Prepared in cooperation with the U.S. Army Corps of Engineers and 

the Oregon Association of Clean Water Agencies

Thermal Effects of Dams in the Willamette River Basin, Oregon 

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5153
Cover: Photograph of Detroit Dam on the North Santiam River.

Back Cover: Photographs, from left to right, by row:

Top row

 Big Cliff Dam on the North Santiam River

 Detroit Dam on the North Santiam River

 Foster Dam on the South Santiam River

 Green Peter Dam on the Middle Santiam River

2nd row:

 Blue River Dam on Blue River

 Cougar Dam on the South Fork McKenzie River

 Fall Creek Dam on Fall Creek

 Dexter Dam on the Middle Fork Willamette River

3rd row:

 Lookout Point Dam on the Middle Fork Willamette River

 Hills Creek Dam on the Middle Fork Willamette River

 Dorena Dam on the Row River

 Cottage Grove Dam on the Coast Fork Willamette River

Bottom row:

 Fern Ridge Dam on the Long Tom River

 River Mill Dam on the Clackamas River

All photographs except that of River Mill Dam are from the U.S. Army Corps of Engineers’ Digital Visual Library

at https://eportal.usace.army.mil/sites/DVL/

Photograph of River Mill Dam is from Portland General Electric.

All photographs are used here with the permission of the source entities.
Thermal Effects of Dams in the Willamette 

River Basin, Oregon

By Stewart A. Rounds

Prepared in cooperation with the U.S. Army Corps of Engineers and the 

Oregon Association of Clean Water Agencies

Scientific Investigations Report 2010–5153

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Rounds, S.A., 2010, Thermal effects of dams in the Willamette River basin, Oregon: U.S. Geological Survey Scientific 

Investigations Report 2010-5153, 64 p.
iii

Contents

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

Introduction.....................................................................................................................................................2

Background............................................................................................................................................2

Description of Study Area ...................................................................................................................3

Purpose and Scope ..............................................................................................................................6

Methods and Models ....................................................................................................................................6

Dam-Site Flow Estimation ...................................................................................................................6

Dam-Site Temperature Estimation.....................................................................................................8

Regressions ..................................................................................................................................8

Mass and Energy Balances .....................................................................................................10

Downstream Warming ..............................................................................................................10

Flow and Temperature Models.........................................................................................................11

Data Sources.......................................................................................................................................12

Thermal Effects of Dams in the Willamette River Basin .......................................................................13

Without-Dam Flow Estimates at Dam Sites....................................................................................13

Without-Dam Temperature Estimates at Dam Sites .....................................................................15

Big Cliff and Detroit Dams ........................................................................................................16

Blue River Dam...........................................................................................................................20

Cottage Grove Dam ...................................................................................................................23

Cougar Dam ................................................................................................................................24

Dexter and Lookout Point Dams..............................................................................................26

Dorena Dam................................................................................................................................28

Fall Creek Dam............................................................................................................................29

Fern Ridge Dam..........................................................................................................................31

Foster Dam..................................................................................................................................32

Green Peter Dam .......................................................................................................................33

Hills Creek Dam..........................................................................................................................35

River Mill Dam ............................................................................................................................36

Downstream Effects of Willamette Basin Dams ...........................................................................37

Downstream Flows....................................................................................................................37

Downstream Temperatures......................................................................................................39

Summary and Conclusions.........................................................................................................................43

Acknowledgments.......................................................................................................................................44

References Cited..........................................................................................................................................45

Appendixes ...................................................................................................................................................47

Appendix A. Graphs of Without-Dam Flow Estimates at Willamette River Basin 

Dam Sites .......................................................................................................................................48

Appendix B. Time/Distance Color Maps Showing the Downstream Thermal Effects of Willamette 

River Basin Dams ...........................................................................................................................52

Appendix C. Graphs of Modeled Temperature Distributions Downstream of Willamette River 

Basin Dam Sites..............................................................................................................................56
iv

Figures

Figure 1. Map of the Willamette River basin, Oregon ............................................. 4

Figure 2. Map of major physiographic regions in the Willamette River basin, Oregon......... 5

Figure 3. Diagram showing imposed seasonal variation in the estimated correction for 

evaporative losses from Willamette River basin reservoirs, Oregon.................. 7

Figure 4. Flow chart showing process for estimating without-dam water temperatures 

at dam sites in the Willamette River basin, Oregon .................................... 9

Figure 5. Graph showing measured and estimated streamflow for conditions with and 

without upstream dams in the Middle Fork Willamette River at Dexter Dam, 

just downstream of Lookout Point Dam in the Willamette River basin, Oregon, 

2001–02 ....................................................................................... 13

Figure 6. Graphs showing measured and estimated water temperatures for conditions 

with and without upstream dams at the Big Cliff / Detroit, Blue River, and 

Cottage Grove dam sites in the Willamette River basin, Oregon, 2001–02............ 17

Figure 7. Graphs showing cumulative frequency curves of measured and estimated 

water temperatures for conditions with and without upstream dams at the 

Big Cliff dam site in the Williamette River basin, Oregon, 2001–02 .................. 18

Figure 8. Graphs showing correlation of water temperatures for Blue River (BR) and 

Blowout Creek (BC) in the Willamette River basin, Oregon, for a period in 

mid-summer of 2000 ........................................................................ 21

Figure 9. Graphs showing measured and estimated water temperatures for conditions 

with and without upstream dams at the Cougar, Dexter / Lookout Point, and 

Dorena dam sites in the Willamette River basin, Oregon, 2001–02 .................. 25

Figure 10. Graphs showing measured and estimated water temperatures for conditions 

with and without upstream dams at the Fall Creek, Fern Ridge, and Foster dam 

sites in the Willamette River basin, Oregon, 2001–02 ................................. 30

Figure 11. Graph showing measured heating effect of Fern Ridge Lake in the Willamette 

River basin, based on temperature measurements upstream and downstream 

of the lake .................................................................................... 31

Figure 12. Graphs showing measured and estimated water temperatures for conditions 

with and without upstream dams at the Green Peter, Hills Creek, and River Mill 

dam sites in the Willamette River basin, Oregon, 2001–02.............................. 34

Figure 13. Graphs showing measured daily mean streamflow at USGS gaging stations 

on the Willamette River at Albany and at Salem ....................................... 38

Figure 14. Graph showing simulated streamflow in the Willamette River at Albany and at Salem 

for conditions with and without upstream dams in the Willamette River 

basin, Oregon, April–October 2002 ...................................................... 39

Figure 15. Time/distance color map showing modeled downstream thermal effects of 

dams on the entire Willamette River, Oregon, June–October 2001 and

 April–October 2002 ........................................................................ 40

Figure 16. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM 

temperatures for conditions with and without dams in the Willamette River 

downstream of the confluence of the Coast Fork and Middle Fork Willamette 

Rivers, Oregon, June–October 2001–02................................................... 42
v

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2)

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

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

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Datums

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

(NAVD 88).

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

Tables

Table 1. Willamette River basin dams included in this study .................................... 3

Table 2. Evaporation rates from selected reservoirs owned and operated by the 

U.S. Army Corps of Engineers in the Willamette River basin, Oregon ............... 7

Table 3. Minimum and maximum streamflows for conditions with and without 

upstream dams at 12 dam sites in the Willamette River basin, Oregon, 2001–02 ... 14

Table 4. Measured with-dam and estimated without-dam annual maximum water 

temperatures as measured by the 7-day average of the daily maximum at 

selected dam sites in the Willamette River basin, Oregon, 2001–02 .................. 18

Table 5. Maximum estimated error ranges for the without-dam water-temperature 

estimates at selected dam sites in the Willamette River basin, Oregon ............ 19
vi

Abbreviations and Acronyms

7dADM 7-day running average of the daily maximum

FERC Federal Energy Regulatory Commission

ODEQ Oregon Department of Environmental Quality

NTP Natural Thermal Potential

PGE Portland General Electric

PSU Portland State University

RM river mile

TMDL Total Maximum Daily Load

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey
 

Abstract

Methods were developed to assess the effects of dams on 

streamflow and water temperature in the Willamette River and 

its major tributaries. These methods were used to estimate the 

flows and temperatures that would occur at 14 dam sites in the 

absence of upstream dams, and river models were applied to 

simulate downstream flows and temperatures under a no-dams 

scenario. The dams selected for this study include 13 dams 

built and operated by the U.S. Army Corps of Engineers 

(USACE) as part of the Willamette Project, and 1 dam on the 

Clackamas River owned and operated by Portland General 

Electric (PGE). Streamflows in the absence of upstream dams 

for 2001–02 were estimated for USACE sites on the basis of 

measured releases, changes in reservoir storage, a correction 

for evaporative losses, and an accounting of flow effects from 

upstream dams. For the PGE dam, no-project streamflows 

were derived from a previous modeling effort that was part 

of a dam-relicensing process. Without-dam streamflows were 

characterized by higher peak flows in winter and spring and 

much lower flows in late summer, as compared to with-dam 

measured flows.

Without-dam water temperatures were estimated from 

measured temperatures upstream of the reservoirs (the 

USACE sites) or derived from no-project model results (the 

PGE site). When using upstream data to estimate without-dam 

temperatures at dam sites, a typical downstream warming rate 

based on historical data and downstream river models was 

applied over the distance from the measurement point to the 

dam site, but only for conditions when the temperature data 

indicated that warming might be expected. Regressions with 

measured temperatures from nearby or similar sites were used 

to extend the without-dam temperature estimates to the entire 

2001–02 time period. Without-dam temperature estimates 

were characterized by a more natural seasonal pattern, with 

a maximum in July or August, in contrast to the measured 

patterns at many of the tall dam sites where the annual 

maximum temperature typically occurred in September or 

October. Without-dam temperatures also tended to have more 

daily variation than with-dam temperatures.

Examination of the without-dam temperature estimates 

indicated that dam sites could be grouped according to the 

amount of streamflow derived from high-elevation, spring-fed, 

and snowmelt-driven areas high in the Cascade Mountains 

(Cougar, Big Cliff/Detroit, River Mill, and Hills Creek 

Dams: Group A), as opposed to flow primarily derived from 

lower-elevation rainfall-driven drainages (Group B). Annual 

maximum temperatures for Group A ranged from 15 to 

20 °C, expressed as the 7-day average of the daily maximum 

(7dADM), whereas annual maximum 7dADM temperatures 

for Group B ranged from 21 to 25 °C. Because summertime 

stream temperature is at least somewhat dependent on the 

upstream water source, it was important when estimating 

without-dam temperatures to use correlations to sites with 

similar upstream characteristics. For that reason, it also is 

important to maintain long-term, year-round temperature 

measurement stations at representative sites in each of the 

Willamette River basin’s physiographic regions.

Streamflow and temperature estimates downstream of 

the major dam sites and throughout the Willamette River were 

generated using existing CE-QUAL-W2 flow and temperature 

models. These models, originally developed for the Willamette 

River water-temperature Total Maximum Daily Load process, 

required only a few modifications to allow them to run under 

the greatly reduced without-dam flow conditions. Model 

scenarios both with and without upstream dams were run. 

Results showed that Willamette River streamflow without 

upstream dams was reduced to levels much closer to historical 

pre-dam conditions, with annual minimum streamflows 

approximately one-half or less of dam-augmented levels. 

Thermal effects of the dams varied according to the time 

of year, from cooling in mid-summer to warming in early 

autumn. Thermal effects diminished with increasing distance 

downstream, from as much as 6 °C or more at most of the 

taller dam sites to a level typically less than 1 °C downstream 

of the Santiam River confluence and less than 0.5 °C 

downstream of the Clackamas River confluence. Modeled 

without-dam temperatures in the Willamette River had a 

higher maximum and a lower minimum than modeled withdam temperatures for the June–October modeled time period. 

Upstream dams have a substantial and measurable effect on 

streamflow and water temperature, both at the dam sites and 

downstream throughout the entire Willamette River.

Thermal Effects of Dams in the Willamette River Basin, 

Oregon

By Stewart A. Rounds
2 Thermal Effects of Dams in the Willamette River Basin, Oregon

Introduction

Background

It is widely recognized that dams have important effects 

on streamflow, water temperature, and fisheries (Collier and 

others, 1996). Often managed to provide protection from 

floods, reservoirs capture large inflows and release them 

over a longer time period, thereby decreasing annual peak 

flows. During dry seasons, reservoir releases for navigation, 

irrigation, municipal uses, and flow augmentation increase the 

annual minimum streamflow above pre-dam levels. Because 

deep reservoirs tend to thermally stratify in summer and keep 

less-dense warmer water at the surface, releases from deep 

in a reservoir typically are colder in mid-summer compared 

to nearby rivers without dams. Similarly, when a reservoir is 

drawn down in autumn to provide storage for flood control, 

the warm surface layer can be brought down to the outlet 

elevation, resulting in warmer-than-normal discharges. These 

alterations in streamflow and seasonal temperature patterns 

downstream of dams can create problems for fish related to 

the timing of migration, spawning, and egg hatching (Caissie, 

2006). Dams also can be a barrier to fish passage, further 

disrupting the patterns and success of migration and spawning.

The Willamette River basin in northwestern Oregon has 

a system of dams that were built to provide flood control, 

power production, storage for agricultural and municipal uses, 

flow augmentation for downstream navigation, and recreation. 

The U.S. Army Corps of Engineers (USACE) operates the 

largest system of dams in the basin, totaling 13 storage and 

flow-regulating dams. Portland General Electric (PGE) owns 

and operates several hydropower projects, many of which 

are located in the Clackamas River basin, a tributary to the 

Willamette River. Fish passage and downstream thermal 

effects have long been an issue for many of these dams. A 

retrofit at the USACE Cougar Dam on the South Fork (SF) 

McKenzie River was completed in 2005 to allow for better 

control of downstream temperatures; the resulting seasonal 

temperature pattern now is more natural and has improved 

conditions for fish spawning and fry emergence in the reach 

downstream of the dam (Greg Taylor, U.S. Army Corps of 

Engineers, oral commun., 2010).

As a result of many factors including the dams, winter 

steelhead (Oncorhynchus mykiss) and Chinook salmon 

(Oncorhynchus tshawytscha) were listed as threatened under 

the Federal Endangered Species Act in March 1999. To protect 

these species, the National Marine Fisheries Service issued 

a “Biological Opinion” in July 2008, providing guidelines 

and timetables that address fish passage and the flow and 

temperature alterations caused by the dams, among other 

recommendations (National Marine Fisheries Service, 2008). 

Some of the recommendations already are being implemented, 

such as modified operations at the USACE Detroit Dam to 

better manage downstream temperatures.

Recognizing that water temperature is a critical factor 

affecting the survival and viability of anadromous fish, the 

Oregon Department of Environmental Quality (ODEQ) 

created a maximum water-temperature standard under the 

authority of the Federal Clean Water Act. To protect fish 

and aquatic resources and meet water-quality standards in 

the Willamette River and several of its largest tributaries, 

ODEQ issued a water-temperature Total Maximum Daily 

Load (TMDL) in September 2006 (Oregon Department 

of Environmental Quality, 2006a, 2006b). Relying on 

models and data, ODEQ attempted to quantify the thermal 

effects of several important factors on the river system. The 

TMDL specified heat wasteload allocations for point-source 

dischargers, called for revegetation of degraded riparian 

zones, and established temperature targets downstream of the 

major dams. The cumulative heating effects of point-source 

discharges and decreased riparian shading were determined 

using detailed calibrated models. Models were not available, 

however, to quantify the thermal effects of the major dams at 

dam sites, and the analysis of those effects in the TMDL was 

limited primarily to a comparison of measured temperatures 

upstream and downstream of the reservoirs.

Thermal effects downstream of tall dams can be large and 

commonly have characteristic seasonal and spatial patterns. 

These patterns and characteristics have been described and 

modeled by Risley and others (2010) for several idealized 

stream, reservoir, and climate conditions, demonstrating that 

altered release temperatures have an effect that dissipates 

with downstream distance, but that flow modifications have 

an increasing thermal effect downstream. In the Willamette 

River basin, a simple comparison of measured temperatures 

upstream and downstream of Detroit and Cougar Dams reveals 

temperature changes as much as 6 °C (cooler downstream in 

mid-summer, warmer in October) prior to actions taken to 

mitigate such effects (Oregon Department of Environmental 

Quality, 2006b; Rounds, 2007; Sullivan and others, 2007). 

If dam releases are not managed to provide for some daily 

variation in water temperature, a downstream pattern in the 

daily range of water temperature also can develop, with 

distinct nodes of minimal daily variation at daily travel-time 

distances (Lowney, 2000). These downstream thermal effects, 

and their mitigation through changes in dam operations, 

can be simulated accurately with models. Examples from 

the Willamette River basin include models of the reservoirs 

impounded by Detroit Dam and Scoggins Dam, which were 

used to evaluate how changes in dam operation can be used 

to meet target downstream water temperatures (Sullivan and 

Rounds, 2006; Sullivan and others, 2007). The downstream 

thermal effects of the Cougar Dam retrofit have been tracked 

downstream using the Willamette River TMDL models 

(Rounds, 2007).
Introduction 3

Description of Study Area

The Willamette River flows for 187 mi from south to 

north past some of the largest cities in northwestern Oregon, 

including Eugene, Corvallis, Albany, Salem, and Portland 

(fig. 1). The river basin covers an area of approximately 

11,500 mi2 and is home to more than 2.6 million people, about 

70 percent of the population of Oregon (U.S. Census Bureau, 

2008). Bounded by the Cascade Range to the east and the 

Coast Range to the west, the basin has a modified maritime 

climate characterized by cool, wet winters and warm, dry 

summers. Eastward-moving Pacific storms deliver plentiful 

rain as well as snow in the mountains during the wet winter 

season; about 70–80 percent of the annual precipitation falls 

between October and March, and less than 5 percent in July 

and August (Wentz and others, 1998). Rainfall can total as 

much as 130 in/yr in the mountains, but typically 35–40 in/yr 

in the valley lowlands. Approximately 70 percent of the basin 

is forested and 20 percent, primarily in the valley bottom, is 

agricultural land (Hulse and others, 2002).

Streamflow in the Willamette River follows seasonal 

rainfall patterns, with the highest flows during winter storms 

and the lowest flows in late summer. A system of floodcontrol dams moderates annual peak flows. The mean annual 

peak flow in the Willamette River at Salem (USGS station 

14191000) for the 1970–2008 post-dam time period was 

111,000 ft3/s but as high as 244,000 ft3/s in 1996. Annual low 

flows are augmented by releases from upstream reservoirs 

during summer, with typical target minimum flows of 

6,000 ft3/s at Salem and 4,000 ft3/s at Albany.

Dams in the Willamette River basin are owned and 

operated by various entities and for a wide range of purposes. 

The USACE operates a system of 13 dams, including the 

tallest dams (463 and 452 ft, Detroit and Cougar Dams, 

respectively) and the largest storage reservoirs (about 

455,000 acre-ft, Detroit and Lookout Point Dams) in the 

basin (table 1). On the Clackamas River, PGE operates a 

series of dams and diversions for power generation; the most 

downstream dam is River Mill Dam.

Streamflow and temperature characteristics in Willamette 

Basin streams are linked to the physiographic region of the 

stream’s headwaters. The basin has four major physiographic 

regions distinguished by their predominant location, elevation, 

and geologic characteristics (fig. 2). Streams with significant 

source areas in the High Cascades derive their flow from 

rainfall and snowmelt, where relatively young and permeable 

volcanic rocks conduct snowmelt to large spring complexes 

that provide cold and consistent flows year round (Conlon and 

others, 2005). In contrast, streams in the Western Cascades 

and the Coast Range are more driven by rainfall and lack the 

large spring complexes of the High Cascades, resulting in 

more variable flows and lower flows and warmer temperatures 

in summer (Tague and others, 2007).

Table 1. Willamette River basin dams included in this study.

[Storage is the reservoir volume at full pool elevation. USACE, U.S. Army Corps of Engineers; PGE, Portland General Electric; ft, feet; na, not available; 

–, none]

Dam name Location Owner Year 

completed

Height 

(ft)

Storage 

(acre-ft)

Upstream 

dams

Big Cliff Dam North Santiam River USACE 1953 191 6,450 Detroit

Blue River Dam Blue River USACE 1969 270 89,500 –

Cottage Grove Dam Coast Fork Willamette River USACE 1942 95 32,900 –

Cougar Dam South Fork McKenzie River USACE 1963 452 219,000 –

Detroit Dam North Santiam River USACE 1953 463 455,100 –

Dexter Dam Middle Fork Willamette River USACE 1954 93 na Lookout Point, Hills Creek

Dorena Dam Row River USACE 1949 145 77,600 –

Fall Creek Dam Fall Creek USACE 1966 180 125,000 –

Fern Ridge Dam Long Tom River USACE 1941 44 116,800 –

Foster Dam South Santiam River USACE 1968 126 60,700 Green Peter

Green Peter Dam Middle Santiam River USACE 1968 327 428,100 –

Hills Creek Dam Middle Fork Willamette River USACE 1961 304 355,500 –

Lookout Point Dam Middle Fork Willamette River USACE 1954 276 455,800 Hills Creek

River Mill Dam Clackamas River PGE 1911 101 na several
4 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig01

5

5

WASHINGTON

COLUMBIA

DOUGLAS

LANE

LINN

MARION

POLK

BENTON

CLACKAMAS

MULTNOMAH

YAMHILL

N Fk

M Fk Willamette River

Mohawk

Marys

Middle

Santiam

N

Santiam River

Pudding

River

S

River

River

Clackamas

River

Molalla

Calapooia

River

River

River

River

River

Fork

Coast Fk

Tualatin

River

Santiam

S

River

Yamhill

RIVER

River

Lookout Point

Lake

Foster

Lake

River

Fern

Ridge

Lake

Willamette Hills

Creek

Lake

McKenzie

Cougar

Reservoir

Dorena

Lake

Cottage

Grove

Lake

Long Tom River

COLUMBIA

N Yamhill

Luckiamute

Detroit

Lake

RIVER

SF

McKenzie R

Row 

R. 

River

Green

Peter

Lake

WILLAMETTE

Middle

N Fk

M Fk Willamette River

Mohawk

Marys

Middle

Santiam

N

Santiam River

Pudding

River

S

River

River

Clackamas

River

Molalla

Calapooia

River

River

River

River

River

Fork

Coast Fk

Tualatin

River

Santiam

S

River

Yamhill

RIVER

River

Lookout Point

Lake

Foster

Lake

River

Fern

Ridge

Lake

Willamette Hills

Creek

Lake

McKenzie

Cougar

Reservoir

Dorena

Lake

Cottage

Grove

Lake

Long Tom River

COLUMBIA

N Yamhill

Luckiamute

Detroit

Lake

RIVER

Fall Creek

Lake

SF

McKenzie R

Row 

River 

River

Green

Peter

Lake

Santiam

River

WILLAMETTE Middle

Blue River

Lake

Fall

Creek

River Mill

Dam

River Mill

Dam

Big Cliff

Dam Detroit

Dam

Foster

Dam

Cougar

Dam

Fall Creek

Dam

Lookout

Point

Dam

Dexter

Dam

Fern

Ridge

Dam

Dorena

Dam

Cottage

Grove Dam

EXPLANATION

River reaches included in the modeling 

 domain—The entire main-stem Willamette

 River and selected major tributaries as 

 far upstream as the first major dam on

 each tributary

Hills

Creek

Dam

Blue River

Dam

Green Peter

Dam

Eugene

Cottage

Grove

Albany

Newberg

Portland

West Linn

Corvallis

Wilsonville

Salem

OREGON

WASHINGTON

C

O

A

S

T

R

A

N

G

E

C

A

S

C

A

D

E

R

A

N

G

E

RM 0

RM 20

RM 10

RM 30 RM 40 RM 50

RM 60

RM 70

RM 80

RM 90

RM 100

RM 110

RM 120

RM 130

RM 140

RM 150

RM 160

RM 170

RM 180

RM 190

RM 200

RM 210

RM 170 River mile

Dam and name

OREGON

Willamette

River Basin

124°00' 123°30' 123°00' 122°30' 122°00' 121°30'

46°

00'

45°

30'

45°

00'

44°

30'

44°

00'

43°

30'

Base map modified from U.S. Geological Survey and other digital data sets (1:2,000,000; 1:100,000). 

Projection: Oregon Lambert Conformal Conic, NAD1983, NAVD1988. 

124°00' 123°30' 123°00' 122°30' 122°00' 121°30'

46°

00'

45°

30'

45°

00'

44°

30'

44°

00'

43°

30'

0 10 20 30 40 Miles

0 10 20 30 40 Kilometers

Figure 1. The Willamette River basin, Oregon (reproduced from Rounds, 2007).
Introduction 5

Figure 2. Major physiographic regions in the Willamette River basin, Oregon (modified from 

Conlon and others, 2005).

ortac10-0490_fig02

OREGON

Willamette

River Basin

g in dduP

N Fk

M Fk Willamette River

Mohawk

Marys

Middle

Santiam

North

Santiam River

Little

Pudding

River

South

River

North

River

Clackamas

River

Molalla

Calapooia

River

River

River

River

River

River

e McKenzi

River

tte Willame

River

Coast

Fork

Tualatin

River

South

River

Yamhill

ette Willam

RIVER

TE AMET WILL

R RIVE

Yamhill

River

Long Tom

River

Waldo

 Lake

Hills

Creek

Lake

Lookout

Point

Lake

Fall

Creek

Lake

Dorena

Lake

Cottage

Grove

Lake

Blue

River

Lake

Cougar

Reservoir

Foster

Lake

Green

Peter

Lake

Detroit

Lake

Timothy

Lake

Henry

Hagg

Lake

Fern

Ridge

Lake

E

G 

N 

A 

R 

T S A O C 

E G N A R 

E D A C S A C 

Basin

boundary

44 ̊

45 ̊

122 ̊

123 ̊

20 Miles

10

10

20 Kilometers

0

0

ma ti naS

Fork le d id M

R

COLUMBIA

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6 Thermal Effects of Dams in the Willamette River Basin, Oregon

Temperatures in Willamette Basin streams generally 

follow the seasonal climate pattern, have characteristics 

derived from their morphology and physiographic source 

area, and are affected by several anthropogenic influences 

such as upstream dams, water withdrawals, point-source 

discharges, and modifications to riparian shading. ODEQ’s 

analysis for the Willamette River water-temperature TMDL 

showed that cumulative heating effects from point sources 

amounted to less than 0.2 °C and that loss of riparian shading 

accounted for a warming of 0.5–1.0 °C (Oregon Department 

of Environmental Quality, 2006b). Downstream of tall dams, 

the temperature can be modified by as much as 6–8 °C, both 

warmer and cooler depending on the time of year (Sullivan 

and others, 2007). The thermal effects of the dams diminish 

with distance downstream, as demonstrated by an analysis 

of the effects of the retrofit to Cougar Dam, where near-dam 

effects were as large as 6.0–6.5 °C, but decreased to a fraction 

of a degree in the Willamette River because of dilution and 

heat exchange with the atmosphere (Rounds, 2007).

Purpose and Scope

Dams have a significant effect on downstream flows 

and temperatures in the Willamette River basin (Oregon 

Department of Environmental Quality, 2006b; Rounds, 2007). 

The primary purpose of this study was to quantify the thermal 

effects of Willamette River basin dams, concentrating on the 

following two objectives:

• Develop and apply methods to estimate the water 

temperatures that would occur at selected Willamette 

River basin dam sites in the absence of those dams, 

and 

• Simulate the downstream flow and thermal effects of 

Willamette River basin dams.

To take advantage of a suite of existing CE-QUAL-W2 

flow and water-temperature models of the Willamette River 

and its major tributaries, the time period for this study was 

aligned with the calibrated time periods for those models—

spring through autumn in 2001 and 2002. Although the 

models were run to determine downstream effects only 

for June through October 2001 and April through October 

2002, without-dam water temperatures at the dam sites were 

estimated for the entire years of 2001 and 2002.

This study focused on 13 USACE dams that make up 

the Willamette Project in northwestern Oregon, as well as the 

combined effects of multiple dams on the Clackamas River, a 

tributary to the lower Willamette River (fig. 1, table 1). Other 

dams in the Willamette River basin, such as Scoggins Dam 

in the Tualatin River basin and the Carmen-Smith Project 

in the upper McKenzie River basin, were not included in 

this analysis because these dams and tributary reaches were 

not included in the models used to assess water-temperature 

issues for the Willamette River water-temperature TMDL; 

these dams also are small enough and far enough upstream 

that their effects on Willamette River flows and temperatures 

are minimal. Downstream effects were simulated only for 

the reaches included in the TMDL models. Where two 

or more dams are located on the same tributary, such as 

Green Peter and Foster Dams in the South Santiam River 

basin, the downstream effects were simulated starting at the 

most downstream dam only, although without-dam water 

temperatures still were estimated for the upstream dam sites.

In addition to water temperatures, without-dam 

streamflows were estimated at each of the dam sites. Those 

flow estimates were not a major focus of the study, but were a 

critical model input for the simulation of downstream thermal 

effects. Although an assessment of the flow effects of the 

dams was not a major objective of this study, those effects 

are substantial and the results provide useful data for future 

resource management.

Finally, the methods and techniques documented in 

this report are not meant to provide definitive equations 

for estimating without-dam water temperatures. Instead, 

the methods are meant to illustrate the types of estimation 

methods that can be used and to provide reasonable starting 

points for similar analyses or models. In the absence of more 

detailed models of water temperature for a without-dams 

condition, the methods documented in this report can be used 

to produce useful estimates.

Methods and Models

Several methods were used to estimate the flow and water 

temperature conditions that would occur at the target dam 

sites in the absence of those dams, and then track those effects 

downstream.

Dam-Site Flow Estimation

The downstream flow and thermal effects of Willamette 

River basin dams cannot be modeled without first estimating 

the flows that would occur at the dam sites in the absence of 

the dams. In some cases, flow estimates also were needed 

to help estimate without-dam water temperatures at the dam 

sites, particularly when more than one reservoir was located 

on the same tributary. One method was used to estimate flows 

at the USACE dam sites, and a different method was used to 

estimate flows at the River Mill dam site on the Clackamas 

River.
Methods and Models 7

For the USACE dam sites, without-dam flows were 

computed using net inflow estimates that are routinely 

calculated by USACE from measured dam releases and 

measured changes in reservoir storage. To estimate the actual 

total inflow rate to each reservoir, it was necessary only to 

adjust the net inflow estimates to account for evaporative 

losses in the reservoir. (The effects of precipitation are 

included in the analysis through an assumption that any 

rainfall collected by the surface of the reservoir would result 

in a similar amount of streamflow in the absence of that 

reservoir.) Evaporation rate measurements at each reservoir 

were available from USACE (see section, “Data Sources”). 

The maximum evaporation rates were measured in July and 

ranged from 4.36 to 6.22 in/mo; minimum evaporation rates 

were assumed to be 1.00 in/mo (table 2). For a reservoir with 

a large surface area such as Fern Ridge Lake (9,000 acres 

at full pool), the maximum evaporative loss is substantial 

(62.2 ft3/s) and can be as large as the incoming streamflow at 

times. Evaporative losses were assumed to vary in a smooth 

sinusoidal-like fashion from a minimum in winter to a 

maximum in summer as in figure 3. Evaporation adjustments 

for the re-regulating reservoirs (Dexter and Big Cliff) were 

negligible and therefore were not included in the without-dam 

flow estimates.

Table 2. Evaporation rates from selected reservoirs owned and operated by the U.S. Army Corps of Engineers 

in the Willamette River basin, Oregon.

[USACE, U.S. Army Corps of Engineers; maximum evaporation rates from July measurements by USACE; minimum evaporation 

rates were assumed]

Reservoir

Evaporation rate 

(inches per month) Full-pool area

(acres)

Evaporative loss 

(cubic feet per second)

Maximum Minimum Maximum Minimum

Blue River 4.36 1.00 1,009 6.0 1.4

Cottage Grove 4.49 1.00 1,156 7.0 1.6

Cougar 5.53 1.00 1,280 9.6 1.7

Detroit 4.59 1.00 3,500 21.8 4.7

Dorena 4.75 1.00 1,749 11.3 2.4

Fall Creek 4.94 1.00 1,820 12.2 2.5

Fern Ridge 5.10 1.00 9,000 62.2 12.2

Foster 4.65 1.00 1,220 7.7 1.7

Green Peter 4.82 1.00 3,720 24.3 5.0

Hills Creek 6.22 1.00 2,735 23.1 3.7

Lookout Point 4.55 1.00 4,360 26.9 5.9

Figure 3. Imposed seasonal variation in the estimated 

correction for evaporative losses from Willamette River 

basin reservoirs, Oregon. Maximum losses were between 

July 1 and August 15. Minimum losses were prior to 

April 1 and after November 15.

tac10-9711-0490_fig 03

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

Minimum

Maximum
8 Thermal Effects of Dams in the Willamette River Basin, Oregon

The Clackamas River has multiple dams and hydropower 

diversions, which complicates the process of estimating 

streamflow in the absence of those features. For this river 

system, however, a model had been constructed for PGE’s 

dam relicensing process, and one of the model scenarios 

was a simulation of conditions in the absence of the dams 

and hydropower projects. That model scenario was provided 

to ODEQ and was used in support of the Willamette River 

temperature TMDL (Oregon Department of Environmental 

Quality, 2006b). The modeled “no-project” flows were used 

as no-dam flow estimates for the River Mill dam site on the 

Clackamas River for the period in 2001 that overlapped the 

period of interest for this study. No-dam flows after September 

30, 2001, were estimated through a correlation of the available 

no-project flows with estimated no-dam flows at Big Cliff 

Dam on the North Santiam River.

Dam-Site Temperature Estimation

The methods used to estimate without-dam water 

temperatures at each of the dam sites of interest varied 

depending on the availability of upstream data and the 

presence of additional dams upstream. The flow chart in 

figure 4 illustrates the general process. For a reservoir 

with only one major inflow, no upstream reservoirs, and a 

complete record of upstream water temperature, the upstream 

water temperatures simply were adjusted to account for any 

warming that would occur during summer as the water travels 

downstream to the dam site. If the upstream water-temperature 

dataset was incomplete, or if data were only available for a 

different time period, then regression techniques were applied 

using measured water temperatures from nearby or similar 

streams to estimate temperatures for the period of interest. For 

reservoirs with more than one major inflow, energy and mass 

balances were applied to generate temperature estimates at the 

confluence of those inflows in the reservoir. Finally, if more 

than one dam is located on the same major tributary, such 

as Green Peter and Foster Dams in the South Santiam River 

basin, then the estimation procedures were applied to the most 

upstream dam first and then to the dam(s) downstream.

Regressions

Regression techniques were used to estimate water 

temperatures at sites upstream of reservoirs if the available 

datasets did not fully correspond to the period of interest. 

The available water-temperature data were correlated with 

water-temperature data from a nearby or similar site, paying 

particular attention to match the elevation and geographic 

characteristics of the two sites. Site elevation and upstream 

geographic characteristics are important in determining a 

stream’s water temperature. A stream that has a significant 

fraction of its headwaters in the High Cascades, for example, 

is likely to be cooler in mid-summer than a stream entirely 

in the Western Cascades or Coast Range because flow in 

High Cascade streams typically is driven by snowmelt and 

high-elevation springs, whereas streams in other geographic 

areas of the Willamette River basin tend to be rainfall driven 

(Conlon and others, 2005; Tague and others, 2007). The best 

water-temperature regressions were obtained when the target 

site and its correlated site were in the same geographic area 

and at similar elevations.

Separate regressions typically were constructed to predict 

a site’s daily mean water temperature and its daily temperature 

range, in order to avoid phase errors that might occur if the 

hourly or half-hourly data were compared directly. Phase 

errors in a time series comparison can be caused by subtle 

timing discrepancies, such as (1) differences in the duration of 

daylight caused by site location or shading characteristics, or 

(2) undocumented errors in the time datum (daylight savings 

time versus standard time). For example, if a site is located far 

enough west relative to the target site, the timing of sunrise 

and sundown will occur a fraction of an hour later and delay 

the daily warming and cooling cycle; direct comparison 

of water-temperature time series from the two sites would 

contain a phase error that can be difficult to remove. 

Producing separate regressions for the daily mean and the 

daily range in water temperature is a simple way to circumvent 

such problems.

Extrapolation of the regression often was necessary to 

extend incomplete temperature datasets (typically available 

only for the warmer summer months) to the rest of the year. 

For some comparisons, a linear regression worked well, 

but extrapolation of a linear model could result in winter 

temperature predictions below freezing under some conditions. 

To remedy that situation, a linear regression could be forced 

to have a zero intercept, but that might result in a poorer fit to 

the data in summer. Instead, a more robust regression could 

be produced for all temperature ranges by fitting a quadratic 

curve with a zero intercept:

2

1 2 2

1

2

,

where

is the daily mean water temperature at the

target site (site 1),

is the measured daily mean water temperature

at the correlation site (site 2), and

and are regression coefficien

m m m

m

m

T aT bT

T

T

a b

= +

ts.

(1)
Methods and Models 9

ortac10-0490_fig04

Start at most upstream dam

Apply

appropriate level of

downstream warming

to dam site.

End

Yes

No

Only one

major

inflow?

Estimate

contributing flow

or drainage area

ratios

No Have flow

data for

inflows?

No

Yes Yes

Q & A data

from nearby

or similar

streams 

Apply downstream warming to mixing

point for measured T data, then apply

flow-weighted or area-weighted mean

to T estimates.

Repeat as necessary for other inflows.

Dams

downstream?

No

Yes

Repeat the process for each dam until the most downstream estimates are completed.

T data

from

nearby or

similar

streams

Correlate

upstream

data 

Correlate upstream

T data to T data

from nearby or

similar streams

Correlate estimated

T data at dam site to

T data from nearby or

similar streams

Apply

appropriate level of

downstream

warming

to dam site.

Correlation

needed? Yes

No

Figure 4. Flow chart showing process for estimating without-dam water temperatures at dam sites in the Willamette River basin, 

Oregon. [Abbreviations: T, water temperature; Q, streamflow; A, upstream drainage area]
10 Thermal Effects of Dams in the Willamette River Basin, Oregon

When this approach was necessary, the coefficient for the 

quadratic term in the equation typically was small, indicating 

that only a slight curvature was needed to force the regression 

curve through zero. Similar approaches were used to correlate 

the daily range in water temperature at pairs of sites.

When separate daily mean and daily range regressions 

were used, they were combined to produce a time series of 

estimated water temperatures by assuming that the timing of 

any daily variation in temperature at site 1 was similar to that 

at site 2. The combined equation, allowing for a quadratic or 

linear regression of the daily means and a linear regression of 

the daily ranges, would take the following general form:

2

1 2 2 2 2 2 2

1

2

2

( )( ) / ,

where

is the estimated water temperature at site 1,

is the daily mean water temperature at site 2,

is the measured hourly or half-hourly water

 temperature at s

m m m r r

m

T aT bT c T T dT e T

T

T

T

= + + + − +

2

ite 2,

is the daily range in water temperature at

site 2, and

– are regression coefficients.

Tr

a e

(2)

The first three terms are from the daily mean regression, 

whereas the other terms are used to superimpose a scaled level 

of daily variation on top of the estimated daily mean. This 

equation illustrates a typical approach, but the method varied 

depending on the site.

Mass and Energy Balances

At the confluence of two streams, both mass and energy 

are preserved in the mixed waterbody downstream. When 

the flow in both streams is known, it is trivial to compute the 

temperature downstream of the confluence by applying an 

energy balance:

( 1 1 2 2 1 2 ) ( )

1 2

1 2

/ ,

where

is the downstream temperature,

and are the temperatures of the two streams

upstream of the confluence, and,

and are the flows in those streams.

d

d

T T Q T Q Q Q

T

T T

Q Q

= + + (3)

If flow data are not available, the upstream drainage areas 

of each stream can be substituted for the flow data as an 

approximation. This energy balance technique is required for 

the estimation of without-dam water temperatures when more 

than one major stream enters the reservoir of interest or if 

one or more inflows have measurement stations upstream of 

tributary junctions. Application of the energy balance is best 

done at each of the relevant stream confluences that would 

occur in the absence of the reservoir.

Downstream Warming

During periods of strong solar heating, particularly in 

summer, water temperatures upstream of a reservoir generally 

are cooler than the without-dam temperatures that would 

occur at the downstream dam site. Some amount of warming 

is likely to occur as the water travels downstream. Several 

methods could be used to estimate the amount of warming 

that would result, including (1) estimates based on trends in 

historical data, (2) measurements of warming in nearby or 

similar streams, and (3) application of a model that simulates 

the heat budget of the stream or a similar or idealized stream.

Temperature models of the rivers upstream of and 

within the reaches now occupied by Willamette River basin 

reservoirs do not currently exist. Until such models are 

constructed, which is beyond the scope of this study, any 

downstream warming must be estimated. CE-QUAL-W2 

models are available for the major rivers downstream of most 

of the dams of interest (see section, “Flow and Temperature 

Models”). Those models could be used to assess the general 

magnitude of warming that occurs during summer months 

just downstream of the dams, provided that the upstream 

flow and temperature boundary conditions imposed on those 

models are representative of a no-dams scenario. When 

that type of no-dam scenario was run for the SF McKenzie 

River, the simulated warming trend in the daily mean water 

temperature for the 4 mi downstream of the Cougar dam site 

was 0.14 °C/mi (range: -0.03–0.28 °C/mi) for June through 

September. For the North Santiam River, the simulated 

warming trend in the first 4 mi downstream of the Big Cliff 

dam site was 0.08 °C/mi (range: 0.00–0.16 °C/mi) for the 

same time period. These rivers, however, may have different 

characteristics (width, shading) downstream of the dam sites 

compared to reaches upstream; therefore, these downstream 

warming trends are only approximate.

Downstream warming rates also can be derived from 

historical data. Water-temperature data collected from 

381 stream sites in Oregon were compiled and documented 

by Moore (1964) for 1947–62, which spans the period when 

about half of the major dams were built in the Willamette 

River basin (table 1). Included in those datasets were pairs 

of sites on unregulated streams that could be used to assess 

the magnitude of warming or cooling that occurs as water 

travels downstream. Results typically showed warming 

in the downstream direction, with periodic cooling trends 

during winter. In the warmest summer months, downstream 

warming could be as much as 0.56 °C/mi (1 °F/mi in 
Methods and Models 11

the original data units), but normally was much less. An 

analysis of the data by Moore (1967) indicated that a 

summertime average downstream warming trend was closer 

to 0.11 °C/mi (0.2 °F/mi). Indeed, a downstream gradient of 

exactly 0.11 °C/mi was measured in the Middle Fork (MF) 

Willamette River for July–September 1951–53, just prior to 

the completion of Lookout Point Dam and within the river 

reach now occupied by Lookout Point Lake (Moore, 1967). 

These measured downstream warming rates are almost 

identical to those that were simulated in the 4 mi downstream 

of the Cougar and Big Cliff dam sites for a no-dams scenario. 

Therefore, a summertime maximum downstream warming 

rate of 0.11 °C/mi was used in this study for estimating 

without‐dam temperatures at the dam sites.

The amount of downstream warming varies seasonally, 

with more warming during summer and proportionally less 

in spring and autumn. Downstream warming in this study 

was applied as a maximum of 0.11 °C/mi during the warmest 

months and a linearly decreasing rate for time periods when 

the stream temperature was cooler. No warming was applied 

outside of the April through October time period. Stream 

temperature is a strong function of flow and meteorological 

conditions and therefore is a good indicator of time periods 

when the stream is warming. Downstream warming, therefore, 

was applied only if the stream’s upstream temperature 

exceeded some lower threshold, with the rate linearly 

increasing until an upper threshold was reached, above which 

the maximum warming rate was applied. For most of the rivers 

in this study, subjective lower and upper thresholds of 10 and 

16 °C were used. For rivers with a substantial drainage area in 

the High Cascades, slightly lower thresholds (6 and 14 °C for 

the North Santiam River; 6 and 12 °C for the SF McKenzie 

River) were applied to account for their colder snowmelt 

sources. Future studies could refine these thresholds by taking 

into account the different physiographic areas of the basin and 

determining how the warming rate of a stream tapers off at 

high temperatures, but no data were available in this study to 

support the exploration of these threshold dependencies.

Water-temperature data upstream and downstream of 

reservoirs can provide additional insights into the seasonal 

pattern of warming and cooling caused by those reservoirs. 

Moore (1967) quantified this effect for several of the dams 

of interest in this study. The warming or cooling effects 

documented by Moore (1967), however, were reported only 

as monthly temperature changes to the nearest whole degree 

Fahrenheit. In the absence of measured upstream temperatures 

(where previously described regression methods could be 

applied), these monthly warming or cooling patterns could be 

used to estimate an upstream temperature from a measured 

temperature downstream of a dam, followed by application 

of a downstream warming rate to compute a without-dam 

temperature at the dam site. This method is subject to larger 

errors than the regression methods used in this study, primarily 

because observed monthly reservoir warming or cooling 

patterns neglect shorter-term variations and may differ 

somewhat from year to year because of variability in dam 

operations. This approach had to be used for Fern Ridge Dam 

on the Long Tom River.

Flow and Temperature Models

The flow and temperature effects of Willamette River 

basin dams downstream of the USACE and PGE dams 

was simulated by imposing the without-dam flow and 

temperature estimates as upstream boundary conditions to 

the Willamette River TMDL models. The suite of Willamette 

models is composed of nine models that can be linked 

together by passing the output of upstream models to the 

input of downstream models. The models simulate the entire 

Willamette River and its largest tributaries as far upstream as 

the first major dam on each tributary (fig. 1). The nine river 

models include the following reaches:

• Lower Willamette River, with part of the Columbia 

River as a downstream boundary condition.

• Middle Willamette River, from river mile (RM) 26.5 

(Willamette Falls) to RM 85.5 upstream of Salem.

• Upper Willamette River, as far upstream as the 

confluence of the Coast Fork and Middle Fork 

Willamette Rivers.

• Clackamas River, the lower 26 mi downstream of River 

Mill Dam.

• Santiam and North Santiam Rivers, downstream of Big 

Cliff Dam.

• South Santiam River, downstream of Foster Dam.

• Long Tom River, downstream of Fern Ridge Dam.

• McKenzie River, as far upstream as its confluence with 

the SF McKenzie River, plus the SF McKenzie River 

downstream of Cougar Dam.

• Coast Fork and Middle Fork Willamette Rivers, 

downstream of Cottage Grove Dam on the Coast 

Fork Willamette River and Dexter Dam on the 

Middle Fork Willamette River, including the Row 

River downstream of Dorena Dam and Fall Creek 

downstream of Fall Creek Dam.
12 Thermal Effects of Dams in the Willamette River Basin, Oregon

All nine river models were constructed using CE-QUAL-W2, 

a state-of-the-art two-dimensional, laterally averaged flow and 

water-quality model from USACE that is jointly maintained 

by Portland State University (PSU) (Cole and Wells, 2002). 

Most of the models were constructed, calibrated, and tested 

by Dr. Scott Wells’ research team at PSU (Annear and others, 

2004a, 2004b; Berger and others, 2004). The South Santiam 

River model was constructed by ODEQ with assistance from 

the PSU team. The Santiam and North Santiam River model 

was built, calibrated, and tested by USGS (Sullivan and 

Rounds, 2004).

These flow and temperature models were calibrated 

and used to simulate periods during 2 years when data were 

available for model calibration; June 1–October 31, 2001, and 

April 1–October 31, 2002. Flow conditions were relatively 

typical in the summer of 2002, whereas 2001 was a drought 

year with some reservoirs that did not fill and streamflows 

at many locations that fell below the lowest 7-day average 

streamflow that would be expected to occur once in 10 years 

(7Q10). Modeled water temperatures were in good agreement 

with measured data; mean absolute errors generally were less 

than 1.0 °C. Much more information about CE-QUAL-W2, its 

application to these river reaches, and its use in the TMDL is 

available from Annear and others (2004a, 2004b), Berger and 

others (2004), and Sullivan and Rounds (2004).

In an attempt to simulate water temperatures that would 

occur if certain anthropogenic influences were minimized or 

eliminated, the Willamette River temperature TMDL included 

a reference condition called Natural Thermal Potential (NTP). 

Under this NTP condition, point sources were removed, 

riparian vegetation was restored with some level of natural 

disturbance, and several instream hydropower projects 

at Willamette Falls and along the McKenzie River were 

eliminated (Oregon Department of Environmental Quality, 

2006b). Upstream dams and instream withdrawals, however, 

remained in the NTP model scenario.

The models used in this study were based on the NTP 

scenario used in the TMDL, but included several small 

modifications to correct errors, remove instabilities, and make 

the models more usable. Some of these model changes were 

documented by Rounds (2007) in a study that used the TMDL 

models to assess the thermal effects of point sources, riparian 

shading, and dam operations. During this study, additional 

modifications were required to run the models under the 

no-dam flow conditions. For example, large withdrawals 

for agricultural and municipal use along the South Santiam 

River downstream of Foster Dam can amount to more 

flow than is present under no-dam conditions. The largest 

withdrawal in that reach, the Lebanon-Santiam Canal (USGS 

station 14187600), by itself averaged more than 100 ft3/s 

during mid-summer in 2001 and 2002. As part of the no-dam 

scenario, withdrawals from the South Santiam River had to 

be decreased to keep the model stable and prevent the river 

from becoming dry. Other modifications also were made to 

the calibrated water-balance flows in the Middle Fork and 

Coast Fork Willamette River model, and a small error in the 

distributed tributary temperatures for several branches of the 

Upper Willamette River model was corrected. The details 

of these corrections are discussed in section, “Downstream 

Effects of Willamette Basin Dams.”

Data Sources

Without-dam streamflows and water temperatures were 

estimated using data from USGS, ODEQ, and USACE. USGS 

streamflow and water-temperature data were obtained from 

measurements stored in the USGS National Water Information 

System database, parts of which can be accessed online (U.S. 

Geological Survey, 2010a). Daily mean data and other daily 

statistics are available from http://waterdata.usgs.gov/or/

nwis/sw/ (U.S. Geological Survey, 2010b), and subdaily data 

from some sites can be accessed at http://or.water.usgs.gov/

grapher/ (U.S. Geological Survey, 2009). USGS streamflow 

and temperature data not available online are available upon 

request. USGS protocols and quality assurance procedures 

for the collection of streamflow and water temperature have 

been published by Rantz and others (1982) and Wagner and 

others (2006), respectively; water-temperature measurements 

typically are accurate to within 0.2 °C. Upstream drainage 

areas, when needed as a surrogate for streamflow data, were 

obtained from the USGS Streamstats program, accessible 

online at http://water.usgs.gov/osw/streamstats/oregon.html

(U.S. Geological Survey, 2007). 

In addition to the USGS data, water-temperature data 

from many sites were obtained from ODEQ’s LASAR 

database (Oregon Department of Environmental Quality, 

2007) at http://deq12.deq.state.or.us/lasar2/. ODEQ has 

detailed protocols for the collection and quality assurance of 

water-temperature data (Oregon Department of Environmental 

Quality, 2009a). Data from the LASAR database were used 

in this study only if those data passed the quality assurance 

checks imposed by ODEQ. USACE data of various types 

(streamflow, reservoir releases, evaporation rates, etc.) were 

obtained either directly from USACE staff in the Portland 

District, or online through the USACE Dataquery tool (U.S. 

Army Corps of Engineers, no date) at http://www.nwd-wc.

usace.army.mil/perl/dataquery.pl.
Thermal Effects of Dams in the Willamette River Basin 13

Thermal Effects of Dams in the 

Willamette River Basin

Without-Dam Flow Estimates at Dam Sites

Estimates of without-dam streamflow at 11 of the 

USACE dam sites were constructed from measured dam 

releases, estimated changes in storage as a function of 

reservoir stage measurements, and a correction for evaporative 

losses from the reservoirs. Net inflow estimates based on 

measured releases and storage changes were obtained from 

USACE. Measured evaporation rates and known reservoir 

surface areas were used to correct the net inflows to produce a 

gross inflow estimate. It was assumed that the flow-regulating 

dams (Big Cliff and Dexter) had little effect on the net inflow 

estimates from the much larger dams directly upstream 

(Detroit and Lookout Point, respectively).

Without-dam streamflows for the Clackamas River 

at the River Mill dam site were estimated from models 

and regressions. Simulated no-project flows at the River 

Mill site were obtained from ODEQ for 2001 through the 

end of September (Oregon Department of Environmental 

Quality, 2006b). After that date, and for 2002, the estimated 

without‐dam streamflows were determined through a 

regression of the available 2001 no-project flows against 

estimated without-dam flows at the Big Cliff dam site on the 

North Santiam River. That regression was:

1.2733 177.5, 0.972

where

is the estimated without-dam streamflow in 

the Clackamas River at the River Mill dam

site, in cubic feet per second,

is the estimated without-dam streamflow in

the N

RM NS

RM

NS

Q Q R

Q

Q

= − =

orth Santiam River at the Big Cliff dam

site, in cubic feet per second, and

R is the correlation coefficient.

(4)

A comparison of with- and without-dam streamflows 

at most of the dam sites reveals the effects of operating 

these dams for wintertime flood control and summertime 

flow augmentation. Dams on the MF Willamette River, for 

example, stored peak inflows for later release, resulting in 

lower peak flows in winter (9,970 versus 21,700 ft3/s) and 

higher minimum flows in summer (approximately 1,020 

versus 633 ft3/s) at the Dexter dam site during 2001–02 

than might have occurred otherwise (fig. 5, table 3). Dam 

operation also typically results in streamflows that are 

relatively constant over periods of weeks, and that change 

abruptly when gates or spillways are opened to augment 

flows or make room for additional flood storage, or closed to 

fill the reservoir or protect downstream areas from flooding. 

500

1,000

2,000

5,000

10,000

20,000

Measured, with dams

Estimated, without dams

STREAMFLOW, IN CUBIC FEET PER SECOND

2001 2002

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

Figure 5. Measured and estimated streamflow for conditions with and without upstream dams in the Middle 

Fork Willamette River at Dexter Dam, just downstream of Lookout Point Dam in the Willamette River basin, 

Oregon, 2001–02. Measured flow data are from USGS streamflow-gaging station 14150000.
14 Thermal Effects of Dams in the Willamette River Basin, Oregon Table 3. Minimum and maximum streamflows for conditions with and without upstream dams at 12 dam sites in the Willamette River basin, Oregon, 2001–02. [Streamflow statistics are rounded to three significant digits. Measured flows at the Foster dam site were obtained by subtracting the flows from Wiley Creek (14187000) from the downstream South Santiam flows (14187200). Measured flows at the Green Peter Dam site were obtained from the U.S. Army Corps of Engineers. GPR, flow gage at Green Peter Dam] Dam site Streamflow, in cubic feet per second Station ID for the measured with-dam data Maximum 95th percentile 5th percentile Minimum Without dams With dams Without dams With dams Without dams With dams Without dams With dams Big Cliff / Detroit 16,400 10,100 4,100 4,690 700 851 641 807 14181500 Blue River 4,500 2,990 1,330 1,170 26.5 49.1 21.2 45.6 14162200 Cottage Grove 1,790 1,570 509 635 23.9 43.0 22.5 40.0 14153500 Cougar 6,590 4,590 1,580 1,990 243 217 231 186 14159500 Dexter / Lookout Point 21,700 9,970 6,000 5,100 709 1,090 633 1,020 14150000 Dorena 7,170 4,160 2,000 1,780 28.2 95.6 24.0 87.8 14155500 Fall Creek 4,910 3,070 1,390 1,420 22.9 38.0 20.9 34.0 14151000 Fern Ridge 9,680 3,030 2,090 2,090 53.8 37.9 36.6 25.8 14169000 Foster 23,700 12,300 7,170 7,400 133 647 99.6 588 14187200 - 14187000 Green Peter 13,500 9,840 4,660 4,580 80.2 50.0 58.0 50.0 GPR

Hills Creek 7,780 3,090 2,190 2,070 304 282 276 83.1 14145500River Mill 20,700 17,400 5,040 5,250 724 680 639 589 14210000
Thermal Effects of Dams in the Willamette River Basin 15

In contrast, streamflow responds quickly to rainfall and 

snowmelt without the dams, and has a characteristic 

exponential transition from high-flow to low-flow conditions 

in the spring to summertime period. Dam operations likely 

will change in the future, however, as flow variations and 

higher peak flows increasingly are recognized as important 

to downstream fish populations and the health of off-channel, 

wetland, and floodplain ecosystems (Gregory and others, 

2007).

The with- and without-dam streamflow comparison 

shows important differences between the 2001 drought 

conditions and the more-normal 2002 hydrologic conditions. 

The winter of 2000–01 was uncharacteristically dry, with 

insufficient precipitation to allow most of the key storage 

reservoirs to fill, including Detroit, Green Peter, Blue River, 

Cougar, Lookout Point, and Hills Creek Lakes. As a result, 

summertime releases in 2001 were less than those in 2002. 

Summertime releases from Dexter Dam (and Lookout Point 

Dam just upstream) on the MF Willamette River in 2001 

were roughly one-half of mid-summer releases in 2002. The 

decreased storage in these dams in 2001 resulted in Willamette 

River flows that were at or near post-dam 7Q10 levels during 

summer. Because 2001 was a drought year and 2002 was a 

more typical year, the without-dam temperature estimates 

from this study reflect a range of hydrologic conditions, which 

is useful for showing a range of thermal responses.

The with- and without-dam streamflow patterns for two 

of the shorter dams included in this study, River Mill Dam on 

the Clackamas River and Fern Ridge Dam on the Long Tom 

River, show less difference than the streamflow patterns for 

the larger storage dams. River Mill Dam in particular is more 

of a run-of-the-river dam. Storage capacities of River Mill 

Dam and the other upstream dams on the Clackamas River are 

minimal, which results in without-dam flow estimates that are 

quite similar to measured flows with the dams in place. The 

storage capacity of Fern Ridge Dam is substantial despite its 

relatively short height (table 1); the dam is operated to provide 

a flood-protection benefit, but its summertime releases for 

downstream irrigators provides less of a contrast in seasonal 

flow compared to the with- and without-dam streamflows for 

some of the larger dams.

Measured with-dam and estimated without-dam 

streamflows for all dam sites included in this study are 

graphed in appendix A for 2001–02. Those comparisons 

illustrate the same general patterns and trends discussed 

above, but two items discernible in those data merit additional 

discussion. First, the similarity of with- and without-dam 

flows at the Cougar dam site during 2002 is a result of 

the reservoir being drawn down for the construction of a 

selective withdrawal tower. The reservoir did not fill in 2001, 

but was drawn down for construction from 2002 through 

2004. Second, the estimated without-dam streamflow in the 

South Santiam River at the Foster dam site decreased during 

summer to as low as 140 ft3/s in 2001 and 100 ft3/s in 2002. 

The autumn rains arrived earlier in 2001 than in 2002, or the 

low‐flow minimum in 2001 might have been comparable 

to that in 2002. Withdrawals from the South Santiam River 

downstream of Foster Dam can easily exceed the low-flow 

levels without the dams, especially the diversion through the 

Lebanon-Santiam Canal, which provides municipal water 

to the city of Albany and was measured at approximately 

90–100 ft3/s during the summers of 2001–02. Clearly, that 

level of surface-water diversion could not be supported 

without the storage provided by upstream dams.

Without-Dam Temperature Estimates 

at Dam Sites

Without-dam water temperatures were estimated at the 

dam sites using a combination of several different methods, 

following the general process shown in figure 4. When 

water‐temperature data were available upstream of the 

reservoirs at each of the major inflows, those temperatures 

were combined using flow- or drainage-area-weighting 

techniques and adjusted to account for warming that typically 

would occur as that water moved downstream to the dam 

site during summer. Measured temperatures upstream of the 

reservoirs, however, usually were not available for the entire 

2001–02 time period. As a result, regressions between the 

measured or adjusted temperatures and those from nearby or 

similar sites were needed to extend the estimates to the entire 

calendar years of 2001 and 2002. In the sections that follow, 

the methods used to estimate without-dam water temperatures 

at each of the dam sites are discussed along with the results. 

The order of presentation is alphabetical, as listed in table 1. 

These estimation methods are neither definitive nor unique, 

but are meant to illustrate the type of estimation method that 

could be applied, and to emphasize the importance of several 

types of data sources that are critical to this type of analysis.
16 Thermal Effects of Dams in the Willamette River Basin, Oregon

Big Cliff and Detroit Dams

Big Cliff and Detroit Dams are located on the North 

Santiam River, which is one of the major Willamette River 

tributaries draining the Cascade Range (fig. 1). Big Cliff Dam 

is a relatively small re-regulating dam that smoothes the flow 

releases from Detroit Dam, which impounds one of the largest 

storage reservoirs in the Willamette River basin (table 1). The 

Big Cliff/Detroit dam complex generates more hydropower 

than any of the other dams in this study. Summertime releases 

from Detroit Dam during 2001–02 primarily were through 

its power penstocks, and at a depth that was below the 

thermocline in mid-summer; therefore, releases at that time 

were relatively cold. The downstream temperature effects 

of these dams have been documented previously (Moore, 

1967; Laenen and Hansen, 1985; Sullivan and Rounds, 2004; 

Sullivan and others, 2007).

Upstream of Detroit Lake, the three major inflows (North 

Santiam River, Breitenbush River, and Blowout Creek) are all 

gaged; streamflow and water-temperature data are available for 

the entire 2001–02 time period. As a result, it was relatively 

simple to compute a flow-weighted mean temperature for the 

major inflows, and adjust for the warming that probably would 

have occurred in the 11.8 mi from the head of Detroit Lake to 

the Big Cliff dam site. The resulting without-dam temperatures 

at the Big Cliff dam site are plotted in figure 6A along 

with measured temperatures just downstream of Big Cliff 

Dam at USGS station 14181500. The comparison shows a 

characteristic shift of the annual maximum water temperature 

from July or August without dams to September or October. 

Such a shift is typical downstream of a large storage reservoir 

that has a mid-level or deeper release point. Releases from 

Detroit Dam are primarily from the power penstocks, which 

are below the lake’s thermocline in mid-summer, resulting in 

relatively cold downstream temperatures in July and August. 

The process of drawing down the lake in September to make 

room for flood storage, however, brings warmer surface waters 

down to the power penstocks, resulting in warmer downstream 

temperatures in September and October.

The comparison of with- and without-dam water 

temperatures clearly shows that the dams have both a cooling 

and a warming effect. During mid-summer, Big Cliff and 

Detroit Dams release water that is as much as 8 °C cooler than 

might occur without the dams, whereas releases in September 

or October can be 6–8 °C warmer than without the dams. 

Because the releases from Detroit Dam are from deep within 

the lake, little daily variation in water temperature occurs with 

the dams in place, compared to a daily range of about 4 °C 

without the dams in mid-summer.

Oregon’s maximum water-temperature standard specifies 

a 7dADM water temperature for comparison to numeric 

criteria (Oregon Department of Environmental Quality, 

2009b). Without the dams, the maximum water temperature 

at the Big Cliff dam site is relatively consistent from 2001 to 

2002, with a 7dADM temperature between 17 and 18 °C in 

late July or early August (table 4). The maximum without-dam 

7dADM temperature was slightly higher in 2001 than in 2002, 

a difference that probably is due to lower streamflow during 

the 2001 drought conditions. With the dams, the maximum 

measured 7dADM temperature was as low as 13.7 °C in 2002. 

Because the lake did not fill in 2001, the warmer surface water 

was closer to the power penstock intake level, resulting in a 

higher maximum release temperature.

Comparing these temperatures as cumulative frequency 

curves, it is clear that the without-dam dataset contains 

both cooler and warmer temperatures (fig. 7). For example, 

temperatures were estimated to be cooler than 8 °C during 

2001–02 about 59 percent of the time without dams, but only 

about 50 percent of the time with dams. The distribution is 

more similar in the 11–15 °C range, above which the curves 

diverge, where the without-dam temperatures increase to a 

maximum that is 1.6 °C higher than the with-dam maximum. 

The shape of the with-dam curve, of course, is greatly affected 

by dam operations and the fact that Detroit Lake did not 

completely fill in 2001. More important than the distribution 

of annual temperatures, however, is the shift of the annual 

maximum from mid-summer without dams to early autumn 

with dams because of the effect of that seasonal pattern on the 

timing of fish migration and spawning. The seasonal pattern 

in water temperature is of critical importance to the life cycle 

and survival of anadromous fish, and the temperature of water 

released by dams is an important controlling factor affecting 

that seasonal pattern (Caissie, 2006).
Thermal Effects of Dams in the Willamette River Basin 17

ortac10-0490_fig06

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

WATER TEMPERATURE, IN DEGREES CELSIUS

2001 2002

C. Cottage Grove Dam

B. Blue River Dam

A. Big Cliff / Detroit Dams

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

Measured, with dams (USGS)

Estimated, with dams (PSU)

Estimated, without dams

Figure 6. Measured and estimated water temperatures for conditions with and without upstream dams at 

the Big Cliff / Detroit, Blue River, and Cottage Grove dam sites in the Willamette River basin, Oregon, 2001–02. 

Estimated with-dam temperatures are from Annear and others (2004a) at Portland State University (PSU). 

Measured temperatures are from USGS stations 14181500, 14162200, and 14153500 for graphs A, B, and C, 

respectively.
18 Thermal Effects of Dams in the Willamette River Basin, Oregon

WATER TEMPERATURE, IN DEGREES CELSIUS

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

90

92

94

96

98

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Measured, with dams

Estimated, without dams

Figure 7. Cumulative frequency curves of measured and estimated water temperatures for conditions with and 

without upstream dams at the Big Cliff dam site in the Williamette River basin, Oregon, 2001–02. Measured withdam temperatures are from USGS station 14181500 just downstream of Big Cliff Dam. The inset graph shows an 

expanded vertical scale at the highest temperatures.

Table 4. Measured with-dam and estimated without-dam annual maximum water temperatures as measured by the 7-day 

average of the daily maximum at selected dam sites in the Willamette River basin, Oregon, 2001–02.

[7dADM, 7-day running average of the daily maximum; na, not available; *, estimate because dataset is incomplete]

Dam site

Annual maximum 7dADM, in degrees Celsius (month/day of maximum 7dADM) USGS station ID

for the measured

with-dam data

2001 2002

Without dams With dams Without dams With dams

Big Cliff / Detroit 17.9 (8/13) 16.3 (9/15) 17.2 (7/23) 13.7 (10/17) 14181500

Blue River 21.1 (8/12) 21.6 (9/01) 21.0 (7/26) 16.3 (10/19) 14162200

Cottage Grove 23.6 (8/12) 21.9 (9/12) 23.4 (7/27) 20.2 (9/21) 14153500

Cougar 15.2 (7/06) 14.5 (10/02) 15.3 (7/12) 17.4 (7/29) 14159500

Dexter / Lookout Point 23.2 (8/10) 20.4 (9/15) 23.6 (7/23) 18.8 (9/22) 14150000

Dorena 25.1 (8/10) 19.3 (9/29) 25.8 (7/27) 18.6 (10/01) 14155500

Fall Creek 21.5 (8/12) 16.6 (10/17)* 21.6 (7/23) 21.3 (8/31) 14151000

Fern Ridge 23.0 (7/10) 23.3 (8/12)* 22.0 (7/16) 24.1 (7/24) 14169000

Foster 24.5 (8/12) 13.2 (8/29) 24.5 (7/22) 13.1 (7/19) 14187200

Green Peter 23.4 (8/12) na 23.9 (7/23) na na

Hills Creek 19.4 (8/13) na 19.9 (7/13) na na

River Mill 19.0 (8/10) 17.9 (8/15) 18.0 (7/23) 18.5 (7/27) 14210000
Thermal Effects of Dams in the Willamette River Basin 19

Without-dam temperature estimates are affected by 

several sources of uncertainty. Because measured flows and 

temperatures were available for the entire 2001–02 time period 

upstream of Detroit Lake, the uncertainty derives primarily 

from the estimated rate of downstream heating for the warmer 

periods during summer. A model was not available to account 

for the actual travel time and the degree of topographic 

shading as the water moved from the head of Detroit Lake 

to the Big Cliff dam site. Downstream of the dam, however, 

a model of the North Santiam River was used to determine 

a downstream warming rate of 0.08 °C/mi (range: 0.00 to 

0.16 °C/mi), which is close to the assumed maximum warming 

rate of 0.11 °C/mi. Applied over a distance of 11.8 mi, a 

warming rate of 0.11 °C/mi produces a maximum warming 

of 1.3 °C. If the warming rate was in error by as much 

as 0.05 °C/mi, an uncertainty of as much as 0.6 °C could 

result. Combined with measurement errors of ±0.2 °C, the 

without‐dam temperature estimates at the Big Cliff dam site 

are likely to have an uncertainty of no more than 0.5 to 0.8 °C 

(table 5).

Table 5. Maximum estimated error ranges for the without-dam water-temperature estimates at 

selected dam sites in the Willamette River basin, Oregon.

[°C, degrees Celsius; na, not applicable]

Dam site

Maximum 

measurement 

error (°C)

Maximum 

regression 

error (°C)

Maximum 

warming rate 

error (°C)

Total maximum 

estimated 

error (°C)

Big Cliff / Detroit 0.2 na 0.6 0.5–0.8

Blue River 0.2 0.3–0.6 0.6 0.5–1.4

Cottage Grove 0.2 0.5–1.0 0.3 0.5–1.5

Cougar 0.2 na 0.3 0.5

Dexter / Lookout Point 0.2 0.6 0.6–1.1 1.5–1.8

Dorena 0.2 0.5 0.3 0.5–1.0

Fall Creek 0.2 0.7 0.6 0.8–1.5

Fern Ridge 0.2 0.8–1.2 0.6 1.0–2.0

Foster 0.2 0.6 0.8 0.8–1.5

Green Peter 0.2 0.4 0.5 0.5–1.0

Hills Creek 0.2 0.5 0.4 0.5–1.0

River Mill 0.2 0.6 na 11.0–1.6

1

Estimated error for River Mill includes an estimate of potential error from the no-project model scenario.
20 Thermal Effects of Dams in the Willamette River Basin, Oregon

Blue River Dam

Blue River Dam is located on Blue River, which is a 

tributary to the McKenzie River just downstream of the SF 

McKenzie River confluence (fig. 1). The entire drainage of 

Blue River is in the Western Cascades physiographic province, 

which means that its flow is generated more from rainfall than 

from snowmelt, and it does not benefit from the cold springs 

and higher baseflow characteristics of High Cascades streams 

(Tague and others, 2007). The temperature effects of Blue 

River and Cougar Dams on McKenzie River temperatures has 

been studied in previous research by Hansen (1988).

Estimation of without-dam water temperatures at the 

Blue River dam site provides an excellent example of the 

regression procedure used in this study. No temperature data 

upstream of Blue River Lake were available for the period 

of interest in 2001 or 2002, but upstream water temperatures 

were measured by ODEQ from June 15 through August 23, 

2000, at a site 11.6 mi upstream of the dam (site 23910, data 

from ODEQ’s LASAR database, Oregon Department of 

Environmental Quality, 2007). A correlation to measured water 

temperatures at a nearby or similar site was needed to estimate 

water temperatures in Blue River for the 2001–02 time 

period. A comparison of the available data showed that Blue 

River temperatures during the summer were not at all similar 

to those from the SF McKenzie River upstream of Cougar 

Reservoir, a nearby site with a long dataset; the dissimilarity 

probably was the result of the SF McKenzie River having its 

headwaters in the High Cascades whereas Blue River has its 

headwaters entirely within the Western Cascades. A nearby 

site on Blowout Creek in the North Santiam River basin 

only about 30 mi to the north, however, has a continuous 

temperature record and is similar to the Blue River site in 

terms of its general elevation, physiographic province, and 

basin size. A comparison between the temperature data from 

2000 for Blue River and Blowout Creek showed a strong 

similarity.

The available Blue River temperature data from the 

summer of 2000 were correlated with measured temperatures 

for the same period from the Blowout Creek site (USGS 

station 14180300). To avoid phase errors derived from the 

fact that the two sites probably are affected by different 

topographic shading characteristics and the timing of local 

sunrise and sundown, the daily means and daily ranges were 

correlated separately. The regressions with the best fit are 

described by the following equations:

2 0.003843 1.041 , 0.986. TmBR = − T T R mBC + mBC = (5)

0.9335 , 0.864,

where

is the predicted daily mean temperature for 

Blue River,

is the measured daily mean temperature for

Blowout Creek,

is the predicted daily temperature range for

Blue

rBR rBC

mBR

mBC

rBR

T T R

T

T

T

= =

 River, and

is the measured daily temperature range for

Blowout Creek.

TrBC

(6)

Both regressions provided good fits to the data with 

correlation coefficients (R) greater than 0.85; the correlation 

of the daily means was particularly good. These equations 

indicate that the daily temperature range of Blue River is 

about 93 percent of the daily temperature range of Blowout 

Creek, and the daily mean temperature of Blue River is 

about 4 percent larger than the daily mean temperature of 

Blowout Creek. A small quadratic term was included in 

the regression of the daily means so that an extrapolation 

from summer to winter conditions would not cause a bias. 

Measured water temperatures at these sites are quite similar, 

and the regressions bear that out. Combining these equations 

and assuming that the timing in the daily variation of water 

temperature at Blue River is similar to that at Blowout Creek, 

the following equation is derived:

2 0.003843 1.041

0.9335( ),

where

is the predicted temperature of Blue River, and 

is the half-hourly temperature of Blowout Creek.

BR mBC mBC

BC mBC

BR

BC

T T T

T T

T

T

= − +

+ − (7)

The resulting predicted temperatures are an excellent match 

for the measured temperatures of Blue River (fig. 8).
Thermal Effects of Dams in the Willamette River Basin 21

ortac10-0490_fig08

0 10 12 14 16 18 20

BLOWOUT CREEK DAILY MEAN, IN DEGREES CELSIUS 

0

2

4

6

8

10

12

14

16

18

20

BLUE RIVER DAILY MEAN, IN DEGREES CELSIUS 

June-01June-05June-09June-13June-17June-21June-25June-29

July-03July-07July-11July-15July-19July-23July-27July-31

Aug-04Aug-08Aug-12Aug-16Aug-20Aug-24Aug-28

Sept-01

7

9

11

13

15

17

19

21

WATER TEMPERATURE, IN DEGREES CELSIUS 

Measured

Predicted

0 1 2 4 5 10

BLOWOUT CREEK DAILY RANGE, IN DEGREES CELSIUS 

0

1

2

3

4

5

6

7

8

9

10

BLUE RIVER DAILY RANGE, IN DEGREES CELSIUS 

TmBR = –0.003843 T + 1.041 TmBC

2000

2 4 6 8 3 6 7 8 9

2

mBC

TrBR = 0.9335 TrBC

Figure 8. Correlation of water temperatures for Blue River (BR) and Blowout Creek (BC) in the Willamette River 

basin, Oregon, for a period in mid-summer of 2000. Daily means and daily ranges in water temperature were 

correlated separately and then recombined to produce the final prediction.
22 Thermal Effects of Dams in the Willamette River Basin, Oregon

The predicted Blue River temperatures from the 

regression analysis do not reflect temperatures that would 

occur 11.6 mi downstream at the Blue River dam site. A 

maximum downstream warming rate of 0.11 °C/mi was 

assumed (see section, “Methods and Models”) and applied 

for those times in summer when the stream was likely to be 

warming. The final estimated without-dam water temperatures 

at the Blue River dam site are plotted in figure 6B along with 

measured water temperatures at that site in 2001–02.

As was the case for temperatures at the Big Cliff dam 

site on the North Santiam River, without-dam temperatures in 

Blue River at the dam site are relatively consistent from year 

to year. An annual maximum 7dADM water temperature was 

estimated to be about 21 °C without the dam and would occur 

in the July to August time period (table 4). With the dam in 

place, in contrast, the measured annual maximum 7dADM 

occurred in September or October. Blue River Lake did not 

fill in 2001, causing warmer releases downstream because 

the warmer surface layer was closer to the elevation of the 

lake outlet. In 2002, the lake did fill and releases were cooler 

throughout the summer and autumn.

The annual maximum temperature estimated for Blue 

River without Blue River Dam (about 21 °C) is warmer than 

the annual maximum without-dam temperature estimated for 

the North Santiam River at the Big Cliff dam site (17–18 °C). 

The difference probably is a result of the upstream drainage 

characteristics, a theme that recurs throughout this report. The 

Blue River drainage is entirely within the Western Cascades 

physiographic province, whereas a large fraction of the North 

Santiam River upstream of Big Cliff Dam drains from the 

High Cascades area (fig. 2). Annual maximum temperatures 

typically are higher for Western Cascade streams than for High 

Cascade streams because the latter derive a large proportion 

of their flow from cold springs and groundwater fed from 

snowmelt, whereas Western Cascade streams lack that 

consistently cold source of water during the low-flow season.

Uncertainties in the without-dam temperature estimates 

for Blue River can be attributed to errors in the regressions as 

well as errors in the estimated rate of downstream warming 

from the correlation site to the dam site. An analysis of the 

regression data from the summer of 2000 shows little error 

in the prediction of the daily mean temperature, with an 

insignificant mean error (ME, bias) of only -0.005 °C and a 

small mean absolute error (MAE, typical error for any data 

point) of 0.25 °C. Errors in the predicted daily minimum 

or maximum temperature are larger, with an MAE of 0.35–

0.58 °C and an ME of -0.21 °C. Estimates of measurement 

error (±0.2 °C ), error in the rate of downstream warming 

(< 0.6 °C), and regression prediction errors (0.3–0.6 °C) 

are difficult to combine, but the overall uncertainty in the 

summertime temperature estimates for Blue River is likely to 

be in the range of 0.5 to 1.4 °C (table 5).
Thermal Effects of Dams in the Willamette River Basin 23

Cottage Grove Dam

Cottage Grove Dam is located on the Coast Fork 

Willamette River just upstream of Cottage Grove, Oregon. The 

upstream drainage mostly lies within the Western Cascades 

physiographic area, with a small fraction in the Willamette 

Lowlands area (fig. 2). Along with Fern Ridge Dam, Cottage 

Grove Dam was one of the first USACE dams built in the 

Willamette River basin. Its storage volume is relatively small 

compared with most of the other USACE reservoirs in the 

basin.

The best estimates of without-dam temperatures at the 

dam sites make use of measured temperatures upstream of the 

reservoirs. Measured temperatures were available from ODEQ 

for the Coast Fork Willamette River upstream of Cottage 

Grove Lake at London (LASAR site 28614) for August 2–

September 26, 2001, and April 21–September 9, 2002. A 

maximum downstream heating adjustment of 0.11 °C/mi 

was applied for the 6.5 mi to the dam site, but only for those 

periods during summer when heating was likely (see section, 

“Methods and Models”).

To estimate without-dam water temperatures for the 

rest of 2001–02, correlations with data from other sites were 

required. In 2001, measured water temperatures in Mosby 

Creek downstream of Row River Trail were available from 

ODEQ (LASAR site 28103) for June 21–October 3. A 

correlation between these temperatures from Mosby Creek 

and the adjusted without-dam temperatures for the Coast Fork 

Willamette River showed an excellent similarity, with no need 

to correlate the daily means and daily ranges separately:

0.9465 0.4053, 0.963,

where

is the estimated without-dam temperature in

the Coast Fork Willamette River, and 

is the measured temperature in Mosby Creek.

CF MC

CF

MC

T T R

T

T

= + = (8)

This regression was used for the periods June 21–August 2 

and September 26–October 3 in 2001. Prior to June 21 and 

after October 3 in 2001, a correlation with measured water 

temperatures in the SF McKenzie River upstream of Cougar 

Reservoir (USGS station 14159200) provided a good fit, 

despite the fact that the SF McKenzie River drains water from 

the High Cascades. A correlation with data from Blowout 

Creek in the Western Cascades did not fit as well. Separating 

the daily means from the daily ranges, the following 

regression was used:

2 0.014909 1.5081

1.2852( ),

CF mSF mSF

SF mSF

T T T

T T

= +

+ − , (9)

where the temperatures on the right-hand side of the equation 

are from the SF McKenzie River site and the m subscript 

indicates the daily mean. The daily mean and daily range 

regressions produced correlation coefficients of 0.934 and 

0.678, respectively.

Measured water temperatures were available from June 

20 to November 19, 2002, for Mosby Creek at Laying Road 

(LASAR site 30638), which is a nearby unregulated tributary 

to the Row River near Cottage Grove. A comparison of these 

water-temperature data from Mosby Creek with temperatures 

from the Coast Fork Willamette River upstream of Cottage 

Grove Lake indicated that the data were similar enough to 

be used without further adjustment. No downstream heating 

adjustment was applied because the elevation of the Mosby 

Creek site is similar to the elevation of Cottage Grove. These 

data were used to fill out the 2002 without-dam temperature 

time series after September 9, 2002. A correlation with 

measured temperatures in the SF McKenzie River upstream 

of Cougar Reservoir was used for the period prior to April 21, 

2002. (As for the 2001 data, a correlation with Blowout Creek 

temperatures was not as successful.) A new regression was 

prepared for the 2002 data, and the results were similar to the 

regression used for 2001:

2 0.010234 1.6169

1.2458( ).

CF mSF mSF

SF mSF

T T T

T T

= +

+ − (10)

This regression was used to estimate without-dam water 

temperatures for the Coast Fork Willamette River for 

January 1–April 21, 2002. Correlation coefficients for the 

daily mean and daily range regressions were 0.964 and 0.591, 

respectively. The 2001 and 2002 datasets could have been 

combined and a single regression constructed, but the results 

would not have been different enough to be of concern.

The without-dam water-temperature estimates at the 

Cottage Grove dam site show a seasonal pattern that is 

similar to those estimated at other dam sites, with peak 

annual temperatures occurring in July or August rather than 

in September with the dam in place (fig. 6C). The annual 

maximum 7dADM without-dam temperature is about 23.5 °C, 

which is warmer than the estimates for the Blue River dam 

site, despite both sites draining areas in the Western Cascades. 

That difference is consistent with the lower elevation at 

Cottage Grove.

Uncertainties in the without-dam temperature estimates 

at the Cottage Grove dam site vary depending on whether 

those estimates were derived from upstream measurements, 

from nearby sites, or from correlations with sites farther 

away. The downstream warming was applied over only 6.5 mi 

compared to about 11.5 mi for the previous two dam sites, so 

the uncertainty in that adjustment might account for an error of 

0.3 °C or less. Errors associated with the regressions are likely 

to be higher (about 0.5–1.0 °C). Overall, these without-dam 

temperatures probably have uncertainties on the order of 0.5 to 

1.5 °C (table 5). 
24 Thermal Effects of Dams in the Willamette River Basin, Oregon

Cougar Dam

Cougar Dam is located on the SF McKenzie River 

about 4.5 mi upstream of its confluence with the McKenzie 

River. Built in 1963, Cougar Dam is the second tallest dam 

in the Willamette River basin and impounds the fifth largest 

volume of water in Cougar Reservoir. USGS maintains a 

streamflow-gaging station upstream of Cougar Reservoir 

(station 14159200) where water temperature also is measured 

and available for the entire 2001–02 period of interest. The SF 

McKenzie River is the major inflow to Cougar Reservoir, with 

minor contributions from Walker Creek and the East Fork of 

the SF McKenzie River.

Without-dam water temperatures at the Cougar dam 

site were estimated by applying a reasonable downstream 

warming rate to the water temperatures measured upstream 

of Cougar Reservoir at USGS station 14159200. A maximum 

downstream heating rate of 0.11 °C/mi was applied for 

the 6 mi from the measurement site to the dam site. This 

maximum heating rate was applied when the upstream 

measured temperatures exceeded 12 °C. No downstream 

heating adjustment was applied when upstream temperatures 

were less than 6 °C, and a linearly interpolated fraction of 

the maximum rate was applied for upstream temperatures 

between 6 and 12 °C. These temperature thresholds are lower 

than those applied at most of the other sites, primarily because 

summertime temperatures in the SF McKenzie River tend to 

be colder than those upstream of the other reservoirs. Because 

the SF McKenzie River derives a substantial fraction of its 

flow from the High Cascades during summer, it tends to be 

relatively cool (Tague and others, 2007).

Measured with-dam and estimated without-dam water 

temperatures at the Cougar dam site show the characteristic 

shift of annual maximum temperatures caused by dam 

operations in 2001 but not in 2002 (fig. 9A). Cougar Reservoir 

did not fill during the drought winter of 2000–01, but releases 

were still cool in mid-summer because water was released 

from deep in the lake. In 2002, the lake had been drawn down 

to allow for the construction of a selective withdrawal tower at 

the dam. With little water in storage, a more natural seasonal 

temperature pattern occurred in 2002, but some warming 

exceeding the without-dam condition occurred as a result of 

the impoundment, capture of solar radiation, and no ability to 

release water from a cooler hypolimnion.

Without-dam annual maximum 7dADM temperatures 

at the Cougar dam site were estimated to be just over 15 °C, 

colder than the estimates from any other dam site in this study 

(table 4). Maximum without-dam temperatures at the Big 

Cliff dam site are the next coolest, but still warmer by more 

than 2 °C. Both sites are located at approximately 1,200 ft 

above sea level and derive much of their flow from the 

High Cascades, but temperatures in the SF McKenzie River 

upstream of Cougar Reservoir (USGS station 14159200) 

are colder in mid-summer by about 2 °C compared to 

temperatures in the North Santiam River just upstream of 

Detroit Lake (USGS station 14178000). Uncertainties in the 

without-dam water temperatures at the Cougar dam site are 

derived primarily from errors in the downstream warming rate, 

which are on the order of 0.3 °C for this site. Overall, errors in 

the without-dam temperatures at this site should be less than 

0.5 °C (table 5).
Thermal Effects of Dams in the Willamette River Basin 25

ortac10-0490_fig09

0

2

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6

8

10

12

14

16

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22

24

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28

Measured, with dams (USGS)

Estimated, with dams (PSU)

Estimated, without dams

0

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0

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WATER TEMPERATURE, IN DEGREES CELSIUS

2001 2002

C. Dorena Dam

B. Dexter / Lookout Point Dams

A. Cougar Dam

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

Figure 9. Measured and estimated water temperatures for conditions with and without upstream dams at the 

Cougar, Dexter / Lookout Point, and Dorena dam sites in the Willamette River basin, Oregon, 2001–02. Estimated 

with-dam temperatures are from Annear and others (2004a) at Portland State University (PSU). Measured 

temperatures are from USGS stations 14159500, 14150000, and 14155500 for graphs A, B, and C, respectively.
26 Thermal Effects of Dams in the Willamette River Basin, Oregon

Dexter and Lookout Point Dams

Dexter Dam is a re-regulating dam located just 

downstream of Lookout Point Dam on the Middle Fork 

(MF) Willamette River not far from Eugene, Oregon (fig. 1). 

Lookout Point Dam impounds one of the two largest storage 

reservoirs in the Willamette River basin and has a large 

upstream drainage area that encompasses portions of both the 

Western Cascades and the High Cascades. The MF Willamette 

River and its major tributaries—the North Fork of the Middle 

Fork (NFMF) Willamette River, Salt Creek, and Salmon 

Creek—all have contributing areas in the High Cascades.

The process of estimating without-dam water 

temperatures at the Dexter dam site had to begin upstream of 

Hills Creek Dam, also on the MF Willamette River. The first 

step was to determine the availability of water-temperature 

data upstream of Hills Creek Dam and in the other major 

tributaries (Salt Creek, Salmon Creek, and the NFMF 

Willamette River), and then work downstream by applying 

energy and mass balances at each major tributary confluence. 

Measured temperatures upstream of Hills Creek Lake were 

available from Hills Creek (LASAR site 27974) and from 

the MF Willamette River upstream (LASAR site 27992) for 

most of June through September 2001. Streamflow data for 

Hills Creek were not available, so the energy balance for the 

Hills Creek and MF Willamette River confluence was done 

by weighting the measured temperatures with their upstream 

drainage areas (59.9 mi2 upstream of the Hills Creek site, and 

330 mi2 upstream of the MF Willamette River site) after first 

accounting for summertime warming as water moved from 

the two temperature-measurement sites to the Hills Creek 

dam site. The Hills Creek and MF Willamette River sites are 

7.5 and 13.4 mi upstream of that confluence, respectively. 

A maximum downstream warming rate of 0.11 °C/mi was 

applied as described previously, using thresholds of 10 and 

16 °C for no warming and maximum warming.

Downstream of the Hills Creek dam site, 

water‐temperature data were available from Salt Creek 

(LASAR site 28010) and from the mouth of the NFMF 

Willamette River (LASAR site 28003), but no temperature 

data from Salmon Creek and no flow data from any of these 

tributaries were available. Temperatures from Salmon Creek, 

therefore, were assumed to be similar to those from Salt Creek 

and the NFMF Willamette River, and the combined flow 

contribution of these tributaries was determined by subtracting 

the without-dam streamflow at the Hills Creek dam site from 

the without-dam streamflow at the Dexter dam site. When 

temperature data were available from Salt Creek and the 

NFMF Willamette River (June 7–September 30, 2001), their 

upstream drainage areas (114 and 248 mi2, respectively) were 

used as weighting factors in an energy balance computation 

after accounting for the downstream warming of the Salt 

Creek water from its measurement site to the confluence of the 

NFMF with the MF Willamette River (6.4 mi). The combined 

flow and temperature effects of these tributaries then were 

merged with the without-dam flow and temperature in the 

MF Willamette River, again accounting for any downstream 

warming that would occur from the Hills Creek dam site 

to the confluence of the NFMF and MF Willamette Rivers. 

Finally, downstream warming estimates from that location to 

the Dexter dam site (20.8 mi) were applied, with a maximum 

warming rate of 0.11 °C/mi and minimum and maximum 

thresholds of 10 and 16 °C as before. This calculation resulted 

in without-dam temperature estimates at the Dexter dam site 

for June 4–September 30, 2001. Far fewer data were available 

from these or similar sites in 2002; therefore, 2002 withoutdam temperatures at the Dexter dam site could not be derived 

from upstream temperature measurements.

To extend the without-dam temperature estimates for 

summer 2001 at the Dexter dam site to the rest of 2001–02, 

regressions of those estimates with measured temperatures 

at other sites were constructed. Daily mean and daily range 

regressions were tested using water-temperature data from 

the two long-term USGS sites used previously in this study: 

Blowout Creek in the North Santiam River basin and SF 

McKenzie River upstream of Cougar Reservoir. Both 

regressions provided relatively good models for predicting 

without-dam temperatures at the Dexter dam site, with 

correlation coefficients greater than 0.91 and 0.76 for the daily 

mean and daily range models, respectively. The SF McKenzie 

River regression, however, produced temperature estimates 

that were biased slightly high in spring and early summer and 

slightly low in late summer. In contrast, the Blowout Creek 

regression produced temperature estimates that were too high 
Thermal Effects of Dams in the Willamette River Basin 27

in late summer and early autumn. Mean absolute errors were 

0.92 and 1.03 °C for the SF McKenzie River and Blowout 

Creek regressions, respectively. Averaging the results from 

the two regressions eliminated some of the bias from each 

and reduced the MAE to 0.59 °C. Therefore, the average of 

the two regressions was used to estimate without-dam water 

temperatures at the Dexter dam site for January 1–June 3 and 

October 1–December 31, 2001 and for the entire year in 2002. 

The final combined regression equation is:

2 (0.080762 0.63471

1.7792( ) 0.13038 1.1733

1.1941( )) / 2,

where

is the without-dam temperature estimate at

 the Dexter dam site, 

and are the measured half-hourly te

DX mSF mSF

SF mSF mBC

BC mBC

DX

SF BC

T T T

T T T

T T

T

T T

= +

+ − + +

+ −

mperatures 

at the SF McKenzie River and Blowout

Creek sites, and

and are the daily mean temperatures at those

two sites, respectively.

T T mSF mBC

(11)

This regression indicates that the daily range in without-dam 

temperature at the Dexter dam site is about 78 percent larger 

than the measured range at the SF McKenzie River site, and 

about 19 percent larger than the measured range at Blowout 

Creek.

For 2002, few measured water temperatures upstream of 

Lookout Point Lake were available, but some data from the 

MF Willamette River upstream of Hills Creek Lake were used 

to check the regression estimates at Dexter. The measured 

MF Willamette River temperatures upstream of Hills Creek 

Lake (LASAR site 30500) were compared to regression 

results at the Dexter dam site after applying the same type of 

downstream warming to the measured temperatures, using 

a maximum warming rate of 0.11 °C/mi over 40.4 mi. The 

comparison was favorable, with relatively good agreement 

for the daily maximum temperature in mid-summer and good 

agreement in general for April through June. The estimated 

without-dam temperatures are compared to the measured 

with‐dam temperatures at the Dexter dam site in figure 9B.

The patterns for with- and without-dam temperatures 

at the Dexter dam site are somewhat similar to those from 

the Cottage Grove dam site. Annual maximum without-dam 

7dADM temperatures at Dexter are approximately 23.5 °C, 

similar to those at Cottage Grove, whereas the with-dam 

annual maximum 7dADM temperatures at Dexter ranged 

from 18.8 to 20.4 °C (table 4). The shift in annual maximum 

temperature from July or August to September as a result of 

the dams is evident at Dexter just as it is at most of the other 

dam sites. Without-dam temperature estimates are warmer 

at Dexter than at the Blue River, Cougar, or Big Cliff dam 

sites because of the lower elevation of Dexter and the greater 

distance downstream from its cold High Cascade headwaters.

The greatest uncertainties in the without-dam temperature 

estimates at the Dexter dam site probably result from the 

downstream warming rate that was applied, despite the fact 

that the maximum warming rate applied in this study was 

an exact match to the warming gradient measured in the MF 

Willamette River in July–September of 1951–53 (Moore, 

1967) prior to the completion of Lookout Point, Dexter, and 

Hills Creek Dams. Given the large distance over which the 

warming rate was applied (20.8 mi from the North Fork 

confluence, as far as 42 mi from the MF Willamette River site 

upstream of Hills Creek Lake), a small error in the warming 

rate might produce a significant error in the estimated withoutdam temperature at Dexter. An error of 0.03 °C/mi, for 

example, would result in a predictive error of 0.6–1.1 °C at 

Dexter during mid-summer, which may be sufficient reason 

to justify the construction of river models to provide a better 

prediction of the warming rate. Still, applying measurement 

error, regression error, and potential errors in the estimated 

downstream warming rate, the uncertainty in the without-dam 

temperatures at Dexter are likely to be no more than 1.5 °C 

most of the time (table 5).
28 Thermal Effects of Dams in the Willamette River Basin, Oregon

Dorena Dam

Dorena Dam is located on the Row River near Cottage 

Grove, Oregon. Completed in 1949, the dam impounds a 

relatively small reservoir compared to most of the other 

USACE dams in the Willamette River basin, but still more 

than twice the storage of nearby Cottage Grove Lake. The 

upstream drainage area is entirely within the Western Cascades 

physiographic area, but the dam site is at an elevation of only 

about 800 ft above sea level, which is only slightly higher than 

the elevation of the Cottage Grove dam site but about 400 ft 

lower than the Blue River dam site.

Measured water temperatures were available for the 

Row River upstream of Dorena Lake (LASAR site 28613) for 

August 2–September 26, 2001, and April 21–September 9, 

2002. A maximum downstream warming rate of 0.11 °C/mi 

was applied to account for the heating that normally would 

occur as the water moved from the temperature-measurement 

site downstream 6 mi to the Dorena dam site.

To help estimate without-dam water temperatures for 

the rest of 2001 and 2002, nearby measurements of water 

temperature were examined. For June 21–October 3, 2001, 

measured temperatures from Mosby Creek downstream of 

Row River Trail (LASAR site 28103) were obtained from 

ODEQ. The Mosby Creek temperatures appeared to have 

a larger daily variation than the Row River temperatures. 

Therefore, the daily means and daily ranges of these two 

datasets were correlated separately, and the resulting 

regression was obtained:

1.8147 0.97575

( )(0.14626 0.58866 ) / ,

where

is the estimated without-dam Row River

temperature, 

is the measured daily mean temperature of

Mosby Creek,

is the half-hourly te

RR mMC

MC mMC rMC rMC

RR

mMC

MC

T T

T T T T

T

T

T

= +

+ − +

mperature of Mosby

Creek, and

is the daily temperature range at the Mosby

Creek site.

TrMC

(12) 

Correlation coefficients for the daily mean and daily range 

regressions were 0.989 and 0.747, respectively. This 

regression was used to estimate Row River without-dam 

temperatures for June 21–August 2 and September 26–

October 3 of 2001, and it produced a good match to both the 

mean and range of the measured Row River temperatures.

Extending the without-dam predictions to winter, 

spring, and autumn, a correlation with SF McKenzie River 

temperature data upstream of Cougar Reservoir for 2001 

was constructed, also using separate regressions for the daily 

mean and daily range. The correlation produced the following 

equation:

1.2996 1.9892

( )(0.29507 0.87489 ) / .

RR mSF

SF mSF rSF rSF

T T

T T T T

= − +

+ − + (13)

where the variables are similar to those from the previous 

equation, except the SF subscript refers to data from the SF 

McKenzie River site. Again, the regression provided a good 

fit (correlation coefficients of 0.945 and 0.852 for the daily 

means and daily ranges, respectively), and this equation was 

used to estimate without-dam Row River temperatures for 

January 1–June 21 and October 3–December 31, 2001.

For 2002, measured water temperatures were available 

from nearby Mosby Creek at Laying Road (LASAR site 

30638) for June 20–November 19. A regression of the Mosby 

Creek data against the without-dam Row River temperatures 

gave the following result:

2.095 0.97023 , 0.935. TRR = + T R MC = (14)

This regression provided a good fit to the available withoutdam temperatures in 2002 and was used to extend the 

estimates from September 9 to November 19. To extend the 

estimates to the rest of 2002, a correlation with SF McKenzie 

River temperatures upstream of Cougar Reservoir again was 

constructed. This time, however, only the 2002 without‐dam 

estimates from April 21 to June 9 were used; using all 

available 2002 without-dam Row River temperatures did 

not produce as good a correlation. The resulting regression 

equation is:

1.37 1.8196 ( ), 0.895. TRR = − + + − = T T T R mSF SF mSF (15)

This regression was used to estimate without-dam Row 

River temperatures in 2002 for January 1–April 21 and 

November 19–December 31.

The final compilation of without-dam temperature 

estimates at the Dorena dam site shows patterns and extremes 

that are similar to those at the nearby Cottage Grove dam 

site (fig. 9C). The annual maximum without-dam 7dADM 

temperatures at Dorena are the warmest of all dam sites 

included in this study, exceeding 25 °C. Cooler temperatures 

occur with the dam in place, despite the lack of a natural 

seasonal pattern. Although these without-dam temperature 

estimates include the warmest temperatures estimated in 

this study, they are based on measured upstream and nearby 

temperatures, which decreases the inherent uncertainty of 

the estimates. Uncertainties associated with the withoutdam temperatures at the Dorena dam site are likely to be 

no more than 0.5 °C in mid-summer (table 5), based on a 

typical measurement error of 0.2 °C and a maximum warming 

estimation error of 0.3 °C (6 mi times a potential error of 

0.05 °C/mi).
Thermal Effects of Dams in the Willamette River Basin 29

Fall Creek Dam

Fall Creek Dam impounds a reservoir on Fall Creek 

about 7 mi from its confluence with the MF Willamette River, 

and that confluence is about 5.5 mi downstream of Dexter 

Dam (fig. 1). The dam site elevation is less than 700 ft above 

sea level, and the upstream drainage area is located entirely 

within the Western Cascades physiographic area. Fall Creek 

Lake captures the inputs from Fall Creek as well as Winberry 

Creek, a major tributary that joins Fall Creek from the south 

and close to the dam site.

Without-dam water temperatures at the Fall Creek 

dam site were estimated first using measured temperatures 

upstream of Fall Creek Lake for the period of interest. 

Measured water temperatures were available from sites 

upstream of Fall Creek Lake on both Winberry Creek 

(LASAR site 28013) and Fall Creek (LASAR site 27969) for 

mid-June through mid-October 2001. Temperature data were 

not available for these creeks during 2002. USGS maintains 

a streamflow gage on Winberry Creek (station 14150800). 

Using measured flows in Winberry Creek and the estimated 

without-dam streamflow at the Fall Creek dam site, estimates 

of streamflow in Fall Creek were obtained by difference. 

Measured temperatures in Winberry Creek (7.7 mi upstream 

of the dam site) and Fall Creek (11.2 mi upstream of the 

dam site) were adjusted to account for a typical amount of 

downstream heating that would occur during warm summer 

periods, using techniques previously described in the section, 

“Methods and Models.”

To extend the without-dam temperature estimates to the 

rest of the 2001–02 period, a correlation of these estimated 

temperatures to measured temperatures at a site with similar 

characteristics was constructed. The only nearby unregulated 

stream site with a complete temperature dataset is the SF 

McKenzie River upstream of Cougar Reservoir; however, 

that river derives a substantial part of its flow from the 

snowmelt and spring-fed High Cascades, which does not 

match the characteristics of the Fall Creek and Winberry 

Creek drainages. A dataset from a site farther away, then, but 

from a stream whose drainage area is entirely in the Western 

Cascades, might be better suited for this correlation. To test 

this hypothesis, the Fall Creek without-dam temperature 

estimates were correlated to measured water temperatures 

from the SF McKenzie River upstream of Cougar Reservoir 

(USGS station 14159200) and from Blowout Creek near 

Detroit (USGS station 14180300). Both tests correlated the 

daily means and daily ranges separately. The Blowout Creek 

regression produced better predictions (MAE of 0.71 °C 

versus 0.93 °C for the SF McKenzie River regression), 

particularly early in the summer. The following equation from 

the Blowout Creek regression, therefore, was used:

2 0.0064233 1.04106

( )(0.11331 0.55893 ) / ,

where

is the estimated without-dam temperature at

the Fall Creek dam site, 

is the measured daily mean temperature at the

Blowout

FC mBC mBC

BC mBC rBC rBC

FC

mBC

T T T

T T T T

T

T

= +

+ − +

 Creek site,

is the half-hourly temperature from Blowout

Creek, and

is the daily temperature range from the Blowout

Creek site.

BC

rBC

T

T

(16)

Correlation coefficients for the daily mean and daily range 

regressions were 0.962 and 0.800, respectively. This 

regression was used to estimate Fall Creek without‐dam 

temperatures for January 1–June 12 and October 17–

December 31 of 2001 and for the entire year of 2002. A 

comparison of the estimated 2002 temperatures with data from 

nearby Little Fall Creek (LASAR site 28092), an unregulated 

stream in the adjacent drainage to the north, demonstrated that 

the without-dam temperature estimates for Fall Creek were 

representative, at least for the July–August period when data 

from Little Fall Creek were available.

Without-dam water temperature estimates for the Fall 

Creek dam site (fig. 10A) compare favorably with those from 

the Blue River dam site, another site with a drainage area 

entirely within the Western Cascades. The annual maximum 

without-dam 7dADM temperatures for these two sites are 

nearly identical, with those from Fall Creek slightly warmer 

at about 21.5 °C (table 4). The without-dam temperatures 

have the same seasonal pattern as those from other sites with 

maximum temperatures occurring in July or August, whereas 

the with-dam temperatures are affected by dam operations and 

the choice of release point. It is obvious, for example, that the 

release point from Fall Creek Dam was changed to a point 

deeper in the reservoir on September 3, 2002, because the 

downstream temperature decreased abruptly at that time.

Errors in the without-dam temperature estimates at the 

Fall Creek dam site are derived from measurement error, 

errors in the application of the estimated downstream warming 

rate, and regression prediction errors. The downstream 

warming was applied over a relatively short distance (7.7 mi 

on Winberry Creek and 11.2 mi on Fall Creek), so an error of 

as much as 0.05 °C/mi would result in an error no larger than 

0.6 °C. Measurement errors typically are no larger than 0.2 °C, 

and the mean absolute error associated with the Blowout 

Creek regression was 0.7 °C. Uncertainties associated with the 

without-dam temperatures, therefore, likely are no larger than 

0.8 to 1.5 °C (table 5).
30 Thermal Effects of Dams in the Willamette River Basin, Oregon

Figure 10. Measured and estimated water temperatures for conditions with and without upstream dams 

at the Fall Creek, Fern Ridge, and Foster dam sites in the Willamette River basin, Oregon, 2001–02. Estimated 

with-dam temperatures are from Annear and others (2004a) at Portland State University (PSU) for Fall Creek 

and Fern Ridge Dams or from correlations by the Oregon Department of Environmental Quality (ODEQ) for Foster 

Dam. Measured temperatures are from USGS stations 14151000, 14169000, and 14187200 for graphs A, B, and C, 

respectively. ortac10-0490_fig10

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Estimated, with dams (PSU, ODEQ)

Estimated, without dams

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WATER TEMPERATURE, IN DEGREES CELSIUS

2001 2002

C. Foster Dam

B. Fern Ridge Dam

A. Fall Creek Dam
Thermal Effects of Dams in the Willamette River Basin 31

Fern Ridge Dam

Fern Ridge Dam is located on the Long Tom River northwest 

of Eugene, Oregon. At only 44 ft high, it is the shortest dam 

included in this study, but it impounds almost as much water as Fall 

Creek Dam, which is four times taller. Upstream of the dam is Fern 

Ridge Lake, a large but shallow reservoir that has high value for 

wildlife and recreation in addition to providing flood control and 

irrigation water for downstream populations. The dam and lake are 

situated in the Willamette Lowlands physiographic area, with an 

upstream drainage area that extends into the Coast Range (fig. 2).

Few measured temperature data were available upstream of 

Fern Ridge Lake for 2001–02. Data were available from Coyote 

Creek, a tributary to the lake, and from a site on the Long Tom 

River 28.3 mi upstream of the dam. The Coyote Creek temperatures, 

however, appeared to be too warm to be representative of Long 

Tom River temperatures upstream of Fern Ridge Lake because 

mid‐summer Coyote Creek temperatures were as warm as 

or warmer than measured temperatures downstream of Fern 

Ridge Dam. Similarly, the available upstream Long Tom River 

data were from a site that was far upstream and in a different 

physiographic province (Coast Range), characteristics making 

those data less representative of temperatures that would occur in 

the valley bottom. An alternate method of estimating without-dam 

temperatures at the Fern Ridge dam site was needed.

Temperature measurements upstream and downstream of 

Fern Ridge Lake from 1946 to 1962 were compiled and analyzed 

by Moore (1964, 1967) to assess the thermal effects of Fern Ridge 

Dam. The analysis was based on spot measurements, and the 

aggregated monthly results illustrate the heating effects of the lake 

in spring through autumn and the cooling effects in winter (fig. 11). 

A nonlinear equation was fitted to those data as an estimate of the 

heating or cooling effect of the lake as a function of the day of year. 

Using measured temperatures just downstream of the dam, 

those heating or cooling effects of the lake could be removed 

by subtracting the fitted function in figure 11 to estimate water 

temperatures that might have occurred upstream of Fern Ridge 

Lake. Measured temperatures near the dam were available for 

all of 2002 and starting August 2001 from the USGS station 

at Alvadore (14169000). Temperatures for June and July 2001 

were estimated for that site by PSU (Annear and others, 2004a).

After applying the reservoir-heating correction to estimate 

water temperatures upstream of Fern Ridge Lake, those results 

were used to estimate without-dam temperatures at the Fern 

Ridge dam site. A maximum downstream warming rate of 

0.11 °C/mi was applied over the 11.9 mi from the upstream 

site to the dam site for those times when warming was likely to 

occur.

With-dam annual maximum 7dADM temperatures at the 

Fern Ridge dam site are warmer than without-dam counterparts 

during 2001 and 2002, demonstrating that Fern Ridge Lake 

typically warms the Long Tom River during summer (table 4, 

fig. 10B). The seasonal temperature patterns for with- and 

without-dam conditions are similar, which is consistent with 

Fern Ridge Dam not being tall enough to keep and release 

any significant quantity of cold water at the bottom of Fern 

Ridge Lake. Errors inherent to the monthly heating and cooling 

pattern in figure 11 are on the order of 0.8 °C. Uncertainties in 

the without-dam temperature estimates for this site, however, 

probably are larger than 0.8 °C because the process used to 

estimate temperatures upstream of the lake retains the daily 

temperature variations measured downstream of the dam, which 

are smaller than those that might occur in the absence of the 

dam. This method, therefore, underestimates the without-dam 

daily temperature range in mid-summer, resulting in errors that 

are likely to be about 1.0 °C and at times perhaps as large as 

2.0 °C (table 5).

JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC

MONTH

-2

-1

0

1

2

3

4

5

Spot measurements

Alvadore minus Noti

Fitted function

RESERVOIR HEATING EFFECT, IN DEGREES CELSIUS

Figure 11. Measured heating effect of Fern Ridge Lake in the Willamette River basin, based on temperature 

measurements upstream and downstream of the lake. The first of two available datasets was documented by Moore 

(1967); the second was derived from measurements during 1946-62 at the USGS stations at Noti (upstream, 14166500) and 

at Alvadore (downstream, 14169000) (Moore, 1964).
32 Thermal Effects of Dams in the Willamette River Basin, Oregon

Foster Dam

Foster Dam is located on the South Santiam River near 

Sweet Home, Oregon, southeast of Albany. Foster Dam serves 

partially as a re-regulating dam downstream of the much larger 

Green Peter Dam on the Middle Santiam River. Foster Lake 

is a popular recreational area during summer. The upstream 

drainage area for both Foster and Green Peter Lakes lies 

entirely within the Western Cascades physiographic region.

Estimation of without-dam water temperatures at the 

Foster dam site relied on methods similar to those used 

for the Dexter dam site. Foster and Dexter Dams have 

other dams upstream, so the estimation process began 

upstream of the most-upstream reservoir. In 2002, hourly 

water-temperature data upstream of Green Peter Lake were 

available from Quartzville Creek (LASAR site 10791) and 

the Middle Santiam River (LASAR site 23805), which are 

the two major inflows to the lake. Downstream of Green 

Peter Dam, the other major inflow to Foster Lake is the 

South Santiam River. Water-temperature data from the South 

Santiam River upstream of Foster Lake were available in 

2002 from ODEQ (LASAR site 30514). Flow in the South 

Santiam River upstream of Foster Lake was estimated as 

the difference between the without-dam streamflows at the 

Foster and Green Peter dam sites, and flow in the Middle 

Santiam River upstream of Green Peter Lake was estimated 

as the without-dam flow at Green Peter minus the measured 

flow in Quartzville Creek at USGS station 14185900. A 

flow-weighted energy balance was used to combine these 

three temperature datasets, but only after accounting for the 

warming that would occur as water from each of these sites 

traveled downstream to the Foster dam site. The Quartzville 

Creek, Middle Santiam, and South Santiam temperature 

measurements sites were 17.6, 19.2, and 9.0 mi upstream of 

Foster Dam, respectively. A maximum downstream warming 

rate of 0.11 °C/mi was applied, with minimum and maximum 

thresholds of 10 and 16 °C as applied previously for other 

sites. This method resulted in without-dam water temperatures 

at the Foster dam site for April 23–September 11, 2002. The 

datasets for Quartzville Creek and the South Santiam River for 

this time period had sizeable gaps. When data for Quartzville 

Creek were absent, the Middle Santiam River data were the 

only measurements of temperature upstream of Green Peter 

Lake. Similarly, when South Santiam River temperatures 

were absent, temperature estimates at the Foster dam site were 

derived only from data upstream of Green Peter Dam.

For 2001, a similar approach was used, but with data 

from slightly different sites. The measured temperatures in the 

Middle Santiam River upstream of Green Peter Lake in 2001 

were rated as poor by ODEQ and therefore were not used. A 

comparison of the 2002 temperatures from Quartzville Creek 

and the Middle Santiam River showed great similarity, both in 

their daily means and daily ranges. Using only the Quartzville 

Creek data to represent temperatures upstream of Green Peter 

Lake, therefore, was justified. The temperature measurement 

site on the South Santiam River was at a different location in 

2001 (LASAR site 28615, 5.1 mi upstream of Foster Dam). 

Applying a flow-weighted energy balance and an appropriate 

level of downstream warming as described previously 

(maximum rate of 0.11 °C/mi, etc.), without-dam temperature 

estimates were obtained for August 3–September 28, 2001.

The without-dam temperature estimates at the Foster dam 

site were extended to the rest of 2001–02 using correlations 

to measured water temperatures at the Blowout Creek site in 

the North Santiam River basin (USGS station 14180300). The 

Blowout Creek site was a good choice because its drainage 

area and the drainage area upstream of Foster Dam are both 

entirely within the Western Cascades. Separate regressions 

were made for the 2001 and 2002 datasets, and the results 

were similar, demonstrating that the input datasets for the 

two time periods showed consistent patterns and magnitudes. 

A combined regression could have been constructed, but the 

results would not have been significantly different. Separate 

regressions were made for the daily means and daily ranges, 

and the following equations were determined:

2 0.0063134 1.1714

0.72656( ), for 2001,

F mBC mBC

BC mBC

T T T

T T

= +

+ − (17)

2 0.0036136 1.2431

0.79928( ), for 2002,

where

is the without-dam temperature estimate at

the Foster dam site, 

is the measured daily mean temperature at the

Blowout Creek site, and

F mBC mBC

BC mBC

F

mBC

T T T

T T

T

T

T

= +

+ −

is the half-hourly temperature from Blowout

Creek.

BC

(18)

Correlation coefficients for the daily mean and daily range 

regressions were 0.977–0.997 and 0.872–0.881, respectively. 

If the datasets had been combined, it is clear that similar 

coefficients would have been derived, with the daily mean 

temperature at the Foster dam site approximately 20 percent 

higher than that at Blowout Creek, and a daily temperature 

range about 76 percent of the range at Blowout Creek.

The estimated without-dam temperatures at the Foster 

dam site are much warmer in mid-summer than the measured 

with-dam temperatures at Foster Dam (fig. 10C). The cooler 

measured temperatures are a direct result of the large volume 

of cold water released from Green Peter Dam upstream. 

The without-dam temperatures are similar to those derived 

for the Dorena, Dexter, and Cottage Grove dam sites, with 

annual maximum 7dADM temperatures of about 24.5 °C 

(table 4). These maxima are substantially warmer than their 

with-dam counterparts (about 13.2 °C). Uncertainties in the 

without-dam temperature estimates at the Foster dam site are 

derived from measurement error (0.2 °C), inaccuracies in the 

applied downstream warming rates (as much as 0.8 °C), and 

regression error (approximately 0.6 °C). An approximate total 

uncertainty, therefore, is likely to be in the range of 0.8–1.5 °C 

(table 5).
Thermal Effects of Dams in the Willamette River Basin 33

Green Peter Dam

Green Peter Dam impounds Green Peter Lake on the 

Middle Santiam River upstream of Foster Lake. The two 

major inputs are Quartzville Creek and the Middle Santiam 

River, with the former providing about 30–40 percent of the 

flow during summer. The entire upstream drainage area lies 

within the Western Cascades (fig. 2). Flow data are available 

from a USGS gaging station on Quartzville Creek upstream 

of the lake (station 14185900). ODEQ temperature data are 

available at or near that streamflow-gaging station (LASAR 

site 10791) during parts of 2001–02, and USGS temperature 

data are available at the gaging station beginning in 2008. 

Some temperature data also are available on the Middle 

Santiam River upstream of the lake (LASAR site 23805), but 

the data from 2001 were rated poor by ODEQ, and the 2002 

data were comparable to the Quartzville Creek temperatures.

Although the 2002 without-dam temperature estimates 

at the Foster dam site were derived from both the Quartzville 

Creek and Middle Santiam River temperatures upstream of 

Green Peter Lake, the similarity of the temperatures at these 

sites and the longer record at the Quartzville Creek site led to 

a decision to base the without-dam temperatures at the Green 

Peter dam site only on the Quartzville Creek data. Using those 

data (August 3–September 28, 2001, April 23–May 31, 2002, 

and July 12–September 11, 2002), a regression was developed 

with measured temperatures at the Blowout Creek site (USGS 

station 14180300), which is located in an adjacent drainage 

to the east. An excellent fit was derived, with a mean absolute 

error of only 0.44 °C (see equation 19).

Without-dam temperatures at the Green Peter dam 

site were estimated using the measured and correlated 

Quartzville Creek temperatures upstream of Green Peter 

Lake. A maximum downstream warming rate of 0.11 °C/mi 

was applied over the 10.2 mi distance from the temperature 

measurement site to the dam site (see section, “Methods 

and Models”). The resulting without-dam temperature 

estimates are plotted along with a limited dataset of measured 

temperatures at the Green Peter dam site in figure 12A. The 

without-dam temperatures at the Green Peter dam site are 

nearly identical to the without-dam temperature estimates 

at the Foster dam site, but slightly cooler. The without-dam 

temperatures at the Green Peter dam site fall neatly into 

a group with those from the Foster, Dexter, Dorena, and 

Cottage Grove dam sites, all of which are similar by having 

a substantial drainage area in the Western Cascades (table 4). 

Estimates for the Blue River dam site are slightly cooler, 

probably as a result of a higher elevation. Uncertainties in the 

Green Peter without-dam temperatures can be estimated on the 

basis of measurement error (0.2 °C), correlation error (0.4 °C), 

and error in the downstream warming rate (<0.5 °C), for a 

total range of approximately 0.5–1.0 °C (table 5).

2

0.2533 1.1681

( )(0.0423 0.1955 0.8695)

/ ,

where

is the estimated temperature for Quartzville

Creek upstream of Green Peter Lake, 

is the measured daily mean temperature 

QC mBC

BC mBC rBC rBC

rBC

QC

mBC

T T

T T T T

T

T

T

= +

+ − + +

at the

Blowout Creek site,

is the half-hourly temperature from Blowout 

 Creek, and

is the daily temperature range from the Blowout

Creek site.

BC

rBC

T

T

(19)

Correlation coefficients were 0.991 and 0.675 for the daily 

mean and daily range regressions, respectively.
34 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig12

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

Measured, with dams (USGS)

Measured, with dams (ODEQ)

Estimated, with dams (PSU)

Estimated, without dams

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

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

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

WATER TEMPERATURE, IN DEGREES CELSIUS

2001 2002

.

C. River Mill Dam

B. Hills Creek Dam

A.. Green Peter Dam

Figure 12. Measured and estimated water temperatures for conditions with and without upstream dams at the 

Green Peter, Hills Creek, and River Mill dam sites in the Willamette River basin, Oregon, 2001–02. Measured withdam temperatures are from the Oregon Department of Environmental Quality (ODEQ) for Green Peter and Hills 

Creek Dams and from USGS station 14210000 for River Mill Dam. Estimated with-dam temperatures for River Mill 

Dam are from Annear and others (2004a) at Portland State University (PSU).
Thermal Effects of Dams in the Willamette River Basin 35

Hills Creek Dam

Hills Creek Dam is located on the MF Willamette River 

near Oakridge, Oregon, and upstream of both Dexter and 

Lookout Point Dams. Major inflows to the lake are from 

Hills Creek and the MF Willamette River. The Hills Creek 

drainage area primarily is in the Western Cascades, but the 

MF Willamette River extends up into the High Cascades. 

As a result, summertime inflows to Hills Creek Lake are fed 

partially by the snowmelt and colder springs of the High 

Cascades. Combined with an elevation of about 1,250 ft above 

sea level, temperatures in the MF Willamette River at the 

Hills Creek dam site should be relatively cool in mid-summer 

compared to lower elevation sites and sites that do not derive 

flow from the High Cascades. 

Without-dam temperatures at the Hills Creek dam 

site were derived from measured temperatures upstream of 

Hills Creek Lake. Temperature data were available for both 

Hills Creek (LASAR site 27982) and the MF Willamette 

River (LASAR site 27986) upstream of the lake for 

June 5–September 30, 2001, but temperature data were not 

available for Hills Creek for 2002. Streamflow data were not 

available for either inflow, but an energy balance could be 

derived by weighting those temperatures with their upstream 

drainage areas (52.8 mi2 for Hills Creek, 258 mi2 for the MF 

Willamette River) after accounting for the warming that was 

likely to occur from those sites to the dam site. A maximum 

downstream warming rate of 0.11 °C /mi was applied over a 

distance of 4.2 mi from the Hills Creek site to the dam, and 

8.5 mi on the MF Willamette River.

To extend these temperature estimates to the entire 

2001–02 time period, the without-dam data were correlated 

against measured temperatures in the SF McKenzie River 

above Cougar Reservoir (USGS station 14159200). A 

correlation to Blowout Creek data also was constructed, but 

the correlation to the SF McKenzie River data was better, with 

a mean absolute error of 0.51 °C as opposed to 0.85 °C. This 

result is not surprising, as SF McKenzie River data are more 

representative of the magnitude and patterns of temperature 

in a High Cascades stream, whereas Blowout Creek data are 

more representative of temperatures in a Western Cascades 

stream. Separate regressions were constructed for the daily 

means and daily ranges, resulting in the following equation:

2 0.0334 0.9345

( )(1.1604 1.2812) / ,

where

is the estimated without-dam temperature

at the Hills Creek dam site, 

is the measured daily mean temperature at the

SF McKenzie 

HC mSF mSF

SF mSF rSF rSF

HC

mSF

T T T

T T T T

T

T

= +

+ − +

River site,

is the half-hourly temperature from the 

SF McKenzie River site, and

is the daily temperature range from the 

SF McKenzie River site.

SF

rSF

T

T

(20)

Correlation coefficients for the daily mean and daily range 

regressions were 0.931 and 0.889, respectively.

The without-dam temperature estimates at the Hills 

Creek dam site have a similar seasonal pattern but are 

several degrees cooler in mid-summer than the withoutdam temperature estimates at the Dexter dam site (fig. 12B, 

fig.9B). Annual maximum without-dam 7dADM temperatures 

at the Hills Creek dam site are less than 20 °C, cooler than 

all dam sites that have substantial drainage areas in the 

Western Cascades (Blue River, Cottage Grove, Dexter, 

Dorena, Fall Creek, Foster, Green Peter), but warmer than 

dam sites that have a substantial fraction of their drainage in 

the High Cascades (Big Cliff and Cougar). Uncertainties in 

the without‐dam temperatures at the Hills Creek dam site are 

relatively small because errors in the downstream warming 

rate are accumulated over a short distance (maximum 

8.5 mi, perhaps as large as 0.4 °C), and the correlation to SF 

McKenzie River temperatures was good (MAE 0.5 °C). A 

typical error in the without-dam estimates then should be no 

more than 0.5–1.0 °C (table 5).
36 Thermal Effects of Dams in the Willamette River Basin, Oregon

River Mill Dam

River Mill Dam is located on the Clackamas River near 

Estacada, Oregon, and is the most downstream of a series 

of hydropower projects owned and operated by Portland 

General Electric in the Clackamas River basin. Like the 

North Santiam and McKenzie Rivers, the Clackamas River 

derives some of its flow from the High Cascades but also 

has a substantial drainage area in the Western Cascades 

(fig. 2). The hydropower projects in the Clackamas River 

system recently underwent a relicensing process overseen 

by the Federal Energy Regulatory Commission (FERC). As 

part of that process and a related water-quality certification 

process through ODEQ, models of the Clackamas River were 

constructed to determine the flow and temperature effects 

of the hydropower projects. One of the model scenarios was 

a “no-project” scenario to simulate the river without the 

hydropower dams and diversions (Oregon Department of 

Environmental Quality, 2006b).

No-project temperature predictions from the Clackamas 

River model at the River Mill dam site from April 9, 2000, to 

September 30, 2001, were used as obtained from ODEQ to 

represent without-dam temperatures at that location. To cover 

the rest of 2001 and all of 2002, the no-project temperatures 

were correlated with measured temperatures at nearby sites. 

Five different correlation sites were tested using the 2001 

no-project estimates including all three major inflows to 

Detroit Lake and a site on the Little North Santiam River in 

the adjacent North Santiam River basin to the south. The daily 

means and daily ranges were correlated separately to reduce 

timing (phase) errors. All tested regressions captured most of 

the seasonal pattern in the no-project temperatures, but the 

regression with North Santiam River temperatures upstream 

of Detroit Lake (USGS station 14178000) produced the best 

fit (MAE 0.61 °C). Expanding the dataset by adding the 2000 

data did not improve the correlation.

The seasonal pattern in the daily temperature range 

from the North Santiam River site was not the best match for 

the seasonal pattern in the daily temperature range from the 

Clackamas River dataset. The North Santiam River regression 

produced a smaller daily range in late summer and early 

autumn than was evident in the Clackamas River dataset. A 

correlation with daily ranges from the Blowout Creek site 

provided a better match. The final regression, therefore, relied 

on daily mean temperatures from the North Santiam River 

upstream of Detroit Lake and daily temperature ranges from 

Blowout Creek, illustrating the wide variety of regression 

models that might be constructed. The final equation was:

0.06208 1.1146 0.85051( ),

where

is the estimated without-dam temperature

at the River Mill dam site, 

is the measured daily mean temperature in the

North Santiam River upstream of D

RM mNS BC mBC

RM

mNS

T T T T

T

T

= − + + −

etroit

Lake,

is the half-hourly temperature from the 

Blowout Creek site, and

is the measured daily mean temperature from 

the Blowout Creek site.

BC

mBC

T

T

(21) 

Correlation coefficients were 0.993 and 0.902 for the daily 

mean and daily range regressions, respectively. This regression 

model produced an MAE of 0.55 °C, and was used to estimate 

without-dam temperatures at the River Mill dam site for 

September 30, 2001–December 31, 2002.

A comparison of the modeled and estimated without-dam 

and measured with-dam temperatures at or near the River Mill 

dam site shows a great similarity in the two datasets (fig. 12C). 

The seasonal patterns are nearly identical, and the annual 

maximum 7dADM temperatures are roughly comparable 

(table 4), but the measured temperatures downstream of 

River Mill Dam have a much smaller daily variation than 

the without-dam temperature estimates. The similar seasonal 

patterns probably are the result of the fact that the hydropower 

projects on the Clackamas River are largely run-of-the-river; 

they have limited storage capacity and therefore cannot store a 

large amount of heat absorbed in mid-summer for release later 

in the year. Most of the dams in the Clackamas River system, 

with the possible exception of North Fork Dam, also are not 

so tall that they generally release water from a hypolimnion 

that is colder than the surface water. The presence of the dams, 

however, may account for the lack of daily variation in the 

with-dams temperatures.

Uncertainties in the without-dam Clackamas River 

temperatures at the River Mill dam site derive from model 

error and correlation error. Although the correlation error was 

calculated to be approximately 0.6 °C, the model error for the 

no-project scenario is somewhat unknown. It is likely that the 

model as calibrated to existing conditions had a typical error 

on the order of 0.5 to 1.0 °C, but the no-project scenario error 

is subject to additional uncertainty inherent to assumptions 

used to construct that model scenario. Uncertainties in these 

without-dam temperature estimates, therefore, are likely to be 

at least 1.0–1.6 °C (table 5).
Thermal Effects of Dams in the Willamette River Basin 37

Downstream Effects of Willamette Basin Dams

To determine the effects of Willamette River basin dams 

on streamflow and water temperature downstream of those 

projects, the suite of CE-QUAL-W2 models used as the basis 

of the Willamette River temperature TMDL were run for 

scenarios with and without the dams in place. These models 

simulate the entire Willamette River and its major tributaries 

up to the first major dam on each tributary (see fig. 1). Both 

scenarios were based on the NTP model runs from the TMDL, 

which specified that riparian vegetation was restored to a 

more natural state, instream hydropower projects and power 

diversions were eliminated (such as those along the McKenzie 

River and at Willamette Falls on the Willamette River), and 

point sources were removed. Water withdrawals, upstream 

dams, and a modern channel shape were retained in the NTP 

model scenario. To create the without-dams scenario, the 

upstream flow and temperature boundary conditions were 

simply switched from the with-dam datasets to without-dam 

datasets, leaving the rest of the models unchanged as much as 

possible.

Other than the upstream boundary conditions that were 

used to specify with- or without-dam conditions, several other 

modifications to the TMDL models were necessary to allow 

them to run under a no-dams scenario. For example, water 

withdrawals from the South Santiam River downstream of 

Foster Dam during mid-summer can exceed the streamflow 

that would occur in a no-dams scenario. To keep the river from 

going dry and enabling the model to run and remain stable 

in a no-dams scenario, those withdrawals had to be reduced. 

All withdrawals were eliminated if the flow at the Foster dam 

site was less than 88 ft3/s (2.5 m3/s), and withdrawals were 

gradually increased to their full amount when flows at Foster 

were equal to or greater than 177 ft3/s (5.0 m3/s). In addition, 

the total withdrawal rate was constrained to be less than 

90 percent of the 5-day minimum flow at Foster.

For the Coast Fork / Middle Fork Willamette River 

model, modifications were needed for some of the ungaged 

tributary inputs that were used in the model calibration 

to adjust river flows to match measured flows. These 

water-balancing flows had been used in part to eliminate 

inconsistencies between measured upstream reservoir releases 

and downstream flow measurements. For the no-dams 

scenario, such adjustments were no longer entirely applicable, 

and their use in losing reaches caused the model to either dry 

up or become unstable. These flow adjustments as used in the 

calibrated model also were far too frequent (half-hourly) and 

contained many abrupt changes (spikes) in flow. To resolve 

these problems, the inputs for these flow-correcting features 

(model tributaries 2–11 and the distributed tributary in model 

branch 10) were smoothed to remove short-term spikes, and 

maximum losses to groundwater were limited late in the 

summer. These changes allowed the Coast Fork / Middle Fork 

Willamette River model to run under a no-dams scenario, but 

the resulting simulated flows in these reaches are far more 

uncertain than those in other parts of the model domain.

Downstream Flows

The presence of large flood-control and storage dams 

typically reduces annual peak flows and increases annual 

minimum flows downstream of those dams. This effect 

is easily discernible in the Willamette River through an 

examination of long-term measurements at the USGS 

streamflow-gaging stations at Albany (14174000) and at 

Salem (14191000), where historical records go back to 1892 

and 1909, respectively (fig. 13). Most of the major dams in 

the Willamette River basin were constructed from 1941 to 

1969 (table 1). That transition period shows a decrease in the 

magnitude of annual peak flows as well as a distinct increase 

in annual minimum flows from approximately 2,500 to 

4,000 ft3/s at Albany, and from 3,000 to 6,000 ft3/s at Salem.

Simulated streamflows for the with- and without-dam 

model scenarios illustrate the expected flow-augmentation 

effects of the dams in mid- to late-summer. Streamflow in 

the Willamette River at Albany decreases to a minimum 

near 2,000 ft3/s without the dams, whereas the with-dam 

scenario results in streamflows greater than 4,000 ft3/s or even 

5,000 ft3/s for most of July–October (fig. 14). A similar effect 

occurs at Salem, where the without-dam streamflow decreases 

to a range near 2,500–3,000 ft3/s, yet the with-dam streamflow 

maintains a minimum greater than 6,000 ft3/s. The operation 

of upstream dams for flood control and downstream navigation 

results in lower annual peak flows and higher minimum 

summer flows in the Willamette River. With the wealth of 

streamflow data going back to the 19th century at Willamette 

River gaging stations, a model was not necessary to verify 

these effects. Streamflow, however, affects travel times 

through the river system and can have an important effect on 

water temperature.
38 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig13

1,000

5,000

10,000

50,000

100,000

300,000

1890189418981902190619101914191819221926193019341938194219461950195419581962196619701974197819821986199019941998200220062010

YEAR

1,000

5,000

10,000

50,000

100,000

300,000

DAILY MEAN STREAMFLOW, IN CUBIC FEET PER SECOND

.

B. Willamette River at Salem (14191000)

A. Willamette River at Albany (14174000)

Figure 13. Measured daily mean streamflow at USGS gaging stations on the Willamette River at Albany 

(14174000, data starting 1892) and at Salem (14191000, data starting 1909). Streamflows during 2001–02 are 

highlighted in blue. The largest upstream dams were constructed in the period 1941–1969 (shaded in gray).
Thermal Effects of Dams in the Willamette River Basin 39

Downstream Temperatures

Upstream dams affect downstream temperatures through 

changes in the release temperatures at the dam sites as well 

as through changes in downstream flows. The presence or 

absence of upstream reservoirs affects downstream flows 

not only through reservoir releases but also as a result of 

changes to the regional groundwater system (Constantz 

and Essaid, 2007). Decreased flows in late summer in the 

absence of upstream dams results in longer travel times, 

allowing more time for river temperatures to respond to the 

influences of weather, water withdrawals, groundwater inputs, 

riparian shading, and point-source heating or cooling effects. 

Decreased flows also mean that less water is in the river, 

providing less thermal mass to resist changes in temperature 

and potentially altering the surface area to volume ratio, an 

important factor in determining the heat exchange flux per 

unit mass; a larger ratio enhances temperature changes due 

to surface heat-exchange processes. Because streamflowmodification effects on water temperature take time to occur, 

the effects are manifested well downstream of the dam sites. 

In contrast, alterations to stream temperature at a dam site 

provides an immediate heating or cooling effect that tends to 

dissipate with distance downstream as the river has time to 

exchange heat with its surroundings. These near- and far-field 

effects were explored and quantified by Risley and others 

(2010) for an idealized river system under several climate 

conditions, with and without upstream reservoirs and with 

varying groundwater inputs or losses, demonstrating that 

the far-field streamflow effects are just as important as the 

near-field release temperatures in determining the downstream 

thermal effects of dams.

For this study, modeled river temperatures for the withand without-dam scenarios were assessed using the 7dADM 

temperature, which is specified by Oregon’s maximum 

water-temperature standard. The 7dADM temperature was 

calculated at every location in the model domain and for 

every day that was modeled in the June–October 2001 and 

April–October 2002 time period. The downstream temperature 

effects of the dams then were determined by subtracting the 

without-dams 7dADM temperatures from the with-dams 

7dADM temperatures. The results are conveniently visualized 

by plotting the 7dADM temperature differences with a time/

distance color map as in figure 15 for the Willamette River. 

Similar plots for most of the other modeled reaches are 

included in appendix B.

ortac10-0490_fig14

APR MAY JUNE JULY AUG SEPT OCT

2002

1,000

2,000

5,000

10,000

20,000

50,000

100,000

With dams

Without dams

Willamette River

at Albany

STREAMFLOW, IN CUBIC FEET PER SECOND

Willamette River

at Salem

Figure 14. Simulated streamflow in the Willamette River at Albany and at Salem for conditions with and 

without upstream dams in the Willamette River basin, Oregon, April–October 2002.
40 Thermal Effects of Dams in the Willamette River Basin, Oregon

Figure 15. Modeled downstream thermal effects of dams on the entire Willamette River, Oregon, June–

October 2001 and April–October 2002. Temperature changes in this figure are the difference in the modeled 

7dADM temperatures for Natural Thermal Potential conditions with upstream dams minus the modeled 7dADM 

temperatures without upstream dams. The McKenzie, Long Tom, Santiam, and Clackamas Rivers join the Willamette 

River at river miles 174.9, 149.4, 108.5, and 24.9, respectively.
Thermal Effects of Dams in the Willamette River Basin 41

Several important results and patterns can be derived 

from the color map showing the downstream temperature 

effects of the dams on the Willamette River. First, the pattern 

of cold-water releases in mid-summer and warmer releases 

in early autumn are apparent throughout the river. This 

pattern is most striking upstream of the McKenzie River 

confluence (RM 174.9) because that reach is closest to a 

series of upstream dams on the Middle Fork and Coast Fork 

Willamette River systems. Second, the magnitude of the 

temperature effect diminishes with downstream distance and 

dilution from tributaries. Upstream of the McKenzie River 

confluence, the effect can be more than 4 or 5 °C, warming 

or cooling at different times of the year. Closer to the dams, 

the effect is even larger and can be sustained for longer 

periods (see appendix B). Downstream of the McKenzie 

River but upstream of the Santiam River (RM 108.5), the 

maximum effect decreases to less than 3 or 4 °C but more 

typically less than 1.5 °C. Downstream of the Santiam River, 

the maximum effect decreases further, to less than 2.5 °C but 

more commonly less than 1 °C. Downstream of the Clackamas 

River confluence, the effect of upstream dams decreases to a 

level usually less than 0.5 or 1 °C. Third, the major tributary 

inflows do not necessarily dilute the effect of upstream dams 

because those tributaries also are affected by upstream dams. 

Several examples are evident in figure 15 where a cooling 

effect switches to warming, or vice versa, at a tributary 

junction because the effect of upstream dams in the tributary 

system can be larger than that in the Willamette River.

The time/distance color map for the Willamette 

River shows several periodic variations with downstream 

distance, particularly in the July through September period. 

These patterns are an artifact of using the daily maximum 

temperature in the analysis. Downstream of dams that 

release a relatively steady flow of water with little daily 

variation in temperature, as is the case for many of the 

larger dams in this study, a distinct downstream pattern in 

the daily maximum temperature can occur (Lowney, 2000). 

This pattern is characterized by minimal daily temperature 

variation at integer travel-time distances (1 day, 2 days, etc.), 

and maximum temperature variation in-between (0.5 day, 

1.5 days, etc.). For example, a larger maximum can occur 

0.5 days downstream of a dam because some water will reach 

that location after exposure only to heating during the day, 

whereas a full heating and cooling cycle is complete when 

that water has traveled a full day. In any case, the results from 

each model scenario contain a downstream nodal pattern in the 

7dADM temperature where the period is related to 1 day of 

travel time. Computing the difference of two scenario results 

with different nodal distances (because the flows are different) 

then produces a new downstream pattern that is an artifact of 

using the 7dADM rather than a mean temperature.

To avoid problems associated with analyzing results that 

contain these travel-time artifacts, the distribution of 7dADM 

temperatures from each model scenario can be examined 

instead. These distributions for all modeled locations along the 

Willamette River are plotted in figure 16 for the June through 

October time periods of 2001–02. The lower plots (B and C) 

are simply histograms of the 7dADM temperatures from the 

without-dam and with-dam model scenarios, respectively, 

whereas the upper plot (fig. 16A) shows the cumulative 

frequency distribution for both scenarios. Similar plots for 

most of the other modeled reaches are included in appendix C. 

The histograms and the cumulative frequency plot for the 

Willamette River show a result that is shared by most of 

its tributaries. The with-dam 7dADM temperatures span a 

narrower range and tend to have higher minimum and lower 

maximum 7dADM temperatures than those without the dams. 

In the case of the Long Tom and Clackamas Rivers, however, 

the without-dam 7dADM temperatures tend to be cooler than 

the with-dam counterparts throughout the entire temperature 

range. These results, of course, depend on the without-dam 

flow and temperature estimates at the upstream dam sites; 

therefore, any errors in those estimates will affect the resulting 

without-dam temperature distributions.

Regardless of any errors in the modeled withoutdam temperatures, the general trends in the predictions are 

consistent with the physics of heat transport processes in 

river systems. The annual maximum temperatures at the dam 

sites are likely to be warmer without the dams because the 

reservoirs tend to buffer seasonal temperature changes and 

dam releases typically are not made from the warmer surface 

layer until later in the year after the reservoir begins to cool. 

Annual minimum temperatures also are likely to be cooler 

without the dams because it is far easier to cool a river in 

winter than it is to cool the bottom of a reservoir, which in 

the Willamette River basin is never likely to be colder than 

4 °C. Given these patterns at the dam sites, and coupled with 

lower flows during mid-summer without the dams, it is logical 

to expect that annual maximum temperatures downstream 

of the dam sites would be warmer without the dams. The 

wider temperature range without the upstream dams is an 

important result from this study. Despite the fact that all of 

the without‐dam models use estimated temperatures at the 

dam sites, the results provide helpful guidelines for resource 

managers as they develop plans to manage dam operations for 

optimal fish habitat, among other goals.
42 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig16

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A.

Willamette River

June–October 2001–2002

4 6 8 10 12 14 16 18 20 22 24 26 28

Cumulative Frequency Plot

Figure 16. Modeled 7dADM temperatures for conditions with and without dams in the Willamette River 

downstream of the confluence of the Coast Fork and Middle Fork Willamette Rivers, Oregon, 

June–October 2001–02.
Summary and Conclusions 43

Summary and Conclusions

Regression techniques, mass and energy balances, 

measured water temperatures upstream of reservoirs, and 

modeled and historic rates of summertime warming with 

downstream distance were used to estimate the streamflows 

and water temperatures that likely would occur at 12 major 

dam sites in the Willamette River basin in the absence of 

those dams and any upstream dams. The dam sites include 

River Mill Dam on the Clackamas River as well as all major 

dams of the Corps of Engineers’ Willamette Project. Because 

of short residence times in two re-regulating reservoirs, the 

effects of Big Cliff and Detroit Dams on the North Santiam 

River, and Dexter and Lookout Point Dams on the Middle 

Fork Willamette River, were combined and treated as single 

dam sites for this analysis. These without-dam streamflows 

and water temperatures were estimated for calendar years 

2001 and 2002 to match the general time frame used in the 

Willamette River water-temperature Total Maximum Daily 

Load (TMDL) analysis. Estimated errors for the without-dam 

temperature estimates ranged from 0.5 to roughly 1.5 °C 

for most of the dam sites in mid-summer, depending on the 

methods used, the availability of measured temperatures 

upstream of the reservoirs, and the amount of summertime 

warming applied through the reservoir reach.

The downstream effects of the dams were evaluated 

with a suite of CE-QUAL-W2 flow and temperature models 

by imposing both with- and without-dam streamflows and 

water temperatures at the dam sites as upstream boundary 

conditions. The models included the entire Willamette River 

and selected major tributaries up to their first major dams, 

were constructed and calibrated originally for the Willamette 

River water-temperature TMDL, and required several small 

modifications to allow them to run under the greatly changed 

without-dam streamflow conditions. The modeling time period 

for the TMDL and this analysis focused on the summertime 

low-flow period from April (2002) or June (2001) through 

October. Results were analyzed using the 7-day average of the 

daily maximum (7dADM) temperature, the statistic specified 

in the State of Oregon maximum water-temperature standard.

Many of the larger, taller dams in the Willamette River 

basin that are operated for flood control and hydropower 

generation have characteristic effects on streamflow and water 

temperature at the dam sites. The dams tend to reduce peak 

flows during precipitation or snowmelt time periods (for flood 

control) and augment streamflow during the summer low‐flow 

period (for downstream navigation, irrigation, and other 

needs). Because the taller dams typically release water from 

a mid-depth or deeper outlet in the upstream impoundment, 

that water tends to be colder in mid-summer than it would be 

without the dam. In September or October, a large amount of 

water is released from many of these dams to make room for 

flood storage, which can bring warmer surface waters down 

to the elevation of the outlet, thus releasing the warmest water 

of the year during a time period when the river without the 

dam would be cooler because of shorter days and colder air 

temperatures. Many of the taller dams, therefore, tend to shift 

the annual maximum water temperature from July/August to 

September/October, and can release water that generally is 6 

to 10 °C cooler or warmer than what would occur without the 

dams.

The temperature effects of each dam are specific to the 

characteristics and operation of each structure. Starting in 

2005, for example, Cougar Dam on the SF McKenzie River 

no longer produces a seasonal thermal shift because it was 

retrofitted with a selective withdrawal tower that allows 

operators to produce a more natural seasonal temperature 

pattern downstream of the dam. Detroit and Big Cliff Dams 

on the North Santiam River produced the typical seasonal 

shift in maximum temperature during the time period of 

this study, but were operated differently in 2008 and 2009, 

releasing water from multiple depths in Detroit Lake to 

produce a more natural pattern of downstream temperatures. 

A similar effect of releasing water from different outlets was 

apparent in the temperatures at Fall Creek Dam during the 

study period, where abrupt changes in the release temperature 

were indicative of a change in dam operations. Shorter dams, 

such as River Mill Dam on the Clackamas River and Fern 

Ridge Dam on the Long Tom River, do not exhibit a seasonal 

maximum temperature shift because the upstream reservoirs 

are not deep enough to provide a significant pool of cold 

hypolimnetic water at the dam outlet.

The degree to which the reservoirs are filled at the 

beginning of the summer season also is important in 

determining the temperature effects of the dams. Precipitation 

during the winter of 2000–01 was abnormally low and 

insufficient to fill most of the larger reservoirs in the 

Willamette River basin. The lower water level resulted in a 

shallower release, closer to the warmer epilimnion during 

summer, when using the normal release point from these 

dams. As a result, water released in mid-summer from several 

of the Willamette Project dams (Big Cliff/Detroit, Blue River, 

Dexter/Lookout Point, and Cottage Grove Dams in particular) 

was significantly warmer in 2001 than in 2002.

Estimation of without-dam water temperatures at the 

dam sites showed that the sites can be grouped according to 

their upstream drainage characteristics and elevation, a finding 

that is consistent with recent research by Tague and others 

(2007) for the McKenzie River basin. Sites that derive most 

of their flow from areas in the High Cascades, for example, 

tend to have cooler mid-summer temperatures because that 

water originates from snowmelt and cold spring complexes 
44 Thermal Effects of Dams in the Willamette River Basin, Oregon

high in the mountains. This is the case for dam sites on the SF 

McKenzie River (Cougar Dam), the North Santiam River (Big 

Cliff and Detroit Dams), and to a slightly lesser degree, the 

upper reaches of the MF Willamette River (Hills Creek Dam) 

and the Clackamas River (River Mill Dam), although the 

latter site is at a lower elevation. Each of these sites produces 

without-dam mid-summer 7dADM temperature maxima less 

than 20 °C. The SF McKenzie River site is the coldest, with 

annual maximum 7dADM temperatures less than 15.5 °C, 

because it derives the greatest amount of flow from the High 

Cascades and is located at a relatively high elevation.

Sites that derive their flow from the Western Cascades, 

Coast Range, and Willamette Lowlands, areas that are affected 

more by rainfall than by snowmelt and lack the cold and 

consistent springs present in the High Cascades, tend to have 

warmer mid-summer temperatures, with annual maximum 

7dADM temperatures greater than 21 °C and as high as 25 °C. 

Sites at higher elevations, such as the Blue River dam site, 

tend to produce mid-summer temperatures at the low end of 

this range, whereas sites at lower elevations, such as Fern 

Ridge on the Long Tom River, Cottage Grove on the Coast 

Fork Willamette River, Foster on the South Santiam River, and 

Dorena on the Row River, tend to have warmer without-dam 

temperature estimates.

In order to address temperature issues related to dams 

and climate change, it is critical to maintain long-term yearround temperature monitors at key unregulated sites in several 

characteristic physiographic areas of the Willamette River 

basin. Currently, Blowout Creek near Detroit (USGS station 

14180300) is one of the only long-term (10+ years), yearround temperature monitoring sites that is representative of 

conditions in the Western Cascades physiographic region. 

Several long-term, year-round temperature monitors exist at 

sites that are representative of conditions in High Cascades 

streams, such as the SF McKenzie River above Cougar 

Reservoir (USGS station 14159200), Breitenbush River 

above French Creek (USGS station 14179000), and the 

North Santiam River below Boulder Creek (USGS station 

14178000). The latter two sites are upstream of Detroit Lake 

in the North Santiam River basin. New sites for temperature 

monitoring are being established at many sites throughout 

the Willamette River basin, including sites upstream of 

key reservoirs. Data from those sites will prove helpful for 

assessing basin-specific thermal characteristics, trends, and the 

effects of dams, but these new sites still have a relatively short 

period of record. Until datasets with longer period of records 

have been collected at these new sites, it is critical to maintain 

the few long-term temperature-monitoring sites that do exist.

Downstream of the dam sites, model results show that 

the effects of the dams persist to the mouth of the Willamette 

River, although the thermal effects diminish with downstream 

distance. The downstream thermal effects are caused by 

changes in both streamflow and water temperature at the 

dam sites. Without the dams, greatly decreased streamflow 

during the late summer months results in longer travel times 

through the river network, allowing additional time for the 

river to gain (or lose) heat from (to) its surroundings. Latesummer streamflow in the Willamette River is augmented by 

dam releases, increasing the flow at Albany and at Salem by 

approximately a factor of 2 or more during August, September, 

and October in 2001 and 2002. No attempt was made to 

separate the effects of altered streamflows relative to altered 

upstream temperatures on downstream water temperatures, but 

the effect of changes in streamflow are likely to be significant.

The thermal effects of the dams are greatest at the dam 

sites, where the 7dADM temperatures are as much as 6 to 

10 °C cooler or warmer compared to what would occur 

without the dams. Downstream, the effects decrease, but are 

still in the 0.5 to 1.0 °C range near the mouth of the Willamette 

River, much larger than the cumulative point-source 

heating effect (< 0.3 °C) regulated by the Willamette River 

temperature TMDL. Upstream of the Clackamas River but 

downstream of the Santiam River, the 7dADM temperature 

changes are on the order of 1 °C, sometimes as high as 2.5 °C. 

Upstream of the Santiam River confluence, the effects of 

upstream dams on 7dADM temperatures in the Willamette 

River are on the order of 1.5 °C but sometimes as high as 3 

to 4 °C. Clearly, the dams have an important effect on both 

streamflow and water temperature in the Willamette River and 

many of its major tributaries. Changes in dam operations have 

been used in recent years to greatly decrease those effects in 

the SF McKenzie River (Cougar Dam) and the North Santiam 

River (Detroit Dam). A quantification of the thermal effects of 

the dams both at the dam sites and downstream will be helpful 

as dam operators, resource managers, and regulators work 

to improve water quality and restore critical fisheries in the 

Willamette River basin.

Acknowledgments

This study was funded partially by the Portland District 

of the U.S. Army Corps of Engineers (USACE). Thanks to 

Jim Britton, Art Armour, and Mary Karen Scullion of USACE 

for providing funding, project coordination, and inflow and 

evaporation data for the USACE Willamette Basin projects. 

Thanks also to Janet Gillaspie and the Oregon Association 

of Clean Water Agencies, which had funded a previous 

Willamette River temperature modeling study that generated 

most of the without-dam temperature estimates. Finally, the 

author is grateful to the group of reviewers who provided 

helpful comments on this report.
References Cited 45

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Appendixes 47

Appendixes
48 Thermal Effects of Dams in the Willamette River Basin, Oregon 

Appendix A. Graphs of Without-Dam Flow Estimates at Willamette River Basin 

Dam Sites 

500

1,000

2,000

5,000

10,000

20,000

Measured, with dams

Estimated, without dams

10

50

100

500

1,000

5,000

10

50

100

500

1,000

5,000

STREAMFLOW, IN CUBIC FEET PER SECOND

C. Cottage Grove Dam

B. Blue River Dam

A. Big Cliff / Detroit Dams

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

2001 2002

M A M J

Figure A1. Measured and estimated streamflow for conditions with and without upstream dams at selected 

dam sites in the Willamette River basin, Oregon, 2001–02. Measured flow data are from USGS streamflow–gaging 

stations 14181500, 14162200, and 14153500 for graphs A, B, and C, respectively.

Measured with-dam and estimated without-dam streamflows at the 12 dam sites of interest in this study are 

graphed in figures A1–A4. Without-dam streamflows were estimated using the techniques described in section, 

“Methods and Models.”
Appendix A 49

Figure A2. Measured and estimated streamflow for conditions with and without upstream dams at selected 

dam sites in the Willamette River basin, Oregon, 2001–02. Measured flow data are from USGS streamflow–gaging 

stations 14159500, 14150000, and 14155500 for graphs A, B, and C, respectively.

ortac10-0490_figA2

100

200

500

1,000

2,000

5,000

10,000

Measured, with dams

Estimated, without dams

500

1,000

2,000

5,000

10,000

20,000

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

10

50

100

500

1,000

5,000

10,000

STREAMFLOW, IN CUBIC FEET PER SECOND

2001 2002

C. Dorena Dam

B. Dexter / Lookout Point Dams

A. Cougar Dam
50 Thermal Effects of Dams in the Willamette River Basin, Oregon

Figure A3. Measured and estimated streamflow for conditions with and without upstream dams at selected 

dam sites in the Willamette River basin, Oregon, 2001–02. Measured flow data are from USGS streamflow–

gaging stations 14151000, 14169000, and (14187200 minus 14187000) for graphs A, B, and C, respectively.

ortac10-0490_figA3

10

50

100

500

1,000

5,000

Measured, with dams

Estimated, without dams

10

50

100

500

1,000

5,000

10,000

J F M A M J J A S O N D J F M A M J J A S O N D 50

100

500

1,000

5,000

10,000

30,000

STREAMFLOW, IN CUBIC FEET PER SECOND

2001 2002

C. Foster Dam

B. Fern Ridge Dam

A. Fall Creek Dam
Appendix A 51

Figure A4. Measured and estimated streamflow for conditions with and without upstream dams at selected 

dam sites in the Willamette River basin, Oregon, 2001–02. Measured flow data in graphs B and C are from USGS 

streamflow–gaging stations 14145500 and 14210000, respectively. Measured releases from Green Peter Dam were 

obtained from the U.S. Army Corps of Engineers.

ortac10-0490_figA4

10

50

100

500

1,000

5,000

10,000

20,000

Measured, with dams

Estimated, without dams

50

100

500

1,000

5,000

10,000

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

500

1,000

2,000

5,000

10,000

20,000

30,000

STREAMFLOW, IN CUBIC FEET PER SECOND

2001 2002

C. River Mill Dam

B. Hills Creek Dam

A. Green Peter Dam
52 Thermal Effects of Dams in the Willamette River Basin, Oregon

Appendix B. Time/Distance Color Maps Showing the Downstream Thermal 

Effects of Willamette River Basin Dams

Figure B1. Time/distance color map showing modeled downstream thermal effects of dams on the entire 

Willamette River, Oregon, June–October 2001 and April–October 2002. Temperature changes in this figure are 

the difference in the modeled 7dADM temperatures for Natural Thermal Potential conditions with upstream 

dams minus the modeled 7dADM temperatures without upstream dams. The McKenzie, Long Tom, Santiam, and 

Clackamas Rivers join the Willamette River at river miles 174.9, 149.4, 108.5, and 24.9, respectively.
Appendix B 53

Figure B2. Time/distance color maps showing modeled downstream thermal effects of dams on (A) the Middle Fork Willamette River, 

(B) the Coast Fork Willamette River, and (C) the Long Tom River, Oregon, June–October 2001 and April–October 2002. Temperature 

changes in this figure are the difference in the modeled 7dADM temperatures for Natural Thermal Potential conditions with upstream 

dams minus the modeled 7dADM temperatures without upstream dams.

A B C
54 Thermal Effects of Dams in the Willamette River Basin, Oregon

Figure B3. Time/distance color maps showing modeled downstream thermal effects of dams on (A) the South Fork McKenzie 

and McKenzie Rivers, and (B) the Clackamas River, Oregon, June–October 2001 and April–October 2002. Temperature changes 

are the difference in the modeled 7dADM temperatures for Natural Thermal Potential conditions with upstream dams minus the 

modeled 7dADM temperatures without upstream dams. Blue River joins the McKenzie River at river mile 54.

A B
Appendix B 55

Figure B4. Time/distance color maps showing modeled downstream thermal effects of dams on (A) the North Santiam and 

Santiam Rivers, and (B) the South Santiam River, Oregon, June–October 2001 and April–October 2002. Temperature changes 

in this figure are the difference in the modeled 7dADM temperatures for Natural Thermal Potential conditions with upstream 

dams minus the modeled 7dADM temperatures without upstream dams. The North Santiam and South Santiam Rivers join at 

river mile 12.1.

A B
56 Thermal Effects of Dams in the Willamette River Basin, Oregon

Appendix C. Graphs of Modeled Temperature Distributions Downstream of 

Willamette River Basin Dam Sites

ortac10-0490fig-c1

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

Willamette River

June–October 2001–2002

8

Figure C1. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the Willamette River downstream of the confluence of the Coast Fork and Middle Fork 

Willamette Rivers, Oregon, June–October 2001–02.
Appendix C 57

ortac10-0490_fig-c2

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 8 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

Coast Fork Willamette River

June–October 2001–2002

Figure C2. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the Coast Fork Willamette River downstream of Cottage Grove Dam, Oregon, June–

October 2001–02. The without-dams histogram is truncated at 28 °C; that part of the distribution (less than 

5 percent of the total) is not shown because of problems modeling very low flows in the Coast Fork Willamette 

River.
58 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig-c3

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

Middle Fork Willamette River

June–October 2001–2002

WATER TEMPERATURE, IN DEGREES CELSIUS

4 6 8 10 12 14 16 18 20 22 24 26 28

Figure C3. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the Middle Fork Willamette River downstream of Dexter Dam, Oregon, June–

October 2001–02.
Appendix C 59

ortac10-0490_fig-c4

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 8 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

SF McKenzie and McKenzie Rivers

June–October 2001–2002

Figure C4. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the South Fork McKenzie and McKenzie Rivers downstream of Cougar Dam, Oregon, 

June–October 2001–02.
60 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig-c5

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 8 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

Long Tom River

June–October 2001–2002

Figure C5. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the Long Tom River downstream of Fern Ridge Dam, Oregon, June–October 2001–02.
Appendix C 61

ortac10-0490_fig-c6

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 8 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

North Santiam and Santiam Rivers

June–October 2001–2002

Figure C6. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the North Santiam and Santiam Rivers downstream of Big Cliff Dam, Oregon, June–

October 2001–02.
62 Thermal Effects of Dams in the Willamette River Basin, Oregon

ortac10-0490_fig-c7

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

4 6 8 10 12 14 16 18 20 22 24 26 28

WATER TEMPERATURE, IN DEGREES CELSIUS

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

South Santiam River

June–October 2001–2002

Figure C7. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the South Santiam River downstream of Foster Dam, Oregon, June–October 2001–02.
Appendix C 63

ortac10-0490_fig-c8

0

10

20

30

40

50

60

70

80

90

100

PERCENT LESS THAN

Without dams

With dams

0

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

2

4

6

8

10

12

14

16

18

20

22

24

FREQUENCY, IN PERCENT

C. Histogram, With Dams

B. Histogram, Without Dams

A. Cumulative Frequency Plot

Clackamas River

June–October 2001–2002

WATER TEMPERATURE, IN DEGREES CELSIUS

0

4 6 8 10 12 14 16 18 20 22 24 26 28

Figure C8. Cumulative frequency plot (A) and histograms (B, C) of modeled 7dADM temperatures for conditions 

with and without dams in the Clackamas River downstream of River Mill Dam, Oregon, June–October 2001–02.
64 Thermal Effects of Dams in the Willamette River Basin, Oregon

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Rounds—Thermal Effects of Dams in the Willamette River Basin, Oregon—Scientific Investigations Report 2010–5153