Chapter 5

HumanEnvironment

Chapter 4BiologicalEnvironment

Chapter 3PhysicalEnvironment

Chapter 2ManagementDirection

Chapter 1Introduction andBackground

Appendices

Chapter 3

Physical Environment

© Ed Bustya
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-1 

Chapter 3 Physical Environment 

3.1 Climate

3.1.1 General Climate 

Tualatin River National Wildlife Refuge (the refuge) is located in Washington and Yamhill Counties 

in northwestern Oregon (see Appendix P, Map 1). The refuge is located at the northern end of the 

Willamette Valley in the Tualatin River watershed, between the Coast Range mountains and the 

Willamette River (see Appendix P, Map 1). Much of the refuge lies within the Tualatin River 

floodplain. Major tributaries that drain into the refuge include Chicken and South Rock Creeks in the 

Sherwood Units (see Appendix P, Map 6), and Wapato, Hill, Ayers, Harris, and Scoggins Creeks at 

the Wapato Lake Unit (see Appendix P, Map 7). Many smaller perennial and ephemeral streams also 

drain into the basin. Elevation within the Sherwood Units ranges from about 105 feet to 300 feet. 

Elevation at the Wapato Lake Unit varies from about 160 to 240 feet, with the majority of the lake 

bed at about 170 feet. The Tualatin River Valley consists of low foothills, terraces, alluvial fans, and 

floodplains. The floodplains are subject to frequent flooding during winter and spring. However, 

changes due to agriculture, urbanization, and flood control projects have altered historic flooding 

patterns. 

Photo 3-1. Frozen wetlands on the Atfálat’i Unit, January 2008. Peter Schmidt/ 

USFWS. 

The northern Willamette Valley exhibits a general maritime climate with cool, wet winters and 

warm, dry summers. Temporal patterns of climate variability in Oregon are primarily influenced by 

the Pacific Ocean, namely the El Niño/Southern Oscillation (Oregon Climate Change Research 

Institute [OCCRI] 2010). Precipitation averages about 42 inches per year (Table 3-1) and occurs 

mainly from November through March. Average temperatures are lowest from December through 

February and highest on average during July and August (Table 3-1). Severe weather events are rare 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-2 Chapter 3. Physical Environment 

but may include summer temperatures above 100°Fahrenheit (F), winter snow and ice storms, 

flooding of the Tualatin River and tributaries, high winds, and periods of subfreezing weather. 

Flooding of the river and tributaries typically occurs during December and January when the ground 

has become saturated and rainfall increases (Figure 3-1). 

Table 3-1. Monthly Average Temperatures and Monthly 

Rainfall April 2008-March 2009

Month Average 

Temperature* 

Average 

Precipitation** 

January 40.0 6.74 

February 41.3 3.63 

March 45.3 4.36 

April 48.5 2.95 

May 58.9 2.59 

June 61.8 1.15 

July 68.8 0.21 

August 69.6 0.74 

September 65.2 1.01 

October 53.5 3.62 

November 49.2 7.12 

December 37.5 8.23 

Total 42.35 

* National Oceanic and Atmospheric Administration (NOAA) National 

Weather Service for Portland, Oregon. 

**Average from refuge rain gauge 2003-2010. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-3 

Average and Peak River Level at Farmington Guage 

2002 - 2010

100.00

105.00

110.00

115.00

120.00

125.00

130.00

135.00

140.00

1-Oct 1-Nov 1-Dec 1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep

Date

River Elevation (Feet MSL)

Flood Stage

Average Stage

Peak flow 

Figure 3-1. Average and peak river levels measured in feet above mean sea level (MSL) at 

Farmington gauge approximately 8 miles upstream from the westernmost parcel in the 

Sherwood Units. 

3.1.2 Climate Change 

As stated in U.S. Department of the Interior (USDOI) Secretarial Order 3226 (USDOI 2009) and the 

Service’s climate change strategic plan (USFWS 2010c), the Service considers and analyzes climate 

change in its decisions, long-range plans, and other activities. Also, refuge efforts contribute to 

carbon sequestration through habitat restoration and protection efforts. A 1999 U.S. Department of 

Energy report, Carbon Sequestration Research and Development (U.S. Department of Energy 1999), 

concluded that ecosystem protection is important to carbon sequestration and may reduce or prevent 

loss of carbon currently stored in the terrestrial biosphere. The report defines carbon sequestration as 

“the capture and secure storage of carbon that would otherwise be emitted to or remain in the 

atmosphere.” 

Greenhouse gases (GHG) occur naturally in the Earth’s atmosphere, but are also added by human 

activities. This happens primarily through the burning of fossil fuels, such as coal, oil, and natural 

gas, which release carbon dioxide into the atmosphere. Over the past century, atmospheric carbon 

dioxide (as measured from ice cores) has increased due to human activities, and the average Earth 

temperature has increased approximately 0.7°Celcius (C) or about 1.3°F (NOAA 2011a; OCCRI 

2010) (Figure 3-2), and it continues to increase. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-4 Chapter 3. Physical Environment 

Figure 3-2. Global surface temperature (NASA 2012). 

The greenhouse effect is a natural phenomenon that assists in regulating and warming the 

temperature of our planet. Just as a glass ceiling traps heat inside a greenhouse, certain gases in the 

atmosphere, called greenhouse gases, absorb heat from sunlight. The primary GHGs occurring in the 

atmosphere include carbon dioxide, water vapor, methane, and nitrous oxide. Carbon dioxide is 

produced in the largest quantities, accounting for more than half of the current impact on the Earth’s 

climate. 

The terms “climate” and “climate change” are defined by the Intergovernmental Panel on Climate 

Change (IPCC). “Climate” refers to the mean and variability of different types of weather conditions 

over time, with 30 years being a typical period for such measurements, although shorter or longer 

periods also may be used (IPCC 2007b:78). The term “climate change” thus refers to a change in the 

mean or variability of one or more measures of climate (e.g., temperature or precipitation) that 

persists for an extended period, typically decades or longer, whether the change is due to natural 

variability, human activity, or both (IPCC 2007b:78). 

Scientific measurements spanning several decades demonstrate that changes in climate are occurring, 

and that the rate of change has been faster since the 1950s. Examples include warming of the global 

climate system, and substantial increases in precipitation in some regions of the world and decreases 

in other regions. (For these and other examples, see Climate Change 2007: The Physical Science 

Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change [IPCC 2007a:30] and Technical Summary [Solomon et al. 2007:35-54, 82-

85].) In the Pacific Northwest, increased GHGs and warmer temperatures have resulted in a number 

of physical and chemical impacts. These include changes in snowpack, stream flow timing and 

volume, flooding and landslides, sea levels, ocean temperatures and acidity, and disturbance regimes 

such as wildfires, insects, and disease outbreaks (United States Global Change Research Program 

[USGCRP] 2009). All of these changes will cause major perturbations to ecosystem conditions, 

possibly imperiling species that evolved in response to local conditions. 

Results of scientific analyses presented by the IPCC show that most of the observed increase in 

global average temperature since the mid-twentieth century cannot be explained by natural variability 

in climate, and is “very likely” (defined by the IPCC as 90 percent or higher probability) due to the 

observed increase in GHG concentrations in the atmosphere as a result of human activities, 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-5 

particularly carbon dioxide emissions from use of fossil fuels (IPCC 2007b:5-6, Figures SPM.3 and 

SPM.4; Solomon et al. 2007:21-35). Further confirmation of the role of GHGs comes from analyses 

by Huber and Knutti (2011:4), who concluded it is extremely likely that approximately 75 percent of 

global warming since 1950 has been caused by human activities. 

Looking toward the future, the University of Washington Climate Impacts Group (CIG) has 

projected changes in mean annual temperature and precipitation for the Pacific Northwest based on 

“statistical downscaling” an ensemble of 20 global climate models and two carbon emissions 

scenarios for each model run (Mote and Salathé 2009, 2010). The CIG also performed projections 

using two regional climate simulations with the same two emissions scenarios (Salathé et al. 2010). 

These two approaches provide the projected changes described below. 

3.1.2.1 Temperature: Historical Trends and Projections 

The Pacific Northwest experiences wide temperature variability based on geography (for example, 

the Olympic, Coast, and Cascade mountain ranges enhance a moderating maritime influence west of 

the Cascades) and seasonal and year-to-year variability (e.g., associated with the El Niño-Southern 

Oscillation) (Salathé et al. 2010). In the Pacific Northwest, regionally averaged temperature rose 

1.5°F between 1920 and 2000, slightly more than the global average. Warming was largest for the 

winter months of January through March. Minimum daily temperatures have increased faster than 

maximum daily temperatures (Mote et al. 2005). 

Historical climate data for Tualatin River National Wildlife Refuge are available from the United 

States Historical Climatology Network (USHCN), a high-quality data set of daily and monthly 

records of basic meteorological variables from 1,218 observing stations across the 48 conterminous 

states. The data have been corrected to remove biases or heterogeneities from non-climatic effects 

such as urbanization or other landscape changes, station moves, and instrument and time-ofobservation changes. 

The USHCN Forest Grove, Oregon, station is the station closest to the refuge, and temperature data 

are shown in Figure 3-3. There has been a statistically significant increase of 1.36°F (p = 0.001) in 

average annual temperature from 1925 to 2010 at the Forest Grove station, similar to the regional 

average. Much of the increase is due to the increase in minimum temperature (increase of 1.53°F, p = 

0.000) rather than maximum temperature (increase of 1.1°F, p = 0.025). In the past 30 years, 

temperature increases have been particularly pronounced during summer months (Table 3-2). 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-6 Chapter 3. Physical Environment 

Figure 3-3. Water year temperature 1925-2010. USHCN Station: Forest Grove, Oregon 

(352997). 

Table 3-2. Seasonal Temperature Trends, 1981-2010 (Monthly Records) 

Monthly Absolute Change 

Temp. Max Temp. Avg. Temp. Min. 

Winter (Dec-Feb) +0.54°F +0.18°F −0.21°F 

Spring (march-May) −0.23°F −0.44°F −0.64°F 

Summer (Jun-Aug) +1.36°F +1.07°F +0.79°F 

Fall (Sept-Nov) −0.36°F −0.41°F −0.48°F 

* Forest Grove, Oregon United States Historical Climatology Network Observation Station 

Looking to future projections, researchers have refined global circulation models so that they are 

useful at a regional scale, in our case, for the Pacific Northwest. One approach involves statistical 

downscaling of 20 global climate models, which projects that average annual temperature would 

increase by 2.0°F by the decade of the 2020s for the Pacific Northwest, 3.2°F by the decade of the 

2040s, and 5.3°F by the decade of the 2080s, relative to the 1970-1999 average temperature (Mote 

and Salathé 2009, 2010). This is substantially greater than the 1.5°F increase in average annual 

temperature observed in the Pacific Northwest during the twentieth century. Seasonally, summer 

temperatures are projected to increase the most. One important note is that actual global emissions of 

GHGs in the past decade have exceeded even the highest emissions scenario (the A2 scenario), a 

scenario that was not modeled by Mote and Salathé in 2009 and 2010, or Salathé et al. in 2010 

(discussed below). The authors used more moderate emissions scenarios. If these emission trends 

continue, temperature increases could turn out to be much greater than projected. 

35

40

45

50

55

60

65

70

1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Temperature (F)

Year

Average WY Max Average WY Mean Average WY Min 4-yr moving average
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-7 

Another approach involves two regional climate simulations (Salathé et al. 2010). These findings 

support the warming increases described above, with small variations. West of the Cascades, both 

regional climate change models predict warming in all seasons (when compared to current average 

air temperatures) (Salathé et al. 2010). Increases in air temperature are forecasted to be the greatest in 

summer and spring (including increased frequency of heat waves, which are three or more 

consecutive days with a daily heat index of over 89.6°F or 32°C), while increased marine cloudiness 

in fall and winter may dampen temperature increases during those seasons (Salathé et al. 2010). 

It should be noted that these projections are not intended to be predictions of actual conditions on any 

given date. Models represent the average response over climatological timescales (i.e., on a multidecadal timescales) rather than precise behavior at exact dates. As discussed above, the Pacific 

Northwest’s long-term climate includes a high degree of interannual and decadal variability, and this 

is projected to continue. However, long-term trends and projections indicate that overall temperatures 

are increasing and will continue to do so. 

3.1.2.2 Precipitation: Historical Trends and Projections 

Longer-term precipitation trends in the Pacific Northwest are more variable than temperature and 

vary with the period of record selected for analysis (Mote et al. 2005). The Pacific Northwest 

experiences this wide precipitation variability due to geography (e.g., the Olympic, Coast, and 

Cascade mountain ranges cause orographic, or upward, lift of air masses and precipitation) and 

seasonal and year-to-year variability (e.g., variation associated the El Niño Southern Oscillation and 

other longer-term oscillations) (Salathé et al. 2010). Looking at the period 1920 to 2000, precipitation 

has increased almost everywhere in the region, though not in a uniform fashion. Most of that increase 

occurred during the first part of the record (Mote et al. 2005). 

The Forest Grove USHCN station (the closest station to the refuge) provides historical precipitation 

data, shown in Figure 3-4. There is no statistically significant trend, but note the year-to-year 

variability, such as flood risk during El Niño years in the late 1990s, which can provide management 

challenges for the refuge. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-8 Chapter 3. Physical Environment 

Figure 3-4. Water year total precipitation 1925-2010. USHCN Station: Forest Grove, Oregon. 

The other significant trend found from the Forest Grove USHCN observation station is a 37 percent 

decline in summer precipitation in the last 30 years (Figure 3-5). 

Figure 3-5. Summer (Jun-Aug) total precipitation, 1925-2010. USHCN Station: Forest Grove, 

Oregon (352997). 

15 

25 

35 

45 

55 

65 

75 

85 

1925 1930 1935 1940 1945 1950195519601965197019751980198519901995 2000 2005 2010

Year

WY Total Pcp (in) 4-yr moving average
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-9 

West of the Cascades, projections of future precipitation levels vary by model, with a majority of 

models projecting an annual increase in precipitation (Mote and Salathé 2010) and some models 

projecting a decrease relative to current observations (Leung et al. 2004). A strong seasonal pattern in 

altered precipitation relative to current levels is foreseen, as well as increases in the number of 

extreme precipitation events (daily precipitation exceeding the twentieth-century 95th percentile) 

(Salathé et al. 2010). Seasonal increases in precipitation are projected for fall, winter, and spring, and 

little change or a slight decline in precipitation for summer (Salathé et al. 2010). Regional climate 

change models project conflicting results for the spring season, with one model showing a substantial 

decline in precipitation while the second shows increases in precipitation (Salathé et al. 2010). 

3.1.2.3 Stream Flow Changes 

In addition to changes in the amount of precipitation, a major concern in the Pacific Northwest is 

change in the form of winter precipitation expected due to warmer temperatures. The CIG has 

modeled changes in current and future peak snowpack versus October through March precipitation 

for watersheds in the Columbia River Basin and the coastal watersheds of Oregon and Washington. 

Generally, there is a large shift in the form of winter precipitation from snow to rain, with lower 

elevation basins affected before upper elevation basins (Elsner et al. 2010). As these changes occur, 

there would likely be a tendency for higher winter flows and possible increased risk of flooding in 

transition watersheds (those not dominated by snow processes and those not rain-dominant), earlier 

snowmelt and runoff from peaks, and lower summer stream flows (Hamlet and Lettenmaier 2007). 

The Tualatin River watershed is a rain-dominant system, and these changes may not be significant to 

the refuge. 

3.2 Hydrology 

3.2.1 Regional Hydrology 

The refuge lies within the Tualatin River watershed, which is a part of the Willamette River Basin 

(see Appendix P, Map 3). The Tualatin River is one of the most northern tributaries in the Willamette 

River watershed and is about 80 miles long (Tualatin River Watershed Council [TRWC] 1998). The 

river flows roughly eastward from the Coast Range mountains in Washington County, Oregon, to its 

confluence with the Willamette River in Clackamas County, Oregon, near the City of West Linn. The 

elevation at the headwaters of the Tualatin River is approximately 1,980 feet, and there are four large 

elevation drops in the river before it enters the Tualatin Valley plain at an elevation of 240 feet. 

Where the Tualatin River meets Wapato Lake, the elevation is about 170 feet, and where the Tualatin 

River meets the Willamette River, it is 58 feet in elevation. The Tualatin River drainage basin is 

approximately 43 miles long and 29 miles wide, covering an area of 712 square miles (TRWC 1998). 

Annually, more than 1.1 million acre-feet of water flow out of this watershed into the Willamette 

River. Nearly 85 percent of this flow is discharged during November through March. 

The river can be characterized in four distinct reaches with varying degrees of flows and gradients 

(Table 3-3). 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-10 Chapter 3. Physical Environment 

Table 3-3. Approximate Reaches of the Tualatin River As it Flows from the Coast Range 

Mountains (river mile 80) to its Confluence with the Willamette River (river mile 0)

Reach Description River mile Elevation Gradient 

(feet/mile) 

Description 

1 Mountain 80-58 80.4 Forested, fast-moving mountain streams, 

rock channels 

2 Meander 58-33 2.8 Transition from timberland to farmland, 

major floodplain storage, logjams 

3 Reservoir 33-3.4 0.2 Flat, “reservoir like” flow with heavy 

sedimentation, recreational use 

4 Riffle 3.4-0.0 10.1 Steep gradient, short series of shallow 

pools and riffles 

The first reach of the river starts in the mountains and is characterized by forested, fast-moving 

mountain streams with rock channels. As the river flows through the basin, it slowly transitions from 

forested areas into more open habitat, and the flow changes from fast to slow flowing. The Tualatin 

River has five major and numerous minor tributary streams. Streams entering the river within the 

refuge include Wapato, Ayers, Hill, Harris, and Scoggins Creeks in the Wapato Lake Unit, and 

Chicken and Rock Creeks in the Sherwood Units. Average daily flow rate in the Tualatin River 

measured at the West Linn gauging station, approximately 1.7 miles from the confluence with the 

Willamette River, ranged from a low of 80 cubic feet per second (cfs) to a high of 4,150 cfs between 

1928 and 2010 (Figure 3-6). These figures do not include water that is diverted upstream of this 

gauging station into Oswego Lake. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-11 

Average Daily Discharge Rate of Tualatin River at West Linn Gauge 

1928-2010

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

1-Jan

1-Feb

1-Mar

1-Apr

1-May

1-Jun

1-Jul

1-Aug

1-Sep

1-Oct

1-Nov

1-Dec

Date

Discharge (CFS)

Figure 3-6. Average daily discharge rate (cfs) measured on the Tualatin River at the West Linn 

gauging station. 

In general, stream flow in the Tualatin River Basin reflects patterns of precipitation with high winter 

and low summer flows (Figure 3-6). River elevation measured at the Farmington gauge is typically at 

summer flows (<110 feet) from late April or early May until late October when precipitation begins 

to fill the river basin). Flooding in tributary streams is typically more prevalent during short-term 

heavy rainfall after the surrounding watershed is saturated by early season rainfall. In particular, the 

Chicken Creek watershed, which includes Cedar Creek, flows from the City of Sherwood and 

experiences heavy flooding. The population of Sherwood has increased from about 3,000 in 1990 to 

over 18,000 in 2010, and this has led to an increase in urban development (U.S. Census Bureau 

2011). This change has created increased impervious surfaces (e.g., roads, rooftops) and has led to an 

increase in runoff and higher peak flows in Chicken Creek that flow into the refuge. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-12 Chapter 3. Physical Environment 

Photo 3-2. Chicken Creek flooding. Peter Schmidt/USFWS. 

There are two dams affecting water flows within the basin: Scoggins dam on Scoggins Creek, 

creating Henry Hagg Lake, and the Lake Oswego diversion dam, which shunts water to Oswego 

Lake. In addition, Barney Reservoir on the Trask River diverts water to the Tualatin River during 

summer to augment flows. Henry Hagg Lake has a capacity of 53,600 acre-feet of water and is 

managed by the Bureau of Reclamation (USDOI 2011). A large part of refuge lands, both in the 

Sherwood Units and the Wapato Lake Unit, lie within the jurisdictional boundary of the Tualatin 

Valley Irrigation District. This district was formed to deliver irrigation water associated with the 

Henry Hagg Lake project. As members of the district with attached water rights, the refuge is 

required to pay approximately $14,500 a year for irrigation on lands that fall within the district. 

3.2.2 Refuge-specific Hydrology 

Water Rights: The refuge has a complex system of irrigation ditches, springs, creeks, and water 

control structures for moving water within the refuge to fill the various wetland cells and for 

irrigation purposes in the cooperative farming program. The refuge has 51 water rights with priority 

dates ranging from 1890 to 2000. Sources of water include wells, ditches, unnamed creeks, springs, 

overland flow and runoff, Chicken Creek, Hill Creek, Rock Creek, Scoggins Creek, and the Tualatin 

River. The purposes of the water rights are primarily for irrigation and wildlife use, and to ensure that 

water is diverted for use on approximately 1,157 acres of wet prairie, scrub-shrub, and herbaceous 

wetlands at the Sherwood Units and the lake bed at the Wapato Lake Unit. In addition, the Service 

voluntarily selected 15 irrigation water rights for in-stream leases, which aids in restoration and 

protection of over 4 cfs of stream flow in Rock Creek and the Tualatin River. 

Table 3-4 summarizes the water rights of the refuge. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-13 

Table 3-4. Summary of Water Rights for the Refuge 

Priority Date Application 

No. 

Certificate 

No. 

Source Name Flow Rate 

(cfs) 

Place of 

Use/Storage 

(Acres/AcreFeet) 

Groundwater 

25-Jul-50 G-2325 33116 Well 0.026 2.9 

30-Sep-68 G-4614 45012 Sump well 0.07 

27-Oct-88 G-11863 72443 Well 0.35 

19-Oct-06 G-16740 Three wells 0.13 

In stream Lease 

02-Mar-51 IL-809 20149 Rock Creek 0.34 

05-Dec-28 IL-867 8117 Rock Creek 0.38 

05-Dec-28 IL-867 8117 Rock Creek 0.38 

1-01-1890 IL-869 29185 Cummings ditch 0.1 

16-Jun-50 IL-868 22918 Tualatin River 0.35 

1-01-1890 IL-870 29186 Rock Creek and 

unnamed ditches 

0.338 

1-01-1890 IL-871 29191 Rock Creek 0.044 

1-01-1890 IL-872 29192 Unnamed ditches 0.037 

1-01-1890 IL-873 29193 Rock Creek 0.1 

1-01-1890 IL-874 29195 Springs and ditch 0.044 

1-01-1890 IL-875 29202 Cummings Ditch and 

unnamed ditches 

0.231 

1-01-1890 IL-876 29282 Rock Creek 0.063 

25-May-53 IL-890 81965 Tualatin River 0.97 

Reservoir 

23-Jun-97 R-83470 Chicken Creek 579 AcreFeet 

01-Jul-97 R-83496 Runoff 180 AcreFeet 

12-Aug-97 R-83529 Runoff and Chicken 

Creek 

 40 Acre-Feet 

08-Aug-00 R-84503 Tualatin River, 

overland flow 

 100 AcreFeet 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-14 Chapter 3. Physical Environment 

Table 3-4. Summary of Water Rights for the Refuge 

Priority Date Application 

No. 

Certificate 

No. 

Source Name Flow Rate 

(cfs) 

Place of 

Use/Storage 

(Acres/AcreFeet) 

25-Jul-01 R-84813 Runoff 10 Acre-Feet 

24-Apr-03 R-85570 84056 Overland flow/runoff 1.6 Acre-Feet 

24-Apr-03 R-85571 84055 Overland flow/runoff 2.9 Acre-Feet 

24-Apr-03 R-85572 84054 Overland flow/runoff 2 Acre-Feet 

24-Apr-03 R-85573 84053 Overland flow/runoff 1.8 Acre-Feet 

06-May-05 R-86279 Runoff 60 Acre-Feet 

Surface Water 

29-Dec-28 S-12500 14092 Tualatin River 6.93 553.8 Acres 

29-Dec-28 S-12500 14093 Tualatin River 2.03 162.3 Acres 

29-Aug-30 S-13702 14096 Chicken Creek (Cedar 

Creek) 

1.8 143.3 Acres 

19-Nov-35 S-16153 24368 Chicken Creek 1.01 80.8 Acres 

04-Sep-36 S-16568 45451 Hill Creek 0.06 4 Acres 

04-Sep-36 S-16568 45451 Tualatin River .44 36 Acres 

01-Aug-39 S-18312 19761 Tualatin River 1.08 21 Acres 

26-Feb-40 S-18591 15099 Tualatin River 0.21 2.1 Acres 

14-Nov-49 S-24270 22317 Rock Creek 0.044 3.5 Acres 

25-Jul-50 S-25034 23148 Drainage ditch and 

unnamed stream 

0.16 7 Acres 

25-Jul-50 S-25035 20132 Unnamed drainage 0.024 2.9 Acres 

12-Mar-51 S-25704 40860 Tualatin River 0.38 30 Acres 

02-Jun-65 S-40957 40726 Tualatin River 0.6 50 Acres 

31-Mar-71 S-48082 43946 Tualatin River 0.96 77.1 Acres 

28-Jun-82 S-63714 64950 Chicken Creek 1.08 86.2 Acres 

25-May-53 T-10246 81965 Tualatin River 0.13 

02-Jan-00 29187 Rock Creek 

Source: USFWS 2012. 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-15 

The main season of water use is usually between November and May at select locations. This varies 

with water conditions as determined by annual precipitation. Adequate water is important to provide 

habitat for migratory birds and for irrigation of habitat restoration sites. During the winter, most 

wetland cells (including the Wapato Lake Unit lake bed) are kept full to provide water for both 

resident and wintering species. 

Photo 3-3. Tualatin River flooding at the Tualatin River Unit, December 2007.

Peter Schmidt/USFWS. 

Sherwood Units: The floodplain of the Tualatin River is the dominant feature within the Sherwood 

Units (see Appendix P, Map 6). The Atfálat’i Unit is bordered by the Tualatin River, but water for 

wetlands in much of this unit comes from a diversion on Chicken Creek. The diversion allows 

gravity flow to fill five managed wetland cells in the unit. One additional wetland cell is filled by rain 

water and is managed as a permanent wetland with no outlet structure. Another wetland cell is also 

filled with rain water and overland runoff, but is managed as a seasonal wetland with a water control 

structure at the outlet. 

On the Tualatin River Unit there are five wetland impoundments that normally fill with rain and 

overland runoff, but the largest wetland also has a pump with a fish screen in the Tualatin River that 

may be used to conduct early season flood-up or to augment rainfall during periods of drought. On 

the Riverboat Unit there are four impoundments, with a seasonal wetland filled using a water control 

structure to back up overland flow from springs and rainfall and, frequently, floods from high water 

in the Tualatin River. In addition, the seasonal wetland has a series of three low-capacity wells that 

are sometimes used to augment overland flow in early fall. There is also a scrub-shrub wetland, 

which is a closed water body and is filled by rainfall alone. During extreme flooding of the Tualatin 

River, the scrub-shrub wetland becomes a flow channel for river water. This unit has extensive 

floodplain connections to the river and is often saturated through the winter. 

The Onion Flats Unit is a low-lying basin bisected by Rock Creek. The creek is channelized and 

straightened through this unit and is deeply incised. This area floods frequently as the Tualatin River 

water backs up Rock Creek, and also during periods of heavy rain. The Rock Creek Unit features its 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

3-16 Chapter 3. Physical Environment 

namesake creek and is also largely channelized. This is a relatively small creek through this unit and 

rarely causes any major flooding. 

During winter as the Tualatin River begins to reach bank full stage (elevation 128 above mean sea 

level), many backwater areas in riparian forest and wetland habitats begin to flood. Vegetation 

communities in these areas are adapted to frequent winter flooding, and fish and wildlife species use 

these areas for foraging and sanctuary. Inundation in these areas is typically intermittent and of short 

duration 

Wapato Lake Unit: The Tualatin River bisects the Wapato Lake Unit acquisition boundary. Inputs to 

Wapato Lake proper were historically from Wapato, Ayers, and Hill Creeks, and from the river 

backing up into the lake (see Appendix P, Map 7). The historical Wapato Lake lake bed was severely 

altered during the late 1800s and early 1900s with the beginning of onion farming that required an 

extensive network of levees, ditches, canals, and a pump station to dewater the lake bed in late winter 

for spring planting. When the infrastructure was originally constructed by the landowners, Wapato, 

Ayers, and Hill Creeks were diverted around the outside of the levees to connect with the Tualatin 

River later. During fall and winter, the lake was allowed to fill with rain water, and it was pumped 

out in early spring for farming. In 1950s, the landowners of the lake bed supported the formation of 

the Wapato Improvement District (WID), a state-sponsored irrigation and drainage district. The WID 

was tasked with the maintenance and operations of the 5.5 miles of levees around the lake, a lift 

pump station, and interior ditches to continue facilitation of drainage of the lake for farming. In 2012, 

the WID voted to dissolve and all assets were donated to the Service. Currently the refuge has a 

cooperative farming program that uses much of the same infrastructure. In the northern part of the 

acquisition boundary, Scoggins and Harris Creeks are tributaries to the Tualatin River. 

3.3 Topography and Bathymetry 

The refuge area lies in the northwestern corner of the Willamette Valley, southwest of Portland, 

Oregon (see Appendix P, Map 1). Elevations on the refuge range from about 105 feet to 300 feet. 

The Tualatin River floodplain is relatively flat, with gradually sloping sides leading to upland forest 

types, agricultural lands, and developed areas. Major mountain features include the Coast Range to 

the west of the Wapato Lake Unit and Chehalem Ridge rising to 1,629 feet (Oregon Parks and 

Recreation Department 2011) between the Wapato Lake and Sherwood Units. The Tualatin River 

channel is deeply incised with steep banks throughout most of its length. Henry Hagg Lake is a 

1,110-acre reservoir (ODFW 2011a) approximately 3 miles west of the Wapato Lake Unit. 

Many refuge land parcels have had detailed topographic surveys completed prior to initiation of 

restoration projects. In the Sherwood Units, surveys were conducted on the Atfálat’i, Tualatin River, 

and Riverboat Units. The refuge is currently working with U.S. Geological Survey (USGS) and other 

partners to conduct a hydrogeomorphic survey at the Wapato Lake Unit that will provide a digital 

elevation model for the lake bed and surrounding area, as well as a water budget for future 

restoration planning. 
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Chapter 3. Physical Environment 3-17 

3.4 Geology and Geomorphology 

3.4.1 Origins and Development 

During the Oligocene epoch roughly 35 million years ago (MYA), a line of sea floor subduction, 

which had run in a northeast direction from west of the Klamath Mountains to the Blue Mountains, 

moved to its current position several hundred miles to the west on its northern end. This change left a 

slab of what had been subducting sea floor attached to the continental margin and also moved the line 

of volcanic activity associated with remelting oceanic crust. Part of this slab, covered by shallow 

seas, ultimately became the floor of the Willamette River basin. Although volcanic eruptions had 

started building the southern Cascade Mountains millions of years earlier in the Eocene epoch, the 

northern Cascades lie on top of the slab and arose as volcanoes. Volcanic rocks in the western 

Cascades date from two principal periods of volcanic activity, one occurring 20 to 30 MYA and the 

other in the Miocene, 10 to 15 MYA; both periods included the entire length of the Cascades 

(Branscomb 2002). The Tualatin River Basin is underlain by 9,000-12,500 feet of sedimentary and 

volcanic rock (Popowski 1996; Wilson 1998). On top of this layer lie up to 900 feet of Columbia 

River basalt and another 1,200 feet of sediments from the late Miocene to Pleistocene. 

The Coast Range arose as continental sediments carried eastward by the subducting oceanic plate 

were forced under the western edge of the slab, pushing it upward. The rise proceeded from south to 

north, the northern Coast Range not appearing until roughly 15 MYA. During this process, the area 

of the future Willamette Valley also rose, becoming dry land and draining its embayment by about 20 

MYA. Although the direction of sea floor spread is southeastward toward the Oregon coast, the 

entire Pacific plate, including the spreading and subduction zones, is moving northward. The net 

effect for the Oregon Coast Range appears to be a slow movement north, evidence for which may be 

seen in the roughly 50-mile northward offset of the Columbia River at Portland and the sharply 

curved Olympic Mountains in western Washington State, where movement of the Coast Range 

encountered the more deeply rooted Vancouver Island. This northward tearing of the Coast Range 

may also have contributed to the formation of the Willamette Valley, the floor of which is now 

composed of multiple fault blocks. In contrast to its south end, the valley is spreading more in the 

north, possibly due to the thinner crust and reduced linkage with the subducting sea floor present 

there (Branscomb 2002). 

3.4.2 Recent Geologic Events 

Defining the northern limit of the Willamette River Basin, the Columbia River has reestablished its 

course from multiple dams created by slides, basalt flows, and ice. Reaching their maximum 13 

MYA, basalt floods from the enormous Grande Ronde volcano in northeast Oregon flowed down the 

Columbia channel all the way to the river’s mouth and into the Willamette Valley. In the Pliocene (3 

to 11 MYA), western Oregon was a desert, and gravels produced during that time eroded into and 

covered the spreading valley floor. 

Work performed by J. Harlan Bretz in the 1920s revealed that between 15,500 and 13,000 years ago, 

ice periodically dammed the Clark Fork River in Idaho east of Spokane, Washington, forming a lake 

in the present-day Missoula Valley of Montana covering some 3,100 square miles and containing 500 

cubic miles of water. In a typical release occurring over less than two weeks, a quantity of water 

equal to half the present volume of Lake Michigan, more than the modern annual volume of all the 
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3-18 Chapter 3. Physical Environment 

world’s rivers, poured down the Columbia channel. These floods were known as the Missoula 

Floods. Backwater from these floods carried ice-rafted “erratic” rocks from Montana repeatedly into 

the Willamette Valley, filling it to about 400 feet above current sea level. Evidence in the geologic 

record suggests that these floods entered the Tualatin River Valley at least 22 times during this time 

period (Wilson 1998). This vigorous overflow southward across the divide between the Willamette 

River and Tualatin River Basins scoured multiple channels in the basaltic bedrock, dug small rockbound basins, and left rock knobs barren of soil, forming a topography in the Rock Creek area just 

east of the City of Sherwood that is a miniature replica of the well-known scablands in eastern 

Washington. The sediments left by these floods form much of the present valley floor (Branscomb 

2002). In the Tualatin River Basin, these overlying sediments have been classified as the Hillsboro 

Formation (Wilson 1998) and consist of about 900 feet of fluvial and lacustrine sediment overlying 

95 feet of lateritic soil and weathered basalt of the Columbia River Basalt Group. This area is still 

geologically active as evidenced by the 1980 eruption of Mount Saint Helens 65 miles to the 

northeast. 

3.5 Soils 

Within the refuge boundaries there are 64 soil types described (U.S. Department of Agriculture 

[USDA] 1982). These can roughly be broken down into the following six categories: 1) clay, 2) 

loam, 3) silt loam, 4) silty clay loam, 5) stony silt loam, and 6) Xerochrepts. A majority of refuge 

lands in the floodplain contain clays, silty clay loams, and silty loams. Most of these lands were in 

agricultural production prior to purchase by the Service. On areas outside the floodplain, most of the 

soils are loams and silty loams that are well drained and primarily support mixed upland forest types. 

There are two areas of particular historical interest—Onion Flats and Wapato Lake. These areas 

contain Labish Mucky Clay, a rare organic peat soil type that historically supported scrub-shrub and 

other wetland types. It is heavy, poorly drained soil, usually associated with old concave lake beds. 

These areas were drained for farming in the early twentieth century and continue to be cooperatively 

farmed by the refuge at present. The major crop in these areas was onion, but with a shift in 

economics, these areas now support small grains such as oats, corn, wheat, barley, and grass and 

clover crops. 

Another area of interest is the Tonquin scablands along the Rock Creek drainage. This area contains 

soils of stony loam to Xerochrepts deposited by the ancient Missoula Floods, and is an area of rocky 

basalt outcrops and deep gravel. This soil varies greatly from highly porous, well-drained soil to 

highly impermeable rock creating perched wetlands. Much of the area surrounding refuge lands in 

the Rock Creek area is extensively mined for rock and gravel. Any organic soil existing within this 

area is relatively shallow, but still supports abundant native plant growth. 

3.6 Fire 

3.6.1 Historic Activity 

For thousands of years prior to European settlement, fire played an important role in maintaining 

various habitat types within the Willamette Valley and in Oregon as a whole. Whether ignited by 

lightning or set by Native Americans, fire helped to maintain native habitat types by preventing 

woody species from encroaching into prairie habitat types, preventing build-up of volatile fuels in 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-19 

forested areas, creating a mosaic of habitat types and successional stages, and recycling nutrients 

(ODFW 2006). Since at least the 1850s, fire suppression actions began to change the landscape of the 

Willamette Valley, and have resulted in a host of unintended consequences for ecological health, 

wildlife populations, and people. 

In more open habitat types such as prairies and oak savanna, fires were essential for removing 

encroaching woody species (especially conifers), reducing thatch left by annual and perennial grasses 

and forbs, and providing habitat for wildlife, such as elk, acorn woodpeckers, and white-breasted 

nuthatch, dependent on open habitat types. Native Americans often burned open habitat types to 

promote food plants such as camas and tarweed, provide for more open travel corridors, and provide 

forage for elk, deer, and other game species that they hunted (ODFW 2006). In part, due to fire 

suppression, less than 1 percent of these habitats remain in the valley, as woody species, such as 

Douglas-fir, have moved in and changed the landscape. 

Photo 3-4. Kalapuya people ignited fires to manage habitats for their needs. 

USFWS. 

In forested areas, fires played an important role in maintaining a diversity of habitat structure. Drier 

forests historically experienced frequent fires (usually at less than 25-year intervals) that mainly 

burned understory plants, but left larger trees undamaged. In wetter forests such as those in the Coast 

Range, fires were less frequent (on the order of once every 100 years), but often burned the entire 

stand, setting the stage for early successional plant growth. 

3.6.2 Recent Activity 

In forested areas, fire suppression has increased the likelihood of wildfires that are unusually large 

and severe (ODFW 2006). In the late nineteenth and early twentieth centuries, logging operations 

combined with fire suppression throughout the Pacific Northwest created conditions for catastrophic 

fires. Vegetation composition within forests has also changed as a result of fire suppression. 

Historically, conifer-dominated forests had little undergrowth because of the shade imposed by the 

closed canopy and frequent fires that cleared brush and downed woody debris. However, with the 

onset of widespread fire-suppression activities, debris was allowed to accumulate, and brush and 

eventually understory trees began to grow and provide “ladder” fuels that led to canopy fires when a 

fire did occur. 
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3-20 Chapter 3. Physical Environment 

In prairie and savanna habitat types, fire suppression often led to a complete change in vegetation 

structure as woody species such as shrubs and conifers invaded and shaded out other species. In 

addition to fire suppression, these open habitat types were often the first to be converted to 

agriculture or developed as home and town sites, thus exacerbating their decline. 

Photo 3-5. Prescribed fire can be used to manage native habitats. Scott Swanson/ 

USFWS. 

3.6.3 Current Conditions 

The refuge currently has a fire management plan (USFWS 2002b) in place that covers wildland fire 

events and pile burning, but does not address prescribed fire. This plan states that all wildland fires, 

whether human-caused or lightning-induced, will be suppressed. Brush and debris piles that result 

from management activities will be burned by fully trained and equipped Service fire crews in 

accordance with national and regional policies. The refuge has updated the fire management plan, 

which now proposes the use of prescribed fire; this information can be found in Appendix O of the 

Comprehensive Conservation Plan (CCP). 

Many of the habitat types on the refuge are fire-dependent vegetation communities. In the absence of 

prescribed fire, these habitat types are maintained with surrogates for fire such as mowing and 

discing. The introduction of non-native and invasive plant species such as reed canarygrass, velvet 

grass, cocklebur, and Bermuda grass during the past century have complicated maintenance of these 

native habitat types. Use of prescribed fire to maintain native habitat types may or may not address 

some of these issues. 

3.7 Environmental Contaminants 

In 1992, prior to formal establishment of the refuge, the Service conducted a pre-acquisition 

contaminant survey (USFWS 1992b) to assess possible contaminants affecting proposed refuge 

lands. The survey found that there were “ ...a variety of contaminant sources on and adjacent to the 

refuge including container nurseries, a tannery, a potato processing plant, an abandoned town dump, 

nonpoint source pollution from highways and industrial areas, agricultural runoff, dairy farms, auto 
Tualatin River National Wildlife Refuge Comprehensive Conservation Plan 

Chapter 3. Physical Environment 3-21 

wrecking yards, electrical power lines, and possible hazardous wastes from farming and industrial 

operations.” 

In the Rock Creek drainage within the refuge acquisition boundary, but on lands not owned by the 

refuge, is the 33-acre site of a former tannery and battery company. The Frontier Leather Company 

(Frontier) began operating a tannery at the site in 1947. Two 3-acre sedimentation lagoons were 

constructed and placed in service in 1965. Frontier discharged wastewater to the sedimentation 

lagoons intermittently between 1965 and 1982. Between 1971 and 1974, chromium-treated hides 

were both stockpiled and buried at the facility. Frontier landfilled an estimated 21,000 cubic yards of 

chromium-containing wastes including chromium-treated hides and chromium-containing vacuum 

filter sludge. Some of the hides were later covered with approximately 2 feet of soil, but a large 

percentage of the landfill was not adequately consolidated and capped to control possible direct 

contact or surface water runoff. Landfilling of hides was discontinued in 1973 after analytical results 

from hide samples collected by the Department of Environmental Quality (DEQ) suggested that the 

hides could adversely impact soil, groundwater, or surface water quality (DEQ 2011c). Additionally, 

contaminants found in surface soil samples included antimony, trivalent chromium, lead, and 

mercury (DEQ 2011b). In addition to contaminants from the tannery, there are also contaminants 

from lead acid batteries at the tannery site, including lead, sulfuric acid, and battery casings (DEQ 

2011b). 

DEQ uncovered records from the former tannery indicating that tannery wastes were land-applied to 

pasture land at Ken Foster Farm (KFF) during the 1960s. The former KFF 40-acre farm site is on 

Murdock Road, in Sherwood, outside the refuge acquisition boundary, but adjacent to and upslope of 

the Rock Creek Unit. Investigations of the KFF confirmed the presence of high concentrations of 

chromium in surface soils consistent with land application of the tannery wastes. These contaminants 

could have migrated into the Rock Creek drainage, which flows through the refuge. In 2007, the U.S. 

Environmental Protection Agency (EPA) published findings from their assessment of the former 

KFF site confirming that several heavy metals, primarily chromium, lead, and mercury were present 

at elevated concentrations in site soils. Sampling performed to date indicates lead and chromium in 

site soils are attributable to historical land application of tannery wastes at the site (DEQ 2011a). 

Additional work at the KFF site is warranted, including completion of a comprehensive remedial 

investigation and risk assessment, feasibility study, and implementation of remedial action as 

necessary (DEQ 2011b). 

3.8 Air Quality 

The EPA has lead responsibility for the quality of air in the United States; through the 1990 Clean 

Air Act, the agency sets limits on the amount of pollutants that can be discharged into the air. More 

than 170 million tons of pollution are emitted annually into the air within the United States, through 

either stationary sources (such as industrial or power plants) or mobile source (such as automobiles, 

airplanes, trucks, buses, and trains). There are also natural sources of air pollution such as fires, dust 

storms, volcanic activity, and other processes. The EPA has identified six principal pollutants that are 

the focus of its national regulatory program: carbon monoxide, lead, nitrogen dioxide, ozone, 

particulate matter, and sulfur dioxide. 

The Oregon DEQ is responsible for protecting Oregon’s air quality. DEQ monitors air pollution to 

ensure that communities meet national ambient air quality health standards (DEQ 2011c). There are 

three main pollutants that are of greatest concern in Oregon. These are: 
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3-22 Chapter 3. Physical Environment 

 Ground-level ozone, commonly known as smog; 

 Fine particulate matter (mostly from wood smoke, other combustion sources, cars, and 

dust) known as PM2.5 (2.5 micrometers and smaller in diameter); and 

 Hazardous air pollutants (also called air toxics). 

According to a 2010 DEQ report (DEQ 2011c), the PM2.5 level was below the Federal health level 

throughout Oregon. Ground-level ozone (smog) exceeded the Federal standards twice in Portland and 

once in Salem. They did not violate the standard, which requires four exceedance days. Forest fire 

smoke was light in 2010 and did not exceed the Federal health standard anywhere DEQ monitored. 

Air toxins, such as benzene and acetaldehyde, remain near or above the health benchmarks. The 

health benchmarks are concentration levels at which, if exposed over a lifetime, an individual’s risk 

of getting cancer is increased by one in one million. Carbon monoxide and PM10 (particulate matter 

10 micrometers in diameter and smaller) remained far below the Federal health standard. The 

pollutants have been trending down in the past 10 to 15 years. 

According to Creative Methods (2011), Washington and Yamhill Counties received poor grades for 

air quality based on tons of emissions per square mile of the following pollutants (Table 3-5): carbon 

monoxide, lead and lead compounds, nitrogen oxides, volatile organic compounds, PM10, PM2.5, 

ammonia, sulfur dioxide, total hazardous air pollutants, and two particular hazardous air pollutants, 

diesel emissions and acrolein emissions. 

Table 3-5. Letter Grades* of Selected Air Quality Measures Based on Tons Emission per 

Square Mile 

County CO Pb NOx VOC PM10 PM2.5 NH3 SO2 HAP Diesel Acrolein

Washington F C D D D F C D D F D 

Yamhill D D C D D D C C C C D 

*The grade “A” represents the best/cleanest emission densities in the United States, and “F” the 

worst/dirtiest in the United States. 

CO: Carbon dioxide 

Pb: Lead 

NOx: Nitrogen oxides 

VOC: Volatile organic compounds 

PM10: Particulate matter less than or equal to 10 micrometers 

PM2.5: Particulate matter less than or equal to 2.5 micrometers 

NH3: Ammonia 

SO2: Sulfur dioxide 

HAP: Hazardous air pollutants 

However, in actual tests of ambient air quality, only Yamhill County exceeded any air quality 

parameters. Based on these tests, EPA levels for quarterly levels of lead were exceeded (Creative 

Methods 2011). 

As discussed in Section 3.6 above, prescribed fire can be a useful management tool, but smoke 

management is a concern and is governed in part by Oregon Administrative Rules (OARs) (OAR 

2011). 
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Chapter 3. Physical Environment 3-23 

3.9 Water Quality 

3.9.1 Overview—Tualatin River Basin 

The Tualatin River watershed lies almost completely within Washington County but also crosses 

Multnomah, Clackamas, Yamhill, Tillamook, and Columbia Counties. This watershed, or subbasin, 

is part of the larger Willamette River Basin. The Tualatin River drains more than 700 square miles of 

land on the west side of Portland’s metropolitan area; its headwaters are in the Coast Range and it 

flows in an easterly direction to the confluence with the Willamette River. The Tualatin River Basin 

supports a growing population of more than 450,000 people and a wide range of urban, agricultural, 

and forest-derived activities (DEQ 2011d). The discharge of the river reflects seasonal rainfall, and 

most precipitation occurs between November and March. Seasonal stream flow is typically highest 

from December through April and lowest from July through October (USGS 2008). The low flow 

period occurs during the summer from May through October. Streams entering the river within the 

refuge include Wapato, Ayers, Hill, Harris, and Scoggins Creeks in the Wapato Lake Unit, and 

Chicken and Rock Creeks in the Sherwood Units. 

The Oregon DEQ is the agency responsible for ensuring that the streams and rivers of Oregon are 

within standards set forth by the Federal Clean Water Act of 1972. In the past, as human activity 

increased in the subbasin, the Tualatin River experienced many water quality problems including 

algal blooms. Many streams in the Tualatin River subbasin do not meet Oregon water quality 

standards as these streams have high water temperatures and low dissolved oxygen levels that can 

harm fish and other aquatic life. Some water bodies have bacteria counts that are higher than 

Oregon’s water quality standards recommend (DEQ 2011d). In response to these streams not meeting 

water quality standards, DEQ listed the Tualatin River as a “water-quality limited” stream. The term 

“water-quality limited” is used in the Clean Water Act to define streams that do not meet established 

water quality standards even after the implementation of standard controls. In 1984 and 1986, 

Tualatin River was listed due to low dissolved oxygen concentrations and nuisance levels of algae. 

Once a river has been designated as water-quality limited, the Clean Water Act requires that the DEQ 

develop thresholds for the amount of pollutants to be released in the river (USGS 2008). 

Since the 1970s, DEQ has worked with other agencies and organizations to ensure actions are taken 

to improve water quality in the Tualatin River. These actions include closure of several water 

treatment plants that historically discharged high concentrations of ammonia and phosphorous, 

upgrades at remaining treatment facilities, implementation of management plans by agencies that 

address potential sources of pollutants, tree plantings along streams to facilitate cooler water 

temperatures, and the release of water from Henry Hagg Lake by Clean Water Services (CWS) 

during the critical summer months when water flow is low. All of these actions have contributed to 

improvement in the water quality of the Tualatin River. 

CWS has worked further with DEQ to improve water quality in the Tualatin River. Water is released 

by CWS from Henry Hagg Lake and Barney Reservoir to improve the river’s water quality. In 2004, 

CWS received a watershed-based permit from DEQ that provided a mechanism to offset a portion of 

the thermal loads from their wastewater treatment facilities with releases of stored water from the 

reservoirs. Reservoir releases during July and August are used to mitigate part of the thermal impacts 

from wastewater treatment facilities. CWS offsets the remainder of its thermal impact by planting 

riparian areas along tributaries, either directly within its service area or through a partnership with the 

Tualatin Soil and Water Conservation District on rural lands. During the rest of the summer, the 
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3-24 Chapter 3. Physical Environment 

water is released to offset the effect of sediment oxygen demand on the dissolved oxygen levels in 

the river. Dissolved oxygen levels in the river downstream of the wastewater treatment facilities 

determine the ammonia limits for wastewater treatment facilities. The higher the dissolved oxygen 

levels, the more operational flexibility the wastewater treatment facilities have (CWS 2011). 

Low dissolved oxygen levels can be a water quality issue in the lower Tualatin River. During the 

early parts of the summer, photosynthetic production of oxygen by algae effectively offsets the 

oxygen consumed by the decaying substances in the sediment of the river (sediment oxygen 

demand). In the fall, however, oxygen production by algae is reduced as the days become shorter and 

it no longer offsets the oxygen consumption by sediment oxygen demand. This can lead to low 

dissolved oxygen levels. Increasing stream flow reduces oxygen consumption by sediment oxygen 

demand because it shortens the contact time between the river water and the river sediments (CWS 

2011). 

In addition to the above actions, DEQ developed total maximum daily loads (TMDLs) as required by 

the Clean Water Act. The TMDLs are developed to meet established water quality standards. They 

define the amount of heat, bacteria, phosphorous, and ammonia that can be added while still being 

protective of the river (DEQ 2011d). DEQ first released TMDL loads (or amounts) in the 1980s for 

various sources in the Tualatin River Basin. There has been a revision of these TMDLs, and they are 

currently undergoing public review at the time of this writing. The establishment of TMDLs in the 

Tualatin River Basin prompted many management agencies to take actions to meet the load 

allocations set forth by the TMDLs. 

3.9.2 Refuge-specific Water Quality 

There are some data available on several creeks that enter the Tualatin River through the refuge. 

Chicken Creek, which supplies most of the water to the managed wetlands on the Atfálat’i Unit, 

enters the refuge through a diversion point from which water is then diverted to several different 

ponds using an irrigation canal. Over the years, as more urban development occurs upstream, the 

refuge has observed higher stream flows at the diversion. Higher flows have caused extra 

management responsibility and burden on the refuge to divert the water to the river to avoid flooding 

or damage to refuge infrastructure. In response to the increases in high flows, the refuge has installed 

some new infrastructure, including an emergency spillway to facilitate water reaching the river. In 

addition to high water flows, another concern of the refuge is the water quality of Chicken Creek, due 

to the runoff from urban development that occurs in the watershed. However, CWS has been 

monitoring the water quality for upper and lower Chicken Creek and as of date, all standards of 

temperature, dissolved oxygen, pH, and toxics are below applicable water quality criteria. 

3.10 Visual Quality 

The scenic beauty of the refuge plays in sharp contrast to the surrounding urban development of 

homes and roads. All features of visitor and administrative facilities have been designed and 

constructed to complement the natural refuge environment. Buildings are placed to welcome visitors 

at the entry point of the refuge, but not detract from wildlife observation. Where possible, structures 

are set into the landscape and/or provide views through or around the structure itself. Overlooks, 

kiosks, benches, signs, and railings are all low profile and colored to match the environment. This 

careful planning allows the visitor to experience the refuge without distractions. 
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Chapter 3. Physical Environment 3-25 

3.11 Surrounding Land Use 

The region surrounding the refuge is primarily a mix of urban and agricultural lands (see Appendix 

P, Map 2). The City of Sherwood in Oregon (population 17,930) abuts the southern and western 

boundaries of the Sherwood Units (U.S. Census Bureau 2011), the City of Tualatin (population 

26,745) is to the east, and King City (population 2,965) is northeast of the refuge. The City of 

Gaston, Oregon (population 792), is adjacent to the west boundary of Wapato Lake Unit, and Forest 

Grove (population 21,448) is north of the refuge boundary by approximately 2 miles. Oregon’s 

largest metropolitan center with Portland at its core is approximately 12 miles from the Sherwood 

Units and 23 miles from the Wapato Lake Unit. The population of Portland is 566,143; the 

population including its surrounding metropolitan area is approximately 2,260,000 (Portland 

Business Journal 2011). The population of Sherwood has increased by more than five times from 

3,093 in 1990 (U.S. Census Bureau 2011) to the current level, and other cities in the surrounding area 

are also experiencing rapid population growth. In fact, some land within the refuge acquisition 

boundary has already been developed with housing and businesses. 

The western part of the Sherwood Units and Wapato Lake Unit are characterized by a more rural 

nature, with active agriculture as well as scattered patches of potential wildlife habitat. Dominant 

agricultural practices include growing of nursery and greenhouse stock; field crops such as winter 

wheat, corn, grass hay, and clover and grass seed; orchards of fruit and nuts; wine grapes; berry 

crops; and row crops such as potatoes and onions. In 2009, Washington County produced $4,106,865 

in agricultural products (Oregon State University 2011). Yamhill County is a more rural county with 

high production of both agricultural commodities and timber products. Yamhill County is the 

nation’s leader in production of hazelnuts and the fifth-largest producer of grass seed; it has the thirdlargest acreage of rhubarb, and is number five nationally for llama production (Oregon Department 

of Agriculture 2012). Active timber harvest occurs both east and west of the Wapato Lake Unit in the 

Chehalem Ridge area and in the Coast Range, respectively. 

Within the Tualatin River Basin there are a number of public and private lands managed for habitat 

and wildlife. The regional Metro government has an active green spaces program and has acquired 

and restored several land parcels. CWS maintains several land parcels that benefit wildlife, and other 

county and public entities maintain suitable wildlife habitat within the basin. In addition, private 

landowners maintain waterfowl hunting clubs throughout the area that benefit fish and wildlife. 

Other private landowners maintain habitat in a natural condition that also benefits fish and wildlife. 

The lower reaches of the Tualatin River are navigable with small boats, and many recreationists take 

advantage of the river using canoes and kayaks. Fishing opportunities are limited; however, a few 

people use motor boats to fish for warm-water species during summer, and at least one person fishes 

for crayfish commercially on the river. 
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