beaver dams allowed gravity flow over an outflow weir of the 

desired target elevation.

The water-budget analyses showed that sufficient water 

should be available in the lake’s tributaries in most years to 

keep the lake inundated to desired levels. Routing flows from 

Ayers, Wapato, and Hill Creeks into Wapato Lake seems to 

be almost enough in a dry year (and more than enough in 

a normal or wet year) to achieve the target lake levels and 

associated water depths used in the scenarios of this study. If 

tributary flows ever become too large, creating deeper water 

than desired or overwhelming the capacity of pumps to evacuate excess water, managers may need to divert excess flows 

around the lake. The WMST allows refuge managers to evaluate such possibilities and to plan accordingly. The WMST 

allows users to define two seasons (wet or dry, irrigation or 

non-irrigation, hunting or non-hunting, etc.) and to specify 

the percentage of flow in each tributary that is routed into the 

lakebed during each of those seasons.

Methods used to manage a restored Wapato Lake will 

depend in part on target water levels as well as water levels in Wapato Creek and the Tualatin River downstream of 

the refuge. If an outflow weir were used as a control on the 

maximum lake level (as in scenario 1), and if water levels on 

the downstream side of the weir ever became higher than the 

weir crest elevation, then water could flow into the lake from 
Implications for Restoration and Water Management 23

downstream and create deeper conditions in the lake than 

desired. Excess water would eventually flow out of the lake 

when downstream water levels receded, but high-water conditions could persist for long periods during winter. Given the 

fact that target lake levels used in scenarios 1 and 2 sometimes 

were lower than historical winter water levels in Wapato Creek 

and the Tualatin River downstream of Gaston Road, importing 

water into the lake from downstream is a possibility if a simple outflow weir were used. The WMST included calculations 

of estimated water levels in Wapato Creek downstream of the 

refuge, based on estimated flows and a historical stage/discharge relation, but those calculations are only rough estimates 

meant to warn the user of the potential for downstream highwater conditions. Routing some tributary flows into the lake 

would decrease water levels in the canals outside the levees, 

and those levels may be low enough during most of the year 

to enable the use of an outflow weir without threat of backflow into the lake. Under high-flow conditions in the Tualatin 

River, however, water may still back up Wapato Creek and 

create water levels that are higher than the weir crest. USFWS 

is evaluating the potential for such conditions to occur through 

the construction of a hydraulic model of the Tualatin River 

and Wapato Creek downstream of the refuge (see Rounds and 

others, 2020). Combined with results from the WMST, the 

hydraulic modeling will be helpful for planning the design of 

any outlet structures for the lake.

Managing lake levels with existing pumps in addition 

to an outflow weir may be a viable strategy, as in scenario 2, 

if the operating and maintenance costs survive a cost-benefit 

analysis. If the outflow weir included a one-way flap gate in 

conjunction with a higher structure, backflow into the lake 

because of high water downstream could be prevented while 

allowing the weir to control the maximum lake level during 

conditions with lower downstream water levels. Use of a oneway gate, however, likely would require increased pumping 

for times when downstream water levels exceed the level of 

that gate. When transitioning from a higher target lake level 

in winter to a lower target level in summer, results from the 

WMST showed that the large pump might be needed in the 

absence of a lower weir outlet, as the small pump was too 

small to export the large volume of water associated with the 

1.5-m (5-ft) decrease in target water level used in these scenarios. Regardless, the use of pumping as a means of exporting 

water from the lake may be necessary if a variable-elevation 

weir cannot be used as a simple control on the lake level.

Restoration of Wapato Lake and implementation of 

water-management strategies likely will have downstream 

effects. Since the 1930s, the lakebed has been pumped dry in 

spring and farmed in summer, and canals around the lakebed 

have been used to deliver irrigation water in summer. As a 

result, little to no water from the Wapato Lake area has flowed 

downstream to the Tualatin River in summer. Depending on 

water-management strategies in the future, flows in Wapato 

Creek downstream of the refuge may be greater than zero 

during summer, particularly if water must be exported from 

the lake to transition from deeper water in winter to shallower 

water in summer. If a variable-elevation weir were used to 

control the lake level, outflows from the lake during a transition from higher to lower lake levels might result in pulses 

of higher flows in Wapato Creek. Assuming that each 0.3 m 

(1 ft) water-level transition occurred over a single day during a weir-height reduction, the WMST predicted daily mean 

outflows of 8.6–11.4 m3/s (302–402 ft3/s) in scenario 1 under 

median flow conditions for the days of transition. Drawdowns 

could be managed over longer time periods, producing lower 

exported flow rates, and could be managed such that they were 

completed before the summer season began. Nevertheless, 

flows exported via Wapato Creek to the Tualatin River in late 

spring or early summer could be substantial and might require 

management.

The water-quality effects of exporting water from Wapato 

Lake and downstream to the Tualatin River are important 

considerations for future water management at Wapato Lake 

NWR. Water pumped out of Wapato Lake during spring 

historically has led to minor issues with drinking-water treatment at the Joint Water Commission plant about 10 km (6.2 

mi) downstream. In addition, an early summer algal bloom in 

Wapato Lake in 2008 combined with pumped export of that 

water led to downstream drinking-water treatment problems 

as well as a large algal bloom and associated water-quality 

issues in the lower Tualatin River (Bonn, 2008; Carpenter and 

Rounds, 2013; Rounds and others, 2015). High organic-matter 

concentrations, phosphorus loads, or plankton populations 

exported from Wapato Lake might cause water-quality and 

drinking-water treatment problems downstream. Since 2008, 

managers of Wapato Lake have been careful to consider the 

downstream water-quality effects of any exported water during 

the May–September dry season. During winter, discharges 

from the Wapato Lake drainage typically are diluted by 

relatively high flows in the Tualatin River, minimizing any 

water-quality effects.

The fact that water-quality issues have occurred in 

association with discharges from Wapato Lake in the past, 

however, does not mean that a properly managed and restored 

lake would produce problems in the future. The most altered 

water-quality conditions in Wapato Lake discharges often have 

occurred when the lake was being pumped out in the spring 

and nearly empty, such that the lake was quite shallow and 

interactions of the water with lakebed soils were maximized; 

these trends are embodied in water-quality data collected 

by USGS on Wapato Creek at Gaston Road (USGS station 

14202650; see U.S. Geological Survey, 2015). Under such 

conditions, concentrations of organic matter and nutrients 

may be elevated. Under a restored condition, however, much 

more water may be in the lake and any rerouted tributary 

inflows might help to “flush” water through the lake, decreasing the time available for algal growth and the accumulation 

of organic matter and nutrients leached from the lakebed. 

Future water-quality conditions and downstream effects with a 

restored Wapato Lake remain unknown, however, and refuge 

managers and planners will need to continually assess and 

adapt as issues arise.
24 Evaluation of Restoration Alternatives Using Water-Budget Tools for Wapato Lake, Oregon

Although the WMST includes all important inputs and 

losses for the Wapato Lake water budget and makes predictions that seem to be sufficiently accurate for the development of water-management plans at the refuge, it makes 

some assumptions and simplifications that could be improved 

upon with additional data and refinement of algorithms. 

For example, estimates of groundwater discharge could be 

improved through measurements of groundwater levels in the 

lakebed combined with measurements of lakebed hydraulic 

conductivity. Compared to some of the other inputs, however, 

groundwater discharge typically is a minor component of the 

water budget.

The combined application of the Water Management 

Scenario Tool and the Shoreline Management Tool to Wapato 

Lake NWR provides a powerful means of helping to design 

potential water-management plans and evaluate their effects on 

lake level and habitat. With new or existing data, these tools 

could be applied to almost any refuge or managed wetland. 

On-the-ground adaptation and response to measured conditions always will be an important means of optimizing restoration strategies, but using the WMST and SMT as planning 

tools is proving to be valuable for evaluating a wide range of 

potential water-management and restoration actions at Wapato 

Lake NWR, and thereby determining in advance which may 

be the most feasible and effective strategies for meeting local 

objectives.

Supplementary Material

Copies of the Wapato Lake WMST and datasets to run 

the SMT for Wapato Lake are available from a USGS website 

at https://or.water.usgs.gov/proj/wapato_lake/. That website 

also provides links to archived flow and water-quality data at 

sites in and around Wapato Lake NWR. All datasets used to 

run the water-budget calculations in the WMST are included 

in the WMST spreadsheet and were obtained from archived 

sources cited in this report.

Acknowledgments

The authors thank everyone who assisted with land 

surveys in support of the many data-collection efforts in the 

Wapato Lake area, especially David Hills of U.S. Fish and 

Wildlife Service for his leadership of the land surveys as well 

as Gary Ball, Molly Cox, David Hoy, Mary Lindenberg, Sheila 

Strachan, and David Taylor of U.S. Fish and Wildlife Service 

and Glen Hess of U.S. Geological Survey. Thanks to Kristel 

Griffith and Jessica Dorsey of the City of Hillsboro for data, 

useful discussions, and support of data-collection programs 

and development of the Water Management Scenario Tool. 

Thanks to Carol Murdock and others at Clean Water Services 

for their support of restoration planning efforts at Wapato Lake 

National Wildlife Refuge.

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