U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020-5014

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

Evaluation of Restoration Alternatives Using Hydraulic 

Models of Lake Outflow at Wapato Lake National Wildlife 

Refuge, Northwestern Oregon
Cover: Wapato Lake in winter when lake has been inundated with rainwater, northwestern Oregon. 

Photograph by Stewart Rounds, U.S. Geological Survey, March 1, 2020.
Evaluation of Restoration Alternatives 

Using Hydraulic Models of Lake Outflow 

at Wapato Lake National Wildlife Refuge, 

Northwestern Oregon

By Stewart A. Rounds, Stephen L. Pilson, Annett B. Sullivan, and Adam J. 

Stonewall

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

Water Commission

Scientific Investigations Report 2020–5014

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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Suggested citation:

Rounds, S.A., Pilson, S.L., Sullivan, A.B., and Stonewall, A.J., 2020, Evaluation of restoration alternatives using 

hydraulic models of lake outflow at Wapato Lake National Wildlife Refuge, northwestern Oregon: U.S. Geological 

Survey Scientific Investigations Report 2020–5014, 21 p., https://doi.org/10.3133/sir20205014.

ISSN 2328-0328 (online)
iii

Contents

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

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

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

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

Methods...........................................................................................................................................................4

Overview.................................................................................................................................................4

HEC-RAS Model Construction ............................................................................................................5

Geometry .......................................................................................................................................5

Flow Data.......................................................................................................................................5

Lake/Creek Flow-Exchange Rates .....................................................................................................6

Open Outlet Addition to the WMST ...................................................................................................7

Model Results and Evaluation of Water-Management Scenarios ........................................................8

HEC-RAS Model Calibration and Validation .....................................................................................8

Lake/Creek Flow-Exchange Rates .....................................................................................................9

Verification of WMST Predictions ...................................................................................................10

Water-Management Scenarios ........................................................................................................12

Scenario 1—Open Outlet Year-Round ....................................................................................12

Scenario 2—Open Outlet Scheduled .....................................................................................15

Implications for Restoration and Water Management ..........................................................................17

HEC-RAS Flow Modeling ...................................................................................................................17

Uncertainties and Sources of Error ........................................................................................18

Accuracy of Flow-Exchange Rates ........................................................................................18

Insights from WMST Scenarios .......................................................................................................19

Supplementary Material .............................................................................................................................20

Acknowledgments.......................................................................................................................................20

References Cited..........................................................................................................................................20

Figures

1. Map showing Tualatin River Basin, including Wapato Lake in the southwestern 

part of the basin, near Gaston, northwestern Oregon ............................................................3

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

northwestern Oregon, winter 2015 ............................................................................................4

3. Photograph showing Wapato Creek looking north (downstream) from the 

Gaston Road bridge at the northern tip of Wapato Lake, northwestern 

Oregon, 2010 ..................................................................................................................................4

4. Map of the study area showing the modeled reaches along the Tualatin River 

and Wapato Creek and nearby stations measuring streamflow and water level, 

near Gaston, northwestern Oregon ...........................................................................................6

5. Map showing modeled reaches of the Tualatin River and Wapato Creek, 

general topographic relief of the floodplain, and river cross sections included 

in the Hydrologic Engineering Center-River Analysis System (HEC-RAS) model 

of the Tualatin River and Wapato Creeks near Gaston, northwestern Oregon ..................7
iv

6. Graph showing comparison of modeled and measured water levels in Wapato 

Creek at Gaston Road, northwestern Oregon (U.S. Geological Survey station 

14202650), for the March–May 2012 calibration period .........................................................8

7. Graph showing comparison of modeled and measured water levels in Wapato 

Creek at Gaston Road, northwestern Oregon (U.S. Geological Survey station 

14202650), for the November 2011 to April 2013 validation period ........................................9

8. Graph showing flow-exchange rates between Wapato Lake and Wapato Creek, 

northwestern Oregon, simulated by the Hydrologic Engineering Center-River 

Analysis System (HEC-RAS) model for water years 1992–2014 ..........................................10

9. Graphs showing streamflow from the Hydrologic Engineering Center-River 

Analysis System (HEC-RAS) model and from the Wapato Lake Water 

Management Scenario Tool (WMST) under a wide range of hydrologic 

conditions in Wapato Creek at Gaston Road, northwestern Oregon, in water 

years 2001, 2007, 2003, and 1997 ...............................................................................................11

10. Graph showing cumulative streamflow volume from the Hydrologic Engineering 

Center-River Analysis System (HEC-RAS) model and from the Wapato Lake 

Water Management Scenario Tool (WMST) in Wapato Creek at Gaston Road, 

northwestern Oregon, during water year 1997 under conditions of an open 

connection between Wapato Lake and Wapato Creek .......................................................13

11. Graph showing predicted water levels for Wapato Lake near Gaston, 

northwestern Oregon, using flow and meteorological conditions from water 

year 2012 and a year-round open connection (scenario 1) between Wapato 

Lake and Wapato Creek and the Tualatin River .....................................................................14

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

Wapato Lake near Gaston, northwestern Oregon, as computed from the daily 

lake water budget for a year-round open lake outlet (scenario 1) and using flow 

and meteorological conditions from water year 2012 ..........................................................14

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

ranges, as predicted with a year-round open lake outlet (scenario 1) 

for Wapato Lake near Gaston, northwestern Oregon, using flow and 

meteorological conditions from water year 2012 ..................................................................15

14. Graph showing predicted water levels for Wapato Lake near Gaston, 

northwestern Oregon, using flow and meteorological conditions from water 

year 2012 and year-round open connection (scenario 1) as well as an open 

connection between Wapato Lake and Wapato Creek and the Tualatin River 

that is closed during part (November 15 to April 15) of the wet winter season 

(scenario 2) ..................................................................................................................................16

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

Wapato Lake near Gaston, northwestern Oregon, as computed from the daily 

lake water budget for scenario 2, in which the lake outlet was closed during 

November 15–April 15, and using flow and meteorological conditions from 

water year 2012 ...........................................................................................................................16

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

ranges for Wapato Lake near Gaston, northwestern Oregon, as predicted for 

scenario 2, in which the lake outlet was closed during November 15–April 15, 

and using flow and meteorological conditions from water year 2012 ...............................17

Figures—Continued
v

Table

1. Hypothetical scenarios tested with an open lake outlet in the Water 

Management Scenario Tool for Wapato Lake near Gaston, northwestern Oregon ........13

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

hectare (ha) 2.471 acre

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)

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.
vi

Abbreviations

DEM digital elevation model

DSS data storage system

GIS geographic information system

HEC-RAS Hydrologic Engineering Center-River Analysis System

lidar light detection and ranging

NWR National Wildlife Refuge

OWRD Oregon Water Resources Department

USFWS U.S. Fish and Wildlife Service

USGS U.S. Geological Survey

WMST Water Management Scenario Tool
Evaluation of Restoration Alternatives Using Hydraulic 

Models of Lake Outflow at Wapato Lake National Wildlife 

Refuge, Northwestern Oregon

By Stewart A. Rounds1, Stephen L. Pilson2, Annett B. Sullivan1, and Adam J. Stonewall1

Abstract

Wapato Lake National Wildlife Refuge near the city of 

Gaston in northwestern Oregon was established in 2013, and 

planning is underway to restore a more natural lake and wetland system after more than 100 years of agricultural activity 

on the lakebed. Several water-management and restoration 

alternatives are under consideration, one of which involves 

opening and reconnecting Wapato Lake’s outlet to allow flow 

in and out of the lake to Wapato Creek and downstream to 

the Tualatin River. The effects of this and other alternatives 

are being evaluated, partly through a detailed examination of 

the lake’s water budget. The water budget for the lake during 2011–13 was quantified by the U.S. Geological Survey in 

partnership with U.S. Fish and Wildlife Service and others. Results were incorporated in a spreadsheet-based Water 

Management Scenario Tool (WMST) for Wapato Lake, which 

predicts the effects of various management actions on daily 

lake level and potential habitat areas for waterfowl or other 

target species. Incorporating the effects of a hypothetical open 

outlet between the lake and the downstream river network in 

the WMST was accomplished by using a hydraulic model to 

simulate the flow-exchange rate between Wapato Lake and 

Wapato Creek over a wide range of lake levels and downstream river conditions. A Hydraulic Engineering Center-River 

Analysis System (HEC-RAS) one-dimensional unsteady flow 

model was constructed and calibrated for Wapato Creek and 

part of the Tualatin River using data from October 2011 to 

April 2013, and then was used to simulate daily lake/creek 

exchange flows in water years 1992–2014 under hypothetically constant lake levels. Results were used to populate a 

table of lake/creek flow-exchange rates for use in the WMST; 

a dynamic link between the WMST and HEC-RAS was unrealistic because it would require hundreds of subroutine calls 

to HEC-RAS and result in long run times for a single wateryear’s WMST calculations with daily time steps. Predictions 

of daily outlet flows from the WMST were checked against 

HEC-RAS simulated flows under daily varying lake levels to 

1U.S. Geological Survey

2U.S. Fish and Wildlife Service

ensure that the timing and magnitude of lake/creek exchange 

flows used by the WMST were consistent with those of the 

hydraulic model. Two scenarios were tested with a hypothetical open lake outlet to show how the WMST could be used to 

inform restoration planning—one scenario used a year-round 

open lake outlet, and the other scenario closed that outlet for 

part of the high-water winter season. Results showed that 

flows in and out of a year-round open lake outlet would dominate the lake’s water budget and produce water depths during 

winter and through mid-summer that might be too deep to 

support waterbird species that require shallow water. Closing 

the lake outlet during large winter storms and high-water 

conditions in the downstream river network would isolate the 

lake from surrounding rivers, keep the lake level lower, and 

retain substantially more shallow-water areas. Because of the 

ease with which management alternatives can be evaluated, 

a water-budget spreadsheet tool such as the WMST has been 

a valuable part of an analysis of potential water-management 

and restoration alternatives for Wapato Lake National 

Wildlife Refuge.

Introduction

National wildlife refuges have complex ecosystems with 

continuously changing interactions among wildlife, vegetation, hydrology, and climate. This complexity can make it difficult to predict the effects of potential management scenarios. 

Models often are used by decision makers to estimate the 

effects of management options prior to carrying out expensive 

or time-consuming restoration activities. Models that include 

and accurately represent the most important processes that 

respond to potential management options and are driven by 

reliable data are the most likely to produce reliable information for decision making. Once a model is built and calibrated 

to represent processes occurring at a specific location, it 

becomes a tool that can be used to examine a wide range of 

situations.

In this study, a hydraulic model of two river reaches 

downstream of Wapato Lake in northwestern Oregon was 

constructed and calibrated and its results were incorporated in 
2 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

an existing water-budget spreadsheet tool to assist in evaluating management options for the recently established Wapato 

Lake National Wildlife Refuge (NWR). Wapato Lake is 

surrounded by levees and external canals that route tributaries around the lake and allow the lakebed to be farmed in 

summer. The hydrology of the system is tightly controlled, 

with no open-flow connections to streams outside the lakebed 

and water export controlled by pumping. Refuge managers 

are considering various options to manage the newly created 

refuge. One option includes the establishment of a morenatural flow connection between the lake and Wapato Creek, 

which flows downstream to the Tualatin River (U.S. Fish and 

Wildlife Service, 2019). This natural connection could provide 

many benefits; however, much uncertainty exists with regard 

to how an open-flow connection could affect the lake and 

downstream rivers.

A range of scenarios for restoration of Wapato Lake 

has been considered with an existing water management 

tool developed by the U.S. Geological Survey (USGS)—the 

Wapato Lake Water Management Scenario Tool (WMST; 

Rounds and others, 2020). The WMST is a water-budget 

spreadsheet tool that uses a daily water budget to predict lake 

water levels, depths, volumes, and potential habitat areas 

under various management options. The daily water budget includes inputs such as precipitation, tributary inflows, 

groundwater discharge, and leakage through levees and the 

pumphouse; and water losses such as evapotranspiration, 

open-water evaporation, groundwater recharge, and pumping. Management options available for analysis in the WMST 

include the simulation of pumping and outflow weirs, tributary 

removal or addition, and imposition of a range of hydrologic 

and meteorological conditions to gain insights into historical 

or future water availability and habitat. One critical limitation 

of the original WMST is that it could not estimate the flow 

exchange through a hypothetical open connection between 

Wapato Lake and Wapato Creek.

Purpose and Scope

The goals of this study were to construct and use a 

hydraulic model to (1) estimate the flow exchange through 

an open downstream outlet at the north end of Wapato Lake, 

and (2) incorporate those results in a daily water budget for 

Wapato Lake so that restoration and water-management 

strategies could be evaluated. The hydraulic model includes a 

section of the Tualatin River and Wapato Creek from its mouth 

to Wapato Lake, and was developed using the U.S. Army 

Corps of Engineers Hydrologic Engineering Center-River 

Analysis System (HEC-RAS). For this study, a HEC-RAS 

unsteady flow model was calibrated and verified for conditions that occurred during 2011–2013, and then was used to 

predict water levels and flows in Wapato Creek at the outlet 

of Wapato Lake under a range of hydrologic conditions. This 

report describes (1) construction and calibration of the HECRAS model to simulate flow conditions that occurred during 

water years 1992–2014; (2) integration of HEC-RAS model 

results in the WMST for an open-outlet condition; and (3) 

results from sample water-management scenarios for Wapato 

Lake using an open lake outlet to evaluate the potential effects 

on lake level and water depth, with ramifications on potential 

habitat for target species.

Study Site

Wapato Lake is a 320-hectare (790-acre) shallow lake 

located in the Tualatin River Basin near the city of Gaston 

in northwestern Oregon (figs. 1–2). Historically, the lake 

received flows from several tributary creeks as well as reverse 

flows from the nearby Tualatin River and Wapato Creek during winter high-water conditions (Washington County, 1872). 

Beginning in 1895, canals were constructed in and around the 

lake to drain it each spring for farming. By the 1930s, 8.9 km 

(5.5 mi) of levees had been constructed around the lakebed 

and its tributaries had been routed to canals outside the levees 

to decrease the lake’s water inputs. In addition, a pump station 

had been constructed at the northern end of the lake to make 

the lake easier to empty (Cass and Miner, 1993). In a typical 

year, rainwater and groundwater seepage would accumulate in 

the lake in autumn and winter, which then would be pumped 

out in spring to provide dry land for agriculture. Years of 

exposing lakebed soils to the atmosphere caused peat deposits to decompose and the lakebed to subside by as much as 

1.5–2.1 m (5–7 ft; Christy, 2015), with the implication that 

the lake could be substantially deeper now than it was in the 

1800s. The lake’s history, soils, topography, and water budget 

are described in more detail by Rounds and others (2020).

In 2007, after a series of land acquisitions from willing 

sellers, the U.S. Fish and Wildlife Service (USFWS) established Wapato Lake as a unit of the Tualatin River National 

Wildlife Refuge. Wapato Lake historically was an important 

site supporting waterfowl in northwestern Oregon, and has 

great potential for the restoration of its lacustrine, wetland, 

and riparian systems (U.S. Fish and Wildlife Service, 2007). 

In 2013, the Wapato Lake Unit was converted to the Wapato 

Lake National Wildlife Refuge as the 562nd refuge in the 

National Wildlife Refuge System. USFWS thereafter began 

planning for future restoration of native systems to support 

migrating and wintering waterfowl, raptors, songbirds, fish, 

amphibians, and reptiles, among other fauna. Among many 

alternatives under consideration is an option to restore an 

open-flow channel at the northern end of Wapato Lake (its 

historical outlet) to connect the lake to Wapato Creek and 

farther downstream to the Tualatin River (fig. 3; U.S. Fish and 

Wildlife Service, 2019). An open-flow connection could simplify water management at the refuge, restore a more-natural 

condition to nearby rivers, and eliminate the operation and 

maintenance costs of pumping.

Restoration of an open lake outlet, however, would 

introduce additional management issues. During periods 

when Tualatin River flows are high, such as during winter 
Introduction 3

WILLAMETTE

TUALATIN RIVER

RIVER

RIVER

COLUMBIA

Hagg

Lake

Wapato

Barney Lake

Reservoir

East

Fork

Dairy

Gales

Creek

Scoggins

Creek

McKay

Creek

Rock

Creek

Fanno

Creek

Beaverton Creek

Creek Chicken

217

26

5

5

5

205

205

84

Lake

Oswego

Tualatin

Sherwood

Portland

Tigard

Beaverton

Cornelius Hillsboro

Forest

Grove

Cherry

Grove

Gaston

WASHINGTON

COLUMBIA

TILLAMOOK

YAMHILL

CLACKAMAS

MULTNOMAH

TUALATIN MOUNTAINS

CHEHALEM MOUNTAINS

COAST RANGE

0

5

10

35 25 15

45

20

50

85

55

90

65 95 60 70 80 75

40

30

125

130

120

110 100 105 115

JWC

WTP

OREGON

Study

Area

EXPLANATION

Designated urban growth boundary (2009)

Basin boundary

Water treatment plant

50 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 1. Tualatin River Basin, including Wapato Lake in the southwestern part of the basin, near Gaston, 

northwestern Oregon. JWC, Joint Water Commission; WTP, water treatment plant.

storms, reverse flow conditions can occur in Wapato Creek; 

an open connection between Wapato Lake and Wapato Creek 

could allow large volumes of river water to enter the lake and 

greatly increase the lake level. At certain times of year, high 

water levels in Wapato Lake may be undesirable, as most 

of the lake could be too deep to provide habitat for some 

waterbird species. Numerous studies have concluded that 

water depth is an important factor linking the abundance and 

foraging success of waterbirds (Bancroft and others, 2002; 

Bolduc and Afton, 2008; Lantz and others, 2011), and that 

relatively shallow water (<0.25 m [<0.82 ft]) is selected by 

many species of shorebirds, wading birds, and dabbling ducks, 

whereas deeper water is preferred by other species such as 

diving ducks (Fredrickson, 1991; Murkin and others, 1997; 

Elphick and Oring, 1998; Colwell and Taft, 2000; Taft and 

others, 2002). An open lake outlet also may allow the introduction of various fish species into the lake, including steelhead trout (Oncorhynchus mykiss) listed as threatened under 

the Endangered Species Act of 1973 (Public Law 93–205, 

87 Stat. 884, as amended), which in turn might require the 

development of plans to manage common carp (Cyprinus 

carpio) or prevent the stranding of steelhead when water 

levels recede in summer (U.S. Fish and Wildlife Service, 

2019). In addition, rapid changes in lake level could disrupt 

plant and wildlife communities (Fredrickson, 1991). If poor 

water-quality conditions were to develop in the lake, an open 

lake outlet could allow that water to be exported and create 

water-quality problems downstream. Indeed, the export of 

poor-quality water from Wapato Lake occurred in the summer 

of 2008 in conjunction with an algal bloom in the lake, with 

detrimental downstream effects on municipal-water treatment 

and recreational uses (Rounds and others, 2015). Therefore, 

it is important to understand the details of an open connection 

between Wapato Lake and Wapato Creek when considering 

alternatives for restoration planning, including the conditions 

when the connection would be active, when reverse flow 
4 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

Figure 2. Wapato Lake when the lake was refilling with rainwater, northwestern Oregon, winter 2015. Photograph by Stewart Rounds, 

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

Figure 3. Wapato Creek looking north (downstream) from 

the Gaston Road bridge at the northern tip of Wapato Lake, 

northwestern Oregon, 2010. Photograph by Stewart Rounds, U.S. 

Geological Survey, July 21, 2010.

conditions could occur, and the magnitude of exchange-flow 

rates. Such information is critical to the water budget of the 

lake and prediction of lake levels and water-depth areas tied to 

potential habitat conditions.

In evaluating a restoration scenario that includes an open 

lake outlet, refuge managers will need to predict and understand lake conditions and availability of target-species habitats 

that may result from a free-flowing connection between 

Wapato Lake and Wapato Creek. With an open connection, the 

lake has the potential to become deep (more than 2 m [6.6 ft]) 

at times, which might limit its value to certain guilds of waterbirds such as dabbling ducks. Simulating an open lake outlet 

with a calibrated hydraulic model and incorporating the results 

in a detailed lake water budget allows managers to evaluate 

the water levels and water-depth fluctuations that might occur 

if the lake were connected to the creek without any kind of 

control structure.

Methods

Overview

To model an open lake outlet in the existing Wapato 

Lake WMST, it was first necessary to develop a dynamic flow 

model of Wapato Creek and part of the Tualatin River using 

HEC-RAS version 4.1.0 (U.S. Army Corps of Engineers, 

2010). HEC-RAS can simulate steady and unsteady (timevariant) streamflow in river channels and floodplains, including flow under bridges and through hydraulic structures. The 

model predicts water-surface elevation, velocity, and flow 

amount and direction at any point in the modeled reach. The 

model was calibrated, and then run iteratively under a range 

of river conditions and lake levels to develop a lookup table 

of exchange flows that the WMST could use to calculate the 

magnitude and direction of flow exchange between Wapato 

Lake and Wapato Creek. A dynamic dependence of WMST on 

the HEC-RAS model was not introduced because such a link 

would require the WMST to make subroutine calls to initiate a 

HEC-RAS model run for every daily time step of the waterbudget calculations, resulting in unrealistically long run times 

for the calculations of a single water-year. To evaluate whether 

the simulation of exchange flows through an open outlet was 

accurately represented in the WMST, the WMST and the 
Methods 5

HEC-RAS model were used to predict lake/creek exchange 

flows under identical conditions for a selection of representative water years (for example, dry, wet, and average rainfall). 

After evaluating the accuracy of WMST predictions, the 

WMST was used to predict lake levels as well as lake areas 

meeting target water-depth criteria under two example restoration scenarios.

HEC-RAS Model Construction

Geometry

The HEC-RAS model domain included 5.75 km (3.57 

mi) of the Tualatin River from Gaston (Oregon Water 

Resources Department [OWRD] station 14202510) to Dilley 

(USGS station 14203500), and Wapato Creek from a point 

south of the bridge at Gaston Road (USGS station 14202630) 

to its confluence with the Tualatin River approximately 3 km 

(1.9 mi) downstream (fig. 4). HEC-RAS requires stream 

cross-sectional data throughout the model extent to characterize the channel and floodplain shape and to compute flow 

and water level at those locations. USFWS contracted with 

Minister-Glaeser, Inc. to survey cross sections along Wapato 

Creek from the Wapato Lake pumphouse (USGS station 

14202630) to the creek’s confluence with the Tualatin River. 

USGS staff surveyed cross sections in the Tualatin River 

channel from Gaston to Dilley. Data from in-channel surveys 

were blended with topographic data from a digital elevation 

model (DEM) derived from high-resolution light detection and 

ranging (lidar) data. Because the airborne lidar laser could not 

penetrate the water column, lidar data were valid only for the 

floodplain and in-channel surveys were needed to characterize 

the stream channels. In-channel data were blended with the 

lidar data by determining coincident channel-edge points and 

replacing invalid lidar-derived channel data with in-channel 

survey data. During this data-blending process, it became 

apparent that dense vegetation such as Himalayan blackberry 

(Rubus armeniacus) and reed canary grass (Phalaris arundinacea) had prevented the lidar laser from reaching ground 

level along the stream banks, causing the lidar dataset to have 

artificially high elevations at stream-channel edges; therefore, 

such elevations were decreased to better match the in-channel 

surveys. An ArcGISTM extension, HEC-GeoRAS (U.S. Army 

Corps of Engineers, 2011), was used to generate the combined 

channel and floodplain cross-sectional geometries for the 

HEC-RAS model (fig. 5).

Survey measurements, photographic information, and 

field observations were used to characterize bridges and channel obstructions in the model. Two bridges and a beaver dam 

were modeled on Wapato Creek, and one bridge was modeled on the Tualatin River. The beaver dam was modeled as a 

“leaky” solid inline structure, allowing 0.014 m3/s (0.5 ft3/s) of 

flow to pass through when water elevation was below the crest 

of the structure. Manning’s n roughness values were estimated 

from a comparison of aerial and field photographs to values 

associated with a range of channel and vegetation types published in the scientific literature (Chow, 1959; Barnes, 1967). 

Ineffective flow areas were designated for overbank areas of 

cross sections where hydraulic connectivity to the main channel was unlikely for the modeled flow.

Flow Data

For model calibration, flow boundary conditions were 

imposed for the upstream boundaries on the Tualatin River 

at Gaston (OWRD station 14202510), on Wapato Creek near 

Gaston Road (USGS station 14202650), and for the Scoggins 

Creek tributary (OWRD and USGS station 14202980; fig. 4). 

The downstream boundary on the Tualatin River at Dilley 

(USGS station 14203500) was modeled using a “normal 

depth” condition based on the friction slope of the channel 

so as not to constrain downstream elevations. The friction 

slope was computed in a geographic information system 

(GIS) by analyzing intervals between topographic contour 

lines generated from a 10-m DEM, with a result of 1.1×10-4. 

Streamflow and water-surface elevation data were available 

from OWRD for the Tualatin River at Gaston (OWRD station 

14202510), and from USGS for Wapato Creek at Gaston Road 

(USGS station 14202650). Scoggins Creek, a large tributary 

that joins the Tualatin River just downstream of the mouth of 

Wapato Creek, was included in the model as a lateral input, 

with streamflow data available from USGS, the Bureau of 

Reclamation, and OWRD at a site farther upstream (OWRD 

and USGS station 14202980). For calibration, streamflow and 

water-surface elevation data were available from USGS for the 

downstream boundary on the Tualatin River at Dilley (USGS 

station 14203500).

For initial construction and calibration of the model, 

30-minute flow data for each location were stored in a data 

storage system (DSS) file to interface with HEC-RAS. The 

streamflow station on Wapato Creek at Gaston Road was operated only from October 2011 to April 2013, which limited the 

analysis window to that period for calibration and validation 

of the model. Because of overbank flows near Gaston during high-water conditions, OWRD did not publish Tualatin 

River streamflows greater than 18.4 m3/s (650 ft3/s) at OWRD 

station 14202510, which resulted in lengthy data gaps during winter. Missing data at OWRD station 14202510 were 

estimated using a water balance approach by subtracting flows 

measured at Scoggins Creek and Wapato Creek from those 

measured at Dilley. Because of slight variations in travel time 

and the presence of some runoff that was not accounted for 

in the water-balance method, small discontinuities sometimes 

occurred where the measured and estimated flows needed to be 

joined; the estimates were adjusted in such cases to ensure a 

seamless transition between the measured and estimated daily 

mean streamflows. Other smaller and intermittent data gaps in 

flow records were filled using simple linear interpolation.
6 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

Wapato

Lake

Hagg

Lake

TUALATIN RIVER

TUALA

TIN

RIVER

Scoggins Creek

O’Neil Creek

Black Jack Creek

Wapato Crk

Wapato

Creek

Hill Creek

TUALATIN RIVER

47

47

TUALATIN

VALLEY HIGHWAY

TUALATIN

VALLEY HIGHWAY

GASTON RD

SPRING HILL ROAD

YAMHILL CO.

WASHINGTON CO.

Gaston

14202650

14202630

14203500

14202980

14202510

OREGON

Tualatin R.

Willamette R.

Columbia River Map

area

14202980

EXPLANATION

Modeled reach

Canal or ditch

Station

Intermittent lake

Typical flow direction

0 0.5 1 1.5 2 MILES

0 1 2 KILOMETERS

Base map modified for USGS and other digital data. 

Coordinate Reference System: NAD 1983 2011 

Oregon Statewide Lambert, horizontal datum is 

North American Datum of 1983. 

123°13' 123°12' 123°11' 123°10' 123°09' 123°08' 123°07'

45°

29'

45°

28'

45°

27'

45°

26'

Figure 4. Study area showing the modeled reaches along the Tualatin River and Wapato Creek and nearby stations measuring 

streamflow and water level, near Gaston, northwestern Oregon.

Lake/Creek Flow-Exchange Rates

After the HEC-RAS model was calibrated, daily flowexchange rates through a hypothetical open connection 

between Wapato Lake and Wapato Creek were simulated 

over a wide range of conditions using hydrologic conditions 

that occurred during water years 1992–2014. To simulate 

these flow-exchange rates, measured flows were applied 

for Scoggins Creek and Tualatin River at Gaston inputs, 

constant lake levels were imposed on the upstream boundary 

on Wapato Creek, and HEC-RAS simulated the downstream 

flows at Dilley using a normal-depth boundary condition. Each 

of the water years was simulated with a range of constant lake 

levels at the upstream boundary on Wapato Creek so that a 

lookup table could be produced for the WMST. The lookup 

table was simply a table of daily open-outlet flow-exchange 

rates as simulated by HEC-RAS, indexed by lake level and 

date (year and day of year), where date was a surrogate for 
Methods 7

GASTON ROAD

SPRING HILL ROAD

TUALATIN VALLEY HIGHWAY

Base map modified from USGS 

and other digital data. Coordinate 

System: OCRS_Portland_NAD 

1983_CORS96_LCC_Feet Intl. Lambert 

Conformal Conic projection. North 

American Datum of 1983. 

47

Scoggins Creek

Wapato

Creek

Tualatin River

Gaston

123°08'30" 123°08' 123°07'30" 123°07'

45°

28'

30"

45°

28'

45°

27'

30"

45°

27'

45°

26'

30"

0 1 MILE

0 1 KILOMETER

Figure 5. Modeled reaches of the Tualatin River and Wapato 

Creek, general topographic relief of the floodplain, and river cross 

sections included in the Hydrologic Engineering Center-River 

Analysis System (HEC-RAS) model of the Tualatin River and 

Wapato Creeks near Gaston, northwestern Oregon.

hydrologic conditions in downstream Tualatin River reaches 

that might constrain open-outlet flows due to backwater 

conditions.

The WMST calculates fluxes of various water-budget 

components on a daily time step. The calibrated HEC-RAS 

model, originally configured to run with 30-minute flow 

data for 2011-2013, was reconfigured to run with daily mean 

boundary conditions and for the longer 1992–2014 wateryear period. Daily mean streamflow data for the Tualatin 

River at Gaston (OWRD station 14202510) were obtained 

from annual reports of the Tualatin River Flow Management 

Technical Committee (years 1992–2004; Oregon Water 

Resources Department, 1992–2004) and from OWRD (years 

2005-2014; Oregon Water Resources Department, 2015a). 

Daily mean streamflow data for Scoggins Creek (OWRD and 

USGS station 14202980) were obtained from OWRD and 

USGS (Oregon Water Resources Department, 2015b; U.S. 

Geological Survey, 2015).

The WMST provides an option to impose hydrologic and 

meteorological conditions that are a precomputed percentile 

of the historical record. The HEC-RAS model, therefore, 

also had to be run with boundary conditions that represented 

the median, 10th, 25th, 75th, and 90th percentiles of streamflows in Scoggins Creek and the Tualatin River at Gaston. 

Percentiles of these daily mean streamflows were computed 

for each site by calculating the daily cumulative streamflow 

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

percentile for each day.

The model boundary condition at the upstream end of 

Wapato Creek, at Wapato Lake, was set to a constant lake 

level in each simulation. Twenty-three different lake elevations from 50.9 to 54.3 m (167 to 178 ft) were simulated at 

intervals of 0.15 m (0.5 ft) with separate model runs for each 

lake elevation, and applying the entire 1992–2014 water-year 

period for each lake level. Although a constant Wapato Lake 

elevation is unlikely to occur for any extended period, these 

constant-elevation model runs allowed HEC-RAS to generate 

a full range of Wapato Lake/Wapato Creek exchange flows 

under a wide range of lake levels and Tualatin River flow 

conditions. The model computed the magnitude and direction of daily streamflow along Wapato Creek in response to 

the imposed lake levels and downstream flow conditions. 

The lake/creek flow-exchange rates that were transferred to 

the WMST were taken from a location 3.18 km (1.98 mi) 

upstream of the mouth of Wapato Creek, at the upstream end 

of the Wapato Creek model domain.

Open Outlet Addition to the WMST

An option to include an open connection between Wapato 

Lake and Wapato Creek was added to the WMST. This option 

essentially added another input or output to the lake’s water 

budget, in which the daily flow-exchange rates were taken 

from a lookup table (based on HEC-RAS model results) in 

the WMST spreadsheet, using that day’s lake level and date 

as indices to find the appropriate exchange rates in the lookup 

table and interpolate a result from the two nearest lake levels 

on that date. The date was a surrogate for the hydrologic and 

meteorological conditions that occurred at that time. Positive 

flow rates represent outflows from the lake, and negative flow 

rates represent inflows to the lake.

The outlet in the WMST can be open for certain date 

ranges specified by the user, and closed at other times. In this 

way, one can imagine a large gate at the lake outlet that could 

be closed under certain conditions. For example, the gate 

could be closed to prevent the lake from importing excessive 

amounts of water during periods of high-water conditions 
8 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

downstream of the lake, or to prevent poor-quality water 

from being exported from the lake during an early summer 

algal bloom.

Model Results and Evaluation of 

Water-Management Scenarios

HEC-RAS Model Calibration and Validation

Model calibration focused on a 3-month period during 

March–May 2012 that included high and low flows. Two 

March storms generated among the highest peak flows and 

water levels recorded at the Wapato Creek at Gaston Road 

site (USGS station 14202650) during the October 2011 to 

April 2013 period of record, followed by recession toward 

base-flow conditions in May 2012. Calibration consisted of 

adjusting model parameters until modeled water levels in 

Wapato Creek at Gaston Road produced the best comparison 

with measured water levels at that location. Several methods 

were used to optimize the model calibration, such as adjusting 

Manning’s n (roughness) values in the floodplain and stream 

channels, applying flow-based roughness factors to account 

for reduced friction during high flows, and using seasonal 

roughness factors to account for the presence or absence of 

vegetation.

Calibration of the HEC-RAS model for March–May 

2012 resulted in a fit to the measured water levels in Wapato 

Creek at Gaston Road that was deemed acceptable for this 

application (fig. 6). Although the measured water level at that 

site varied as much as 2.55 m (8.35 ft) during that 3-month 

period, the mean absolute error (typical error) of the model 

was only 0.22 m (0.71 ft), with a mean error (bias) of 0.10 m 

(0.32 ft). The modeled water level matched almost all patterns 

in the data, rising and falling in response to storms at similar 

times and with similar magnitudes, with an exception in midto late-May when the measured water level increased on May 

17, but the modeled water level showed no response (fig. 6). 

This discrepancy may have been caused by changes in pumping activity at the upstream Wapato Lake pumphouse, or by 

installation of flash boards in Wapato Creek downstream of the 

measurement site—activities that were not well documented 

and not included in the model. The measured water level also 

decreased during the last 10 days of April 2012 to a point 

that the model was not able to reproduce, perhaps because 

of uncertainties in the bathymetric data used to construct 

the model, and (or) because the model included a difficultto-characterize beaver dam (a leaky weir) in Wapato Creek 

downstream of the measurement station. Varying the model’s 

roughness coefficients was not sufficient to retain a good fit for 

the peak water levels while also decreasing the baseline water 

level between storms.

Model performance was tested further, and somewhat 

independently, by comparing modeled and measured water 

levels for almost the full period of record of the Wapato 

Creek at Gaston Road station without further adjustment to 

the model calibration. For November 2011 to April 2013, the 

model reproduced the measured water levels even better than 

for the calibration period (fig. 7), with a mean absolute error of 

165.7

167.3

169.0

170.6

172.2

173.9

175.5

177.2

50.5

51.0

51.5

52.0

52.5

53.0

53.5

54.0

Water level, in feet above NAVD 88

Water level, in meters above NAVD 88

2012

March April May

1 15 30 1 15 30 1 15 30

Calibrated model

Measured

EXPLANATION

Figure 6. Comparison of modeled and measured water levels in Wapato Creek at Gaston Road, northwestern Oregon (U.S. 

Geological Survey station 14202650), for the March–May 2012 calibration period. NAVD 88, North American Vertical Datum of 1988.
Model Results and Evaluation of Water-Management Scenarios 9

HEC-RAS model

Measured

EXPLANATION

165.7

167.3

169.0

170.6

172.2

173.9

175.5

177.2

50.5

51.0

51.5

52.0

52.5

53.0

53.5

54.0

Water level, in feet above NAVD 88

Water level, in meters above NAVD 88

2011 2012 2013

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

Figure 7. Comparison of modeled and measured water levels in Wapato Creek at Gaston Road, northwestern Oregon (U.S. 

Geological Survey station 14202650), for the November 2011 to April 2013 validation period. HEC-RAS, Hydrologic Engineering 

Center-River Analysis System; NAVD 88, North American Vertical Datum of 1988.

0.16 m (0.52 ft) and a mean error of 0.065 m (0.21 ft). Despite 

the inclusion of eight large storms with high water levels, 

as well as relatively rapid declines in water level between 

those storms, the model captured the timing and magnitude of 

water-level variations during November 2011–April 2013. The 

model did not reproduce subtle variations in measured baseflow water levels during summer 2012. Summertime water 

levels were known to be affected by factors whose variability 

was not included in the model, such as the periodic installation of flash boards downstream of the water-level measurement site and periodic dam-building activities of beavers and 

resulting changes in the height and rate of leakage through 

their dam. It was beyond the scope of this work to fully 

capture those variations in base flow in Wapato Creek, and 

the resulting model likely would have been over-calibrated 

and therefore less able to respond accurately to future changes 

in hydrologic conditions. The model was based on accurate 

bathymetric data and reliable streamflow measurements at the 

boundary conditions, and it reproduced patterns in the data 

with sufficient accuracy for the ultimate purpose of supporting the Wapato Lake water budget and its use for evaluating water-management strategies. Therefore, the model was 

deemed suitable for further integration with the WMST and 

the population of lake/creek exchange flows for that waterbudget tool, with the understanding that model modifications 

might be required if Wapato Creek channel characteristics are 

altered in the future.

Lake/Creek Flow-Exchange Rates

The calibrated HEC-RAS model was used to simulate 

daily streamflow in Wapato Creek at Gaston Road in response 

to measured conditions in the Tualatin River during water 

years 1992–2014. Synthetic statistical water years (10th, 25th, 

50th, 75th, and 90th percentiles) also were simulated. For 

each of these 23 water years and 5 statistical years, the HECRAS model was run with a constant upstream water level on 

Wapato Creek to simulate a hypothetical lake level on an open 

connection between the lake and creek. Twenty-three different lake levels were imposed, resulting in a range of predicted 

lake/creek flow-exchange rates (fig. 8). The pattern in the predicted flow-exchange rates is easy to understand, with higher 

flows discharged from the lake when lake levels are high and 

downstream river flows are low, and reverse flows when river 

streamflows are high and lake levels are low. These results 

were tabulated by lake level and date, producing a lookup 

table that was imported into the WMST, thus allowing lake/

creek flow-exchange rates for an open-outlet condition to be 

estimated from a range of pre-computed conditions. The date 

was essentially a surrogate for river conditions that occurred 

at that time; thus, lake level and river flow (by the date) were 

indices for the lookup table.

The lookup tables generated from HEC-RAS model 

results covered a range of lake levels from 50.9 to 54.3 m 

(167 to 178 ft) at intervals of 0.15 m (0.5 ft), but potential lake 

levels ranged from a low of 49.4 m (162 ft) when the lake is 

empty to a high of 56.1 m (184 ft), at which point the lake 
10 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

-80

-60

-40

-20

0

20

40

60

80

Tualatin River flow at Gaston, in cubic feet per second

Wapato Creek flow, in cubic 

meters per second

EXPLANATION

50.6 50.9 51.2 51.5 51.8 52.1 52.4 52.7 53.0 53.3 53.6 53.9 54.3 54.6

166 167 168 169 170 171 172 173 174 175 176 177 178 179

Wapato Lake elevation, in meters above NAVD 88

Wapato Lake elevation, in feet above NAVD 88

0

20

40

60

80

100

120

140

160

180

0

706

1,413

2,119

2,825

3,531

4,238

4,944

5,650

6,357

Tualatin River flow at Gaston, in cubic meters per second

Figure 8. Flow-exchange rates between Wapato Lake and Wapato Creek, northwestern Oregon, simulated 

by the Hydrologic Engineering Center-River Analysis System (HEC-RAS) model for water years 1992–2014. 

Wapato Creek at Gaston Road streamflow is shown as a function of Wapato Lake water level and Tualatin 

River streamflow at Gaston, Oregon. Negative Wapato Creek flows (in shades of red) represent a reversed 

flow direction, with water flowing upstream and into the lake due to high water downstream. NAVD 88, North 

American Vertical Datum of 1988.

would begin to overflow its levees. HEC-RAS model results 

essentially converged at the low end of simulated lake levels, 

with results nearly identical at the two lowest simulated lake 

levels. Outflows in the lookup table were set to zero at and 

below a lake level of 50.1 m (164.5 ft), which was roughly the 

elevation of the bottom of Wapato Creek near Gaston Road. 

Flow-exchange rates between lake levels of 50.3 m (165 ft) 

and 50.7 m (166.5 ft) in the lookup table were set identical to the rates at 50.9 m (167 ft). If negative flow-exchange 

rates at that lake level were less than or equal to -0.03 m3/s 

(-1 ft3/s), then that rate was imposed on all lake levels less 

than that level. Exchange rates at lake levels greater than 

54.3 m (178 ft) were set to those resulting from that maximum 

simulated lake level. The lookup table in the WMST was 

populated at lake-level intervals of 0.15 m (0.5 ft); an interpolated exchange rate was computed in the WMST by finding 

the exchange rates for the nearest two lake levels (above and 

below), and calculating a linear interpolation between the two 

rates based on the current lake level.

Verification of WMST Predictions

To verify that results from the open-outlet-enabled 

WMST closely followed those calculated directly by the 

HEC-RAS unsteady flow model, the WMST lake water budget 

and the HEC-RAS model were run under identical river flow 

and lake-level conditions to compare flow outputs at the lake 

outlet. In short, the WMST simulated a daily water budget for 

selected water years with an open lake outlet, using the openoutlet flow-exchange rate table generated previously by the 

HEC-RAS model. A time-series of daily lake levels predicted 

by the WMST then was imposed as the upstream water-level 

boundary condition on Wapato Creek for the HEC-RAS 

model for those same years. Four water years were tested: a 

dry year (2001), an average rainfall year (2007), a wet winter 

year (2003), and a wetter year with a large storm (1997). The 

WMST was run with an initial lake level of 50.6 m (166 ft), an 

open outlet year-round, and routing the three largest tributaries 

(Ayers, Wapato, and Hill Creeks) into the lake. An option to 

limit evapotranspiration in the WMST water budget based on 

the unsaturated zone’s water content was turned on. The initial 

lake level and rerouted tributaries were chosen in an attempt 

to reflect potential refuge-management goals, and also to minimize instabilities in the HEC-RAS model that occurred when 

lake level was less than 50.9 m (167 ft).

WMST lake outflow rates were compared to simulated 

HEC-RAS streamflows in Wapato Creek at the Gaston Road 

station (HEC-RAS river mile 1.85 cross section) for each of 

the water years tested (fig. 9). For the low-flow year (2001, 

fig. 9A), exchange rates were relatively small
Model Results and Evaluation of Water-Management Scenarios 11

EXPLANATION

HEC-RAS

WMST

-53.0

-42.4

-31.8

-21.2

-10.6

0

10.6

21.2

31.8

-1.5

-1.2

-0.9

-0.6

-0.3

0

0.3

0.6

0.9

-353

-283

-212

-141

-71

0

71

141

212

283

-10

-8

-6

-4

-2

0

2

4

6

8

-212

-141

-71

0

71

141

212

283

353

-6

-4

-2

0

2

4

6

8

10

-530

-353

-177

0

177

353

530

706

883

1,059

1,236

-15

-10

-5

0

5

10

15

20

25

30

35

A. Water year 2001

B. Water year 2007

C. Water year 2003

D. Water year 1997

Flow, in cubic

meters per second

Flow, in cubic feet per second

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

Figure 9. Streamflow from the Hydrologic Engineering Center-River Analysis System (HEC-RAS) model and from the 

Wapato Lake Water Management Scenario Tool (WMST) under a wide range of hydrologic conditions in Wapato Creek at 

Gaston Road, northwestern Oregon, in water years (A) 2001, (B) 2007, (C) 2003, and (D) 1997. Positive flows are downstream 

(out of the lake) and negative flows are upstream (into the lake).
12 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

(less than 1.2 m3/s [42 ft3/s]) and characterized by a single 

notable storm in late December. The typical pattern during 

many such storms was for water levels in the Tualatin River 

to rise more quickly than water levels in the Wapato Creek 

drainage, causing streamflow to reverse in Wapato Creek during the early part of the storm as Tualatin River water backed 

up in the creek. Shortly thereafter, Wapato Creek water levels 

increased in response to storm runoff upstream and the direction of flow was restored to the downstream direction. These 

relatively low exchange rates from the WMST and HEC-RAS 

in 2001 were similar in their timing and magnitude, providing 

some validation of the accuracy of open-outlet results from 

the WMST for a low-flow year. For average rainfall (2007, 

fig. 9B) and wet winter (2003, fig. 9C) conditions, streamflow 

patterns from the WMST and HEC-RAS matched well in 

terms of timing and magnitude, but WMST flows tended to 

have a higher magnitude during times of flow reversals, with 

larger negative and positive flows. Outside of a few discrepancies, results from the WMST and HEC-RAS agreed well during medium and small storms as well as base-flow conditions.

During a winter with a large storm and high river levels 

in late December and early January (1997, fig. 9D), greater 

discrepancies between WMST and HEC-RAS streamflows 

occurred in terms of timing and magnitude. Regardless of 

the cause of these timing issues, the total volume exchanged 

between lake and creek evened out after the storm, with cumulative streamflow volumes from the two models that were 

within about 1 percent by the end of the water year (fig. 10). 

The Wapato Lake WMST is meant to be used for the evaluation of water-management strategies as opposed to real-time 

water management during large storms; therefore, these results 

indicate that the approach of using a lookup table of flowexchange rates in the WMST to represent lake/creek outflows, 

as generated from a series of HEC-RAS model runs, is a valid 

approach and accurate enough to use in WMST water-budget 

scenarios for an open lake outlet.

Water-Management Scenarios

Two water-management scenarios were explored with the 

WMST to illustrate how an open lake outlet might affect the 

Wapato Lake water budget and resulting lake levels, and to 

show how that information might be used to evaluate potential 

restoration scenarios for Wapato Lake. The objective was to 

evaluate how an open outlet might affect the management of 

water levels and the amount of shallow and deep water that 

could be critical in determining available habitat for target 

species in the lake. In these scenarios, all the tributaries were 

routed around the outside of the lakebed, similar to how they 

have been treated for years, but the pumphouse at the historical outlet of the lake was removed to create an open connection between Wapato Lake and Wapato Creek downstream. 

In scenario 1, the lake outlet remained open year-round. In 

scenario 2, a gate was used to close that outlet during the 

time of highest downstream water levels (mid-November to 

mid-April). Although several water years were evaluated, 

water year 2012 was chosen as an example because that winter 

had about a half-dozen distinctly separated and moderate 

storms that provided a good illustration of how river and 

lake levels would respond to an open outlet. A summary of 

the water-management details for these scenarios is given in 

table 1.

Scenario 1—Open Outlet Year-Round

With a lake outlet open year-round, water could enter or 

leave the lake through that outlet at any time in response to 

changes in water levels in the lake and downstream. During 

water year 2012, a series of distinct and separate storms 

increased water levels in the Tualatin River downstream and in 

Wapato Creek, causing the WMST to predict large volumes of 

water being imported into the lake through the open connection. When water levels downstream receded after each storm, 

some of that imported water was exported and lake levels 

decreased (fig. 11). At their peak in mid-March, lake levels 

were 2.5 m (8.1 ft) deeper than they were at the start of the 

water year, and lake levels slowly receded as the wet winter 

season gave way to dry summer conditions. Overall, the lake’s 

water budget was dominated by inflows and outflows through 

the open outlet, with 81 percent of all water inputs and 86 percent of all water losses from the lake flowing through the open 

outlet (fig. 12). The only other substantial input was precipitation (18 percent), and the only other substantial loss process 

was open-water evaporation (12 percent).

A major factor in the availability and quality of habitat 

for waterbirds is water depth, with research showing that 

many wading birds, shorebirds, and dabbling duck species 

favor shallow water, whereas diving birds favor deeper water 

(Colwell and Taft, 2000). Optimal water depths for a managed 

lake/wetland complex depend on the habitat requirements of 

the target species, which may vary over the course of a year. 

For any day in its water-budget computations, the WMST 

calculated the area of the lakebed divided among several 

user-defined water-depth categories. Because the desired depth 

ranges required to support a range of target species vary, depth 

ranges of 0–1 m (0–3.3 ft), 1-2 m (3.3–6.6 ft), and more than 

2 m (6.6 ft) were used as simple categories that encompass 

shallow, moderate-depth, and deep-water conditions. For lake 

levels that resulted from a year-round open outlet in scenario 

1, deep-water conditions dominated the lake for 1 to several 

weeks at a time during the largest storms of winter (fig. 13). 

Moderate depths characterized most of the lake most of the 

time from late November through July, and shallow-water 

conditions dominated only prior to the start of the wet season 

and late in the summer. At other times, shallow water was 

fragmented and limited to narrow margins around the edge of 

the lake.
Model Results and Evaluation of Water-Management Scenarios 13

EXPLANATION

HEC-RAS

WMST

0

2,432

4,864

7,296

9,729

12,161

14,593

17,025

19,457

0

3

6

9

12

15

18

21

24

Cumulative volume, in millions of

cubic meters

Cumulative volume, in acre-feet

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

Water year 1997

Figure 10. Cumulative streamflow volume from the Hydrologic Engineering Center-River Analysis System (HEC-RAS) model 

and from the Wapato Lake Water Management Scenario Tool (WMST) in Wapato Creek at Gaston Road, northwestern Oregon, 

during water year 1997 under conditions of an open connection between Wapato Lake and Wapato Creek.

Table 1. Hypothetical scenarios tested with an open lake outlet in the Water Management Scenario Tool (WMST) for Wapato Lake 

near Gaston, northwestern Oregon.

[Symbol: —, option not used]

Input category Details Scenario 1 

(open)

Scenario 2

(open/closed)

Initial level Set the lake’s initial water-level elevation on October 1 to 50.67 meters (166.25 feet). X X

Hydrology

Test a range of hydrologic and meteorological conditions (water years 

1992–2014), but use conditions from water year 2012 for illustration purposes as a “typical” year.X X

Evapotranspiration Activate the option to limit evapotranspiration based on the unsaturated zone’s water content. X X

Tributaries Divert all tributaries around the lake. X X

Pumps Do not use pumps to manage water levels. X X

Lake outlet

Open the lake outlet year-round. X —

Open the lake outlet, but close it during periods (November 15 

through April 15) when high-water conditions occur downstream. — X
14 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

EXPLANATION

Lake bottom

Estimated Wapato Creek stage

Lake level

Oct. Nov. Dec. Apr. Sept.

Water year 2012

Jan. Feb. Mar. May June July Aug.

160.8

164.0

167.3

170.6

173.9

177.2

180.4

49

50

51

52

53

54

55

Elevation, in feet above NAVD 88

Elevation, in meters above NAVD 88

Figure 11. Predicted water levels for Wapato Lake near Gaston, northwestern Oregon, using flow and meteorological 

conditions from water year 2012 and a year-round open connection (scenario 1) between Wapato Lake and Wapato Creek 

and the Tualatin River. NAVD 88, North American Vertical Datum of 1988.

18.2% 0.4%

0.6%

80.8%

2.2%

11.6%

86.2% Precipitation

Runoff to lake

Seepage/leakage

Open outlet

Open-water evaporation

Seepage/leakage

Open outlet

EXPLANATION EXPLANATION

A. Water inputs B. Water losses

Figure 12. Percentages of total water (A) inputs to and (B) losses from Wapato Lake near Gaston, northwestern Oregon, as computed 

from the daily lake water budget for a year-round open lake outlet (scenario 1) and using flow and meteorological conditions from water 

year 2012. Total inputs were 18.2 million cubic meters (m3) (14,800 acre-feet). Total losses were 18.5 million m3 (15,000 acre-feet).
Model Results and Evaluation of Water-Management Scenarios 15

EXPLANATION

Habitat 1: 0 to 1 m depth

Habitat 2: 1 to 2 m depth

Habitat 3: 2 to 7 m depth

Oct. Nov. Dec. Apr. Sept.

Water year 2012

Jan. Feb. Mar. May June July Aug.

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

Figure 13. Potential habitat areas associated with three water-depth ranges, as predicted with a year-round open lake 

outlet (scenario 1) for Wapato Lake near Gaston, northwestern Oregon, using flow and meteorological conditions from water 

year 2012.

Scenario 2—Open Outlet Scheduled

Although some advantages, such as a greater capacity 

for temporary floodwater storage, might be realized by having the lake’s outlet open year-round, the deeper lake water 

that results during winter storms might not provide the water 

depths that refuge managers are seeking to create and maintain 

to benefit various aquatic plants and animals. As an alternative 

that might also have low maintenance costs, an open connection could be combined with a gate structure at the lake outlet, 

thus allowing the outlet to be closed temporarily during storms 

or during part of winter to prevent the import of excess floodwaters. In scenario 2, the lake outlet was kept open except 

for a period of storms and high water from November 15 to 

April 15.

Closing the lake outlet from November 15 to April 15 for 

conditions that occurred in water year 2012 decreased water 

inputs through the lake outlet by 96 percent relative to an open 

outlet year-round. Because the lake outlet was closed during 

the largest storms and the highest downstream water levels, 

the lake level was controlled mainly by an accumulation of 

rainwater. At a peak in early April, lake levels in scenario 2 

were only 1.0 m (3.3 ft) deeper than they were at the start of 

the water year, a full 1.5 m (4.8 ft) decrease from scenario 1 

(fig. 14). In scenario 2, precipitation accounted for 78 percent 

of the lake’s water inputs, with the open outlet accounting for 

only 13 percent of water inputs (fig. 15). Water losses were 

dominated by open-water evaporation (52 percent) and exports 

through the open outlet (42 percent).

By keeping lake levels lower, water depths greater than 

2 m (6.6 ft) were virtually eliminated, and a greater amount 

of shallow water was retained throughout the year in scenario 

2 (fig. 16). Shallow-water conditions in winter would still be 

limited to the margins of the lake because the lake bottom 

is relatively flat, and the area of shallow water was largely 

unchanged from scenario 1 to scenario 2 for the time frame 

outside November 15–April 15. When the lake outlet was 

closed, however, water less than 1 m (3.3 ft) in depth was 

predicted to cover about 50 hectares (124 acres) more than in 

scenario 1.
16 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

EXPLANATION

Lake bottom

Estimated Wapato Creek stage

Lake level: Lake outlet open all year

Lake level: Lake outlet closed 11/15–4/15

Oct. Nov. Dec. Apr. Sept.

Water year 2012

Jan. Feb. Mar. May June July Aug.

160.8

164.0

167.3

170.6

173.9

177.2

180.4

49

50

51

52

53

54

55

Elevation, in feet above NAVD 88

Elevation, in meters above NAVD 88

Figure 14. Predicted water levels for Wapato Lake near Gaston, northwestern Oregon, using flow and meteorological 

conditions from water year 2012 and year-round open connection (scenario 1) as well as an open connection between 

Wapato Lake and Wapato Creek and the Tualatin River that is closed during part (November 15 to April 15) of the wet winter 

season (scenario 2). NAVD 88, North American Vertical Datum of 1988.

77.7%

2.3%

7.3%

12.7%

52.4%

5.5%

42.1%

Precipitation

Runoff to lake

Seepage/leakage

Open outlet

Open-water evaporation

Seepage/leakage

Open outlet

EXPLANATION EXPLANATION

A. Water inputs B. Water losses

Figure 15. Percentages of total water (A) inputs to and (B) losses from Wapato Lake near Gaston, northwestern Oregon, as computed 

from the daily lake water budget for scenario 2, in which the lake outlet was closed during November 15–April 15, and using flow and 

meteorological conditions from water year 2012. Total inputs were 4.1 million cubic meters (m3) (3,300 acre-feet). Total losses were 4.1 

million m3 (3,300 acre-feet).
Implications for Restoration and Water Management 17

EXPLANATION

Habitat 1: 0 to 1 m depth

Habitat 2: 1 to 2 m depth

Habitat 3: 2 to 7 m depth

Oct. Nov. Dec. Apr. Sept.

Water year 2012

Jan. Feb. Mar. May June July Aug.

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

Figure 16. Potential habitat areas associated with three water-depth ranges for Wapato Lake near Gaston, northwestern 

Oregon, as predicted for scenario 2, in which the lake outlet was closed during November 15–April 15, and using flow and 

meteorological conditions from water year 2012.

Implications for Restoration and Water 

Management

HEC-RAS Flow Modeling

The calibrated HEC-RAS model reproduced patterns in 

streamflow and flow direction in Wapato Creek in response to 

storms and downstream Tualatin River conditions. A characteristic flow reversal often was measured at the beginning 

of storms in Wapato Creek at Gaston Road (USGS station 

14202650), with water in the creek temporarily flowing 

upstream because of a rapid increase in downstream water 

levels, followed by a return to a normal flow direction as 

runoff in the Wapato Creek drainage accumulated and moved 

downstream (fig. 9). This dependence of flow magnitude and 

direction on upstream water level (tributary conditions or lake 

level) and downstream flow condition was critical to capture 

and incorporate in the WMST water-budget tool.

HEC-RAS was used to simulate lake/creek exchange 

flows to populate a lookup table for the WMST based only on 

lake level and date, using the date as a surrogate for downstream river flow and related antecedent conditions, but the 

lake level in each of the HEC-RAS model runs was constant. 

It was important, therefore, to determine whether the daily 

exchange flows used by the WMST were sufficiently accurate 

to capture the general timing, variability, and magnitude of 

lake/creek flow-exchange rates under a dynamically changing lake-level scenario. Using a WMST-generated time-series 

of daily lake levels as an upstream boundary condition on 

Wapato Creek, the HEC-RAS model was run to compare 

the magnitude and timing of the lake/creek exchange rates 

from the two models (fig. 9). The general magnitude of the 

exchange flows was expected to be similar because the ultimate source of those flows was from the HEC-RAS model, 

but it was unknown whether the WMST would capture the 

timing and variability of the dynamic exchange flows without 

an iterative solution technique. Agreement between WMSTgenerated lake outflows and HEC-RAS simulated flows was 

good for base-flow conditions and small to moderate storms, 

but large storms sometimes showed some lack of agreement 

in the timing, magnitude, and direction of flow response 

(fig. 9D). Some overshoot and a bit of oscillation in the stormassociated exchange rates was obvious in the results, but the 

comparison showed no systematic timing or lag problems. In 

addition, despite the presence of some disagreement in the two 

models at time scales of 1 or 2 days, the overall volume of 

water moving in or out of the lake was predicted to be similar 

from both the WMST and HEC-RAS for time frames longer 

than individual storms (fig. 10).

Tualatin River flows and Wapato Lake levels seemed to 

be the most important factors accounting for flow and flow 

direction in Wapato Creek, but other factors in the HEC-RAS 

model had minor effects. The flow from Scoggins Creek, a 

tributary that enters the Tualatin River downstream of the 

Wapato Creek confluence, was one such factor. When flow 

in Scoggins Creek was high, that additional water provided 

some resistance to downstream Tualatin River flow, and 

thus increased the downstream resistance to Wapato Creek 

flows entering the Tualatin River. Scoggins Creek is a large 

and important tributary, with flows that can reach more than 

30 m3/s (1,060 ft3/s) and account for a substantial fraction of 

the total flow in the Tualatin River. Farther downstream, HECRAS results showed that the Spring Hill Road bridge structure 
18 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

at the downstream Dilley boundary also provided some resistance to downstream Tualatin River flows, contributing to high 

water levels when Tualatin River flows were high.

In Wapato Creek, the HEC-RAS model was particularly 

sensitive to creek bathymetry and to a beaver dam that was 

included in the model. About 0.50 km (0.31 mi) downstream 

from Gaston Road, the highest bottom elevation of the creek 

was roughly 50.4 m (165.3 ft), which was one factor restricting lake/creek water exchange when lake levels were low. The 

other factor was the beaver dam at a location 1.1 km (0.69 

mi) downstream from Gaston Road, modeled as a “leaky” 

structure with a top elevation of 51.2 m (168.1 ft) and allowing 0.014 m3/s (0.5 ft3/s) of flow to pass through when water 

levels were below its crest. The HEC-RAS model was used 

to simulate flows in Wapato Creek at lake levels from 50.9 

to 54.3 m (167–178 ft), but Wapato Lake levels can decline 

below that range. Largely because of the beaver dam and the 

elevation of the creek bottom, simulated flows in Wapato 

Creek decreased greatly and converged at a lake elevation 

of 50.9 m (167 ft), with modeled flows essentially the same 

at that level as at 51.1 m (167.5 ft). The effect of the beaver 

dam was apparent during model calibration/validation, with 

simulated water levels at Gaston Road remaining constant at 

a minimum lake elevation of 51.3 m (168.2 ft) during summer 

2012 low-flow conditions (fig. 7), only slightly higher than the 

crest elevation of the beaver dam downstream.

Uncertainties and Sources of Error

Although the calibrated HEC-RAS model reproduced 

patterns in streamflow measurements with acceptable accuracy, application of such models is best done with a knowledge of model uncertainties and any sources of potential 

error. Accurate HEC-RAS models are built upon plentiful and 

representative channel and floodplain cross-sectional information and accurate boundary conditions. In this case, the 

lower reaches of Scoggins Creek were not surveyed because 

access to the land could not be obtained; therefore, that creek 

was modeled only as a tributary input even though its channel crossed the Tualatin River floodplain in the model reach. 

On Wapato Creek, a relatively large number of cross sections 

was surveyed, but those cross sections may not have contained 

an optimal number of points in the narrow active channel, 

perhaps resulting in some mischaracterization of the channel 

shape or its deepest point. In addition, more bank surveying 

would have helped to resolve uncertainties stemming from 

potential vegetation bias in the “bare-earth” lidar data that 

were used to help construct model cross sections.

Continuous streamflow data always have uncertainty, 

and some of the flow data used in the model were estimated. 

Streamflow data from the Tualatin River at Gaston (OWRD 

station 14202510) were collected by OWRD and presumed to 

have an accuracy rating of “good” because previous data collected by USGS at the same site had that rating; such a rating 

implies an uncertainty of about 10 percent. Streamflow data 

from Dilley (USGS station 14203500) also were rated “good” 

for the model calibration and validation period. Flows at both 

stations on the Tualatin River (Gaston, Dilley) included overbank conditions where excess flow moved to side channels 

and the floodplain. Streamflow rates under such conditions 

were estimated for the Gaston station using a water-balance 

method; at the downstream Dilley station, those overbank 

flows were measured and included by USGS in the published 

data. The Scoggins Creek streamflow data had similar uncertainties, but those flows tended to be less variable because of 

reservoir regulation.

Some additional model error may be traced to inaccuracies in the representation of instream structures (bridges, 

beaver dams), generalizations in frictional roughness factors, 

and the need to keep the model’s response general. To meet 

the objectives of the study, the model needed to reproduce 

the timing, patterns, and magnitude of flows and water levels 

under a variety of flow conditions and over a period of many 

years. Model parameters can be fine-tuned to optimally fit 

the response to specific storms, but over-calibration to fit 

every detail likely would not maintain fidelity to measurements across a wider range of storms, climate conditions, and 

vegetation cycles. Some amount of model error, therefore, 

was expected based on uncertainties in the data and model 

configuration, and was deemed acceptable for any single storm 

because the general response across a wider range of conditions was reproduced well.

Accuracy of Flow-Exchange Rates

Lake/creek flow-exchange rates predicted by HECRAS and the WMST showed good agreement under most of 

the conditions tested (fig. 9). The WMST overpredicted the 

exchange rate during storms, sometimes predicting a larger 

negative flow rate at the beginning of a storm compared to 

HEC-RAS results, followed by a larger positive flow rate 

later in the storm. Part of this discrepancy occurred because 

the WMST computed the lake water balance on a daily basis, 

whereas HEC-RAS was run with a 15-minute time step. 

Typically, these errors decrease over subsequent time intervals and the two models generally agreed on the cumulative 

volume of water moving in or out of the lake (fig. 10). During 

large multi-day storms with some of the highest streamflows 

in this study (December 1996–January 1997), predicted flowexchange rates from the two models were not in good agreement (fig. 9D). These rare and large winter storms typically 

do not affect water levels or potential habitat for target species 

during summer, and are not expected to skew lake waterbudget predictions later in the water year. The discrepancy in 

results during such large storms serves as a good reminder that 

it is best to focus not on individual storm conditions, but rather 

on longer-term patterns in water levels and habitat.
Implications for Restoration and Water Management 19

Insights from WMST Scenarios

With the addition of an open lake-outlet option in the 

Wapato Lake WMST, resource managers have a tool to 

explore a wide range of potential options for restoration and 

lake-level management. The WMST predicts daily water 

levels and depth-related habitat areas over the course of a chosen water year and in response to user settings for pumping, 

tributary flow routing, and outlet options. In the two scenarios 

highlighted in this report, a hypothetical restored outlet of 

Wapato Lake was opened year-round (scenario 1) or opened 

most of the year (scenario 2, table 1). Pumps were not used 

to try to control the lake level and all tributaries were routed 

around the lake. As a result, flows in and out of the lake’s outlet dominated the lake’s water budget in scenario 1 (fig. 12), 

with most of that water moving in and out of the lake during winter. Closing the lake’s outlet for most of the winter in 

scenario 2 kept water levels in the lake shallower and greatly 

reduced the total volume of water conveyed through the lake 

outlet (figs. 14–15).

Scenario 1 caused most of the lake to become relatively 

deep (> 2 m [>6.6 ft]) during large winter storms in the 

example water year (2012) and caused most of the lake to be 

deeper than 1 m (3.3 ft) for the entire rainy season and well 

into mid-summer (fg. 13). These relatively deep waters could 

be unsuitable for waterbird species that require shallow-water 

habitat. In addition, deep water in the early part of summer 

could retard the growth of, or prevent the establishment of, 

native plant communities that could be critical to the future 

restoration of Wapato Lake. In that case, one way to keep 

water levels lower and avoid some of the maintenance and 

operational costs associated with pumps would be to close the 

lake outlet for part of the winter, as in scenario 2. In that scenario, the lake outlet was closed for 5 months, but the choice 

of when to close the lake outlet in any given year would 

depend on downstream flows and water levels relative to target 

lake levels and the habitat requirements of target species.

Optimal water depths in Wapato Lake will depend on the 

habitat requirements of the fish and wildlife species of greatest 

interest. Research has shown that water depth is an important 

factor in determining usage by different species of waterbirds, with shallow-water habitats favoring a wide diversity 

of wading birds, shorebirds, and dabbling duck species; and 

deeper water favoring diving ducks and coots, for example 

(Murkin and others, 1997; Colwell and Taft, 2000; Taft and 

others, 2002). Wapato Lake has been an important resource 

for hundreds of tundra swans (Cygnus columbianus) as well as 

mallards (Anas platyrhynchos), northern pintails (Anas acuta), 

canvasbacks (Aythya valisineria), ring-necked ducks (Aythya 

collaris), and Canada geese (Branta canadensis), along with 

various species of shorebirds (U.S. Fish and Wildlife Service, 

2007). When Wapato Lake has deep water, the shape of the 

lakebed dictates that shallow water is limited to small areas 

around the margin of the lake; such shallow areas might have 

been more extensive historically when the lakebed was not 

constrained by levees. To continue to support a variety of 

waterbirds requiring a range of water depths, USFWS may 

wish to avoid creating some of the deeper depths resulting 

from the year-round open outlet in scenario 1. A lake outlet 

that is closed for part of the winter high-water season would 

produce lower lake levels and more shallow-water areas in 

the lake.

In the scenarios of this study, Wapato Creek was connected to a hypothetical restored outlet of Wapato Lake and 

all flows in Wapato Creek immediately downstream of the 

lake resulted from flows moving through that outlet. If any 

Wapato Lake tributaries were reconnected to the lake, those 

inputs would directly contribute to the lake’s water budget and 

most of that water eventually would leave the lake through 

the lake’s outlet. Any tributaries not reconnected to the lake 

were assumed to be routed elsewhere, perhaps directly to the 

Tualatin River through a connection near Gaston, or perhaps 

to Wapato Creek farther downstream, such that the extra flow 

from the tributaries was assumed to not greatly affect Tualatin 

River flows or Wapato Creek water levels. If these externally routed tributary waters were to join Wapato Creek near 

the lake outlet, then they would affect the lake/creek flowexchange rates, such that the rates in the WMST lookup table 

would no longer be valid. If such a water-management choice 

is made in the future, the HEC-RAS model would need to be 

re-run under those conditions to better simulate the lake/creek 

flow-exchange rates.

If all of Wapato Lake’s tributaries were to be routed into 

the lakebed, predicted lake levels with an open lake outlet 

would be higher than those in the example scenarios. For a 

year-round open outlet, routing all tributaries into the lake 

would result in an average increase in water depth of 0.25 m 

(0.83 ft) during November–May. If diverse water depths and 

some meaningful amount of shallow-water waterbird habitat is 

desired, then routing all tributaries into the lakebed under such 

conditions might not achieve the desired goals.

Results from this study show that opening the outlet of 

Wapato Lake would not allow the lake to drain by gravity flow 

below a lake level of about 51.2 m (168.1 ft), mainly because 

of the elevation of the bed of Wapato Creek and the presence 

of a beaver dam in that creek downstream of Gaston Road. 

Predicted lake levels in scenarios 1 and 2 during May–July are 

higher with these flow restrictions in place than without them. 

If these obstacles were to be removed or modified, perhaps a 

scenario with an open lake outlet could achieve more shallowwater areas earlier in the summer season, which could benefit 

the establishment of wetland plants as well as provide habitat 

for waterbirds that require shallow water. If such obstacles 

were removed, however, the lake/creek flow-exchange rates 

would need to be re-modeled with HEC-RAS and a new 

lookup table imported into the WMST to evaluate the results 

of that action. For the purposes of exploring potential watermanagement and restoration scenarios at this time, however, 

the current lookup tables and modeling approach is sufficient 

to provide useful insights and results.
20 Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon

Supplementary Material

The archived HEC-RAS models and copies of 

the WMST 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 the 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 the Wapato Lake advisory group for 

helpful feedback and guidance, especially Kristel Griffith 

with the city of Hillsboro; Erin Holmes, Mary Lindenberg, 

Tim Mayer, Curt Mykut, Erin Stockenberg, and David Hills 

with the U.S. Fish and Wildlife Service; Carol Murdock with 

Clean Water Services; Dan Snyder and Glen Hess with the 

U.S. Geological Survey; and Zach Freed with The Nature 

Conservancy.

References Cited

Bancroft, G.T., Gawlik, D.E., and Rutchey, K., 

2002, Distribution of wading birds relative to vegetation and water depths in the northern Everglades 

of Florida, USA: Waterbirds, v. 25, no. 3, p. 265–277, 

accessed July 20, 2019. https://doi.org/10.1675/1524-

4695(2002)025[0265:DOWBRT]2.0.CO;2.

Barnes, H.H., Jr., 1967, Roughness characteristics of 

natural channels: U.S. Geological Survey WaterSupply Paper 1849, 213 p., accessed July 20, 2019, at 

https://pubs.usgs.gov/wsp/wsp_1849/pdf/wsp_1849.pdf.

Bolduc, F., and Afton, A.D., 2008, Monitoring waterbird abundance in wetlands—The importance of controlling results 

for variation in water depth: Ecological Modelling, v. 216, 

p. 402–408, accessed July 20, 2019, at https://doi.org/

10.1016/j.ecolmodel.2008.05.007.

Cass, P.L., and Miner, J.R., 1993, The historical Tualatin River 

Basin: Oregon Water Resources Research Institute, Oregon 

State University Tualatin River Basin Water Resources 

Management Report Number 7, 59 p. [Also available 

at: https://ir.library.oregonstate.edu/concern/defaults/

5q47rt00r.]

Christy, J.A., 2015, Historical conditions at Wapato Lake—

Washington and Yamhill Counties: Oregon, Portland, 

Oregon Biodiversity Information Center, Institute for 

Natural Resources, Portland State University, 7 p.

Chow, V.T., 1959, Open-channel hydraulics: New York, 

McGraw-Hill Book Company, Inc., 680 p.

Colwell, M.A., and Taft, O.W., 2000, Waterbird communities in managed wetlands of varying water depth: 

Waterbirds, v. 23, p. 45–55, accessed July 20, 2019, at 

https://www.jstor.org/stable/4641109.

Elphick, C.S., and Oring, L.W., 1998, Winter management of 

Californian rice fields for waterbirds: Journal of Applied 

Ecology, v. 35, p. 95–108, accessed July 20, 2019, at 

https://doi.org/10.1046/j.1365-2664.1998.00274.x.

Fredrickson, L.H., 1991, Strategies for water-level manipulations in moist-soil systems: Waterfowl Management 

Handbook, U.S. Fish and Wildlife Service Leaflet 13-4-6, 8 

p., accessed July 20, 2019, at https://ecos.fws.gov/ServCat/

DownloadFile/58461?Reference=57616.

Lantz, S.M., Gawlik, D.E., and Cook, M.I., 2011, The effects 

of water depth and emergent vegetation on foraging success 

and habitat selection of wading birds in the Everglades: 

Waterbirds, v. 34, p. 439–447, accessed July 20, 2019, at 

https://doi.org/10.1675/063.034.0406.

Murkin, H.R., Murkin, E.J., and Ball, J.P., 1997, Avian 

habitat selection and prairie wetland dynamics—A 10-year 

experiment: Ecological Applications, v. 7, p. 1144–1159, 

accessed July 20, 2019, at https://doi.org/10.1890/1051-

0761(1997)007[1144:AHSAPW]2.0.CO;2.

Oregon Water Resources Department, 1992–2004, 

Annual reports of the Tualatin River Flow Management 

Technical Committee: Clean Water Services and 

Oregon Water Resources Department, District 18 

Watermaster [variously paged], accessed July 20, 

2019, at https://www.co.washington.or.us/Watermaster/

SurfaceWater/tualatin-river-flow-technical-committeeannual-report.cfm.

Oregon Water Resources Department, 2015a, Near real time 

hydrographics data—Tualatin River at Gaston, Oregon: 

Oregon Water Resources Department web page, accessed 

July 20, 2019, at https://apps.wrd.state.or.us/apps/sw/hydro_

near_real_time/display_hydro_graph.aspx?station_nbr=

14202510.

Oregon Water Resources Department, 2015b, Near real time 

hydrographics data—Scoggins Creek below Henry Hagg 

Lake near Gaston, Oregon: Oregon Water Resources 

Department web page, accessed July 20, 2019, at 

https://apps.wrd.state.or.us/apps/sw/hydro_near_real_time/

display_hydro_graph.aspx?station_nbr=14202980.
References Cited 21

Rounds, S.A., Carpenter, K.D., Fesler, K.J., and Dorsey, J.L., 

2015, Upstream factors affecting Tualatin River algae—

Tracking the 2008 Anabaena algae bloom to Wapato Lake, 

Oregon: U.S. Geological Survey Scientific Investigations 

Report 2015–5178, 41 p., https://doi.org/10.3133/

sir20155178.

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. 

[Also available at https://doi.org/10.3133/sir20205013.]

Taft, O.W., Colwell, M.A., Isola, C.R., and Safran, R.J., 

2002, Waterbird responses to experimental drawdown—

Implications for the multispecies management of wetland 

mosaics: Journal of Applied Ecology, v. 39, p. 987–1001, 

accessed July 20, 2019, at https://doi.org/10.1046/j.1365-

2664.2002.00763.x.

U.S. Army Corps of Engineers, 2010, HEC-RAS river 

analysis system user’s manual, version 4.1: U.S. Army 

Corps of Engineers, Hydrologic Engineering Center, Davis, 

California, Report CPD-68 [variously paged], accessed 

July 20, 2019, at https://www.hec.usace.army.mil/software/

hec-ras/documentation/HEC-RAS_4.1_Users_Manual.pdf.

U.S. Army Corps of Engineers, 2011, HEC-GeoRAS GIS 

tools for support of HEC-RAS using ArcGIS, user's manual, 

version 4.3.93: U.S. Army Corps of Engineers, Institute for 

Water Resources, Hydrologic Engineering Center, Davis, 

California, Report CPD-83 [variously paged], accessed 

July 20, 2019, at https://www.hec.usace.army.mil/software/

hec-georas/documentation/HEC-GeoRAS_43_Users_

Manual.pdf.

U.S. Fish and Wildlife Service, 2007, Tualatin River National 

Wildlife Refuge Wapato Lake Unit addition—Land conservation plan and environmental assessment, Washington and 

Yamhill Counties, Oregon: U.S. Fish and Wildlife Service 

[variously paged]. [Also available at 

https://www.fws.gov/pacific/planning/main/docs/

OR/Wapato/Final%20LCP%20EA/2%20Wapato%20

FONSI%20EA%20LCP.pdf.]

U.S. Fish and Wildlife Service, 2019, Wapato Lake National 

Wildlife Refuge—Environmental assessment, Washington and Yamhill Counties, Oregon: U.S. Fish and Wildlife 

Service, 94 p. plus attachments, accessed March 4, 2020, 

at https://www.fws.gov/refuge/Wapato_Lake/final_restoration_EA/.

U.S. Geological Survey, 2015, Data inventory page for site 

14202980—Scoggins Creek below Henry Hagg Lake, 

near Gaston, Oregon: U.S. Geological Survey web page, 

accessed July 20, 2019, at https://waterdata.usgs.gov/nwis/

inventory/?site_no=14202980.

Washington County, 1872, Washington County road records—

Establishment of County Road A 143 (Gaston Road), 

August 1872: Washington County, Oregon.

Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Oregon Water Science Center

U.S. Geological Survey

2130 SW 5th Avenue

Portland, Oregon 97201

https://www.usgs.gov/centers/or-water
Rounds and Others—Evaluation of Restoration Alternatives Using Hydraulic Models of Lake Outflow at Wapato Lake, Oregon—SIR 2020-5014

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205014