Prepared in cooperation with Clackamas River Water and the City of Lake Oswego

Sources and Characteristics of Organic Matter in the 

Clackamas River, Oregon, Related to the Formation of 

Disinfection By-Products in Treated Drinking Water

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5001
Cover: The lower Clackamas River at Carver, Oregon, during high streamflow 

(25,000 cubic feet per second) and elevated turbidity (50 formazin turbidity units). 

(Photograph taken by Kurt Carpenter, U.S. Geological Survey, December 14, 2010.)
Sources and Characteristics of Organic 

Matter in the Clackamas River, Oregon, 

Related to the Formation of Disinfection 

By-Products in Treated Drinking Water

By Kurt D. Carpenter, Tamara E.C. Kraus, Jami H. Goldman, John Franco Saraceno, 

Bryan D. Downing, Brian A. Bergamaschi, U.S. Geological Survey; and 

Gordon McGhee and Tracy Triplett, Clackamas River Water

Prepared in cooperation with Clackamas River Water and the City of Lake Oswego

Scientific Investigations Report 2013–5001

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: 2013

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

Suggested citation:

Carpenter, K.D., Kraus, T.E.C., Goldman, J.H., Saraceno, J.F., Downing, B.D., McGhee, Gordon, and Triplett, Tracy, 

2013, Sources and characteristics of organic matter in the Clackamas River, Oregon, related to the formation of 

disinfection by-products in treated drinking water: U.S. Geological Survey Scientific Investigations Report 2013–5001, 

78 p.
iii

Contents

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................3

Disinfection By-Products and Historical Trends in Disinfection By-Product 

Concentrations ........................................................................................................................7

Sources of Organic Matter and Disinfection By-Product Precursors.........................................9

Use of Optical Properties to Characterize Dissolved Organic Matter.........................................9

Absorbance and Fluorescence Spectroscopy ......................................................................9

In-Situ Fluorometers as Proxies for Dissolved Organic Carbon and Disinfection 

By-Product Precursor Concentrations..............................................................................11

Study Objectives and Approach................................................................................................................12

Study Site Descriptions .....................................................................................................................16

Discrete Sampling...............................................................................................................................16

Watershed Samples ..................................................................................................................16

Source-Water and Finished-Water Samples........................................................................17

Benthic Algae (Periphyton)......................................................................................................17

Laboratory Analytical Procedures...................................................................................................17

Organic Carbon ..........................................................................................................................17

Absorbance and Fluorescence ...............................................................................................18

Disinfection By-Product Formation Potentials .....................................................................18

Disinfection By-Product Concentrations in Finished Water...............................................18

Water-Column Chlorophyll-a....................................................................................................19

Benthic Algal Chlorophyll-a and General Species Composition .......................................19

Nutrients......................................................................................................................................19

Treatability Experiments Using Jar Tests...............................................................................19

Continuous Real-Time Measurement of Streamflow, Field Parameters, and Fluorescent 

Dissolved Organic Matter....................................................................................................20

Statistical Analyses.....................................................................................................................................22

Correlations and Exploratory Data Analyses .................................................................................22

Carbon and Disinfection By-Product Formation Potential Loads and Yields............................22

Parallel Factor Analysis (PARAFAC)................................................................................................22

Overview of River Conditions During 2010–11 ........................................................................................23

Dissolved and Particulate Carbon Concentrations................................................................................30

Disinfection By-Product Concentrations in Finished Drinking Water.................................................30

Storm Effects on Organic Carbon Concentrations and Disinfection By-Products ..................34

Disinfection By-Product Formation Potentials .......................................................................................36

Specific Disinfection By-Product Formation Potentials...............................................................39

Loads and Yields of Organic Carbon and Disinfection By-Product Precursors.......................41

Carbon Characterization Using Optical Properties................................................................................47

Proxies for Carbon and Disinfection By-Product Precursor Concentrations....................................53

Water Treatment and Removal Efficiencies for Dissolved Organic Carbon and Disinfection 

By-Product Precursors .................................................................................................................56

Custom In-Situ Fluorescence Sensors.....................................................................................................59
iv

Contents—Continued

Sources of Organic Carbon that Contribute Disinfection By-Product Precursors...........................61

Terrestrial Inputs........................................................................................................................61

Algae .........................................................................................................................................62

Phytoplankton Blooms .....................................................................................................64

Benthic Algae and Periphyton Sloughing Events........................................................64

Wastewater ................................................................................................................................68

Strategies for Managing and Reducing Disinfection By-Products.....................................................68

Fluorescence Technology for Drinking-Water Management...............................................................69

Conclusions, Implications for Drinking-Water Treatment and River Management, and Possible 

Future Studies.................................................................................................................................69

Acknowledgments.......................................................................................................................................70

References Cited..........................................................................................................................................71

Appendixes and Data Quality Assurance................................................................................................77

Figures

Figure 1. Map showing land ownership of the Clackamas River basin, Oregon, and the

sampling-site locations ..................................................................... 4

Figure 2. Schematic diagram showing the direct-filtration process at the Clackamas

River Water drinking-water treatment plant, Clackamas, Oregon..................... 5

Figure 3. Map and photographs showing land cover and potential sources of organic

carbon that contribute disinfection by-product precursors to the Clackamas

River, Oregon................................................................................. 6

Figure 4. Graphs showing historical trends (1982–2011) in disinfection by-product

concentrations in finished drinking water from the Clackamas River Water and

City of Lake Oswego direct-filtration treatment plants, Clackamas River basin,

Oregon ....................................................................................... 8

Figure 5. Example excitation–emission matrix showing the general locations of selected

fluorescence peaks ........................................................................ 12

Figure 6. Boxplot showing relation between the average monthly streamflow in the

Clackamas River at Estacada, Oregon, in 2010 and 2011 and the 103-year

period of record (1908–2011) ............................................................... 23

Figure 7. Graphs showing time series of streamflow and dissolved organic carbon,

turbidity and chlorophyll-a, and dissolved oxygen and pH in the lower

Clackamas River, Oregon, 2010–11 ...................................................... 27

Figure 8. Boxplot showing relation between the average monthly water-column

chlorophyll-a concentrations in the Clackamas River at Oregon City, Oregon, in

2010 and 2011, and the 9-year period of record (2001–11) .............................. 28

Figure 9. Graphs showing streamflow and concentrations of dissolved and particulate

organic carbon in the tributaries and main-stem Clackamas River, Oregon,

2010–11 ....................................................................................... 31

Figure 10. Graphs showing seasonal patterns in dissolved organic carbon, total carbon,

and percentage of total carbon in particulate fraction in the main-stem

Clackamas River, Oregon, 2010–11 ...................................................... 32
v

Figures—Continued

Figure 11. Graphs showing seasonal patterns in total trihalomethanes (THM4) and total

haloacetic acids (HAA5) in finished water from the Clackamas River Water

(CRW) and City of Lake Oswego (LO) drinking-water treatment plants,

Clackamas River basin, Oregon, 2010–11 ................................................ 33

Figure 12. Graphs showing seasonal patterns in disinfection by-product benchmark

quotients for four trihalomethanes and five haloacetic acids in finished water

from the Clackamas River Water and City of Lake Oswego drinking-water

treatment plants, Clackamas River basin, Oregon, 2010–11 ........................... 34

Figure 13. Graphs showing streamflow, turbidity, and dissolved organic carbon in the

lower Clackamas River at the Clackamas River Water drinking-water treatment

plant intake, October–November 2010, highlighting two different responses to

storms ....................................................................................... 35

Figure 14. Graphs showing disinfection by-product formation potentials for total

trihalomethanes (THM4) and total haloacetic acids (HAA5) for samples

collected in the Clackamas River basin, Oregon, 2010–11.............................. 37

Figure 15. Boxplots showing range of disinfection by-product formation potentials for

filtered and unfiltered samples from the Clackamas River basin, Oregon, 

2010–11 ....................................................................................... 38

Figure 16. Graphs showing specific disinfection by-product formation potential for total

trihalomethanes (THM4) and total haloacetic acids (HAA5) for samples

collected in the Clackamas River basin, Oregon, 2010–11.............................. 40

Figure 17. Graphs showing streamflow and instantaneous loads of dissolved organic

carbon, particulate carbon, and associated disinfection by-product formation

potentials for total trihalomethanes (THM4) and total haloacetic acids (HAA5)

for samples collected in the Clackamas River basin, Oregon, 2010–11 ............... 42

Figure 18. Graphs showing instantaneous yields of streamflow, dissolved organic carbon,

particulate carbon, and associated disinfection by-product formation

potentials for total trihalomethanes (THM4) and total haloacetic acids (HAA5)

for samples collected in the Clackamas River basin, Oregon, 2010–11 ............... 44

Figure 19. Graphs showing seasonal patterns in specific ultraviolet absorbance and

fluorescence index values for tributary, reservoir, and main-stem sites in the

Clackamas River basin, Oregon, 2010–11 ................................................ 47

Figure 20. Graphs showing excitation–emission matrices (EEMs) for water samples from

selected main-stem, tributary, and reservoir sites across a range of stream

conditions, Clackamas River basin, Oregon, 2010–11 ................................. 49

Figure 21. Graphs showing principal fluorescence components identified in the PARAFAC

model for the Clackamas River basin, Oregon, 2010–11 .............................. 51

Figure 22. Graphs showing relative percentages of PARAFAC components in water

samples collected from all sites, and source water at the Clackamas River

Water drinking-water treatment plant, Clackamas River basin, Oregon, 2010–11... 52

Figure 23. Graphs showing relation between concentrations of dissolved organic carbon

and laboratory measurement of fluorescent dissolved organic matter (FDOM),

laboratory measurement of UVA254, laboratory measurement of FDOM for

main-stem samples, laboratory measurement of UVA254, and n-situ FDOM,

from the Clackamas River basin, Oregon, 2010–11 .................................... 54
vi

Figures—Continued

Figure 24. Graphs showing elation between in-situ dissolved organic matter fluorescence

(FDOM) and total haloacetic acid (HAA5) concentrations in finished water,

and predicted HAA5 concentrations in finished water from in-situ FDOM at the

Clackamas River Water drinking-water treatment plant, 2010–11..................... 55

Figure 25. Graphs showing the seasonal pattern in source- and finished-water dissolved

organic carbon concentrations and percent DOC removal at Clackamas River

Water and City of Lake Oswego drinking-water treatment plants, Clackamas

River basin, Oregon, 2010–11............................................................... 56

Figure 26. Graphs showing effect of water treatment on component loading, and

percentage of each component in source and finished water during July 2010

low flow, October 2010 initial storm, and November 2010 major-flush event at

the Clackamas River Water drinking-water treatment plant, Clackamas River

basin, Oregon .............................................................................. 57

Figure 27. Graphs showing results of treatability jar-test experiments showing the effect of

coagulation and powdered activated carbon (PAC) on concentrations of

dissolved organic carbon and disinfection by-product precursors at the

Clackamas River Water drinking-water treatment plant, Clackamas River basin,

Oregon, 2010–11 ........................................................................... 58

Figure 28. Graphs showing continuous fluorescence from the standard and three custom

fluorescent dissolved organic matter sensors, streamflow and in-situ

Fluorescence Index, and daily median and maximum water-column

chlorophyll-a in the lower Clackamas River, Oregon, 2011–12 ........................ 60

Figure 29. Graphs showing relations between water-column chlorophyll-a and filtered and

unfiltered disinfection by-product formation potentials for main-stem

Clackamas River sites, Oregon, 2010–11 ................................................ 63

Figure 30. Graph showing daily maximum concentrations of chlorophyll-a in the lower

Clackamas River at Oregon City, Oregon, 2002–12, showing the higher

concentrations that can result from sloughed benthic algae ........................ 65

Figure 31. September 2011 data showing concentrations of total particulate carbon and

trihalomethane formation potential for unfiltered samples in the Clackamas

River at the beginning and near the peak of the Timothy Lake drawdown, time

series of streamflow and chlorophyll-a, daily median values of streamflow and

chlorophyll-a, and daily median turbidity and chlorophyll-a in the lower

main-stem Clackamas River at Oregon City ............................................. 66

Figure 32. Diagrams showing water-displacement hypothesis, showing the proposed

mechanism by which the drawdown of Timothy Lake increases transport of

bottom water from North Fork Reservoir, Clackamas River basin, Oregon ......... 67
vii

Tables

Table 1. Optical properties commonly used as indicators of carbon composition ............ 10

Table 2. Description of continuous monitors and sites sampled during monthly sampling

and basin-wide surveys, Clackamas River basin, Oregon.............................. 13

Table 3. Field and laboratory data collected during monthly samplings and basin-wide

surveys, Clackamas River basin, Oregon, 2010–11 .................................... 15

Table 4. Summary of data collection including continuous dissolved organic matter

fluorescence deployments and discrete water-sample collection in the

Clackamas River basin, Oregon, 2010–11 ................................................ 16

Table 5. Description of in-situ fluorescing dissolved organic matter (FDOM) 

fluorometers sensors deployed in the Clackamas River basin, Oregon, 

April 2010–September 2011 ............................................................... 20

Table 6. Summary statistics for select streamflow, water quality, optical properties, and

disinfection by-products for the upper and lower main-stem Clackamas River,

tributaries, and source water, Clackamas River basin, Oregon........................ 24

Table 7. Benthic algal conditions in the Clackamas River and select tributaries, Oregon ... 29

Table 8. Trends in source-water and finished-water quality during two storms in the

Clackamas River basin, Oregon, October–November 2010 at the Clackamas

River Water drinking-water treatment plant ............................................. 36

Table 9. Top ten highest carbon-normalized (specific) disinfection by-product formation

potentials for filtered and unfiltered samples .......................................... 39

Table 10. Percentage of dissolved organic carbon, total particulate carbon, and

disinfection by-product formation potentials for individual sites relative to

instantaneous loads at the Clackamas River Water drinking-water treatment

plant intake during basin-wide sampling events, Clackamas River basin, 

Oregon ....................................................................................... 46

Table 11. Spearman rank correlations between carbon concentrations, optical

properties, and disinfection by-product concentrations in finished water and

DBP formation potentials for filtered and unfiltered samples from the Clackamas

River basin, Oregon, 2010–11............................................................... 48

Table 12. Percent removal of dissolved organic carbon and disinfection by-product

formation precursors during treatability jar-test experiments conducted at the

Clackamas River Water drinking-water treatment plant, Clackamas River basin,

Oregon, 2010–11 ........................................................................... 59
viii

Conversion Factors, Datums, and Abbreviations

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

Area

acre 4,047 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

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

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Mass

grams (g) 0.035274 ounce, avoirdupois (oz)

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

°F=(1.8×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Turbidity values in water are given either in formazin nephelometric units (FNUs) or 

nephelometric turbidity units (NTUs). 

Ultraviolet-light absorbance is measured in nanometers.

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).
ix

Abbreviations

Alum Aluminum sulfate

ACH Aluminum chlorhydrate

BQ Benchmark quotient

CRW Clackamas River Water

DBP Disinfection by-product

DBPFP Disinfection by-product formation potential

DHBA 3,5-dihydroxy-benzoic acid

DO Dissolved oxygen

DOC Dissolved organic carbon

DOM Dissolved organic matter

DWTP Drinking-water treatment plant

ex Excitation

em Emission

FDOM Fluorescent dissolved organic matter 

FI Fluorescence index

HAA Haloacetic acid

HAA5 Total of five haloacetic acids

HAAFP Haloacetic acid formation potentials

HIX Humic index

LED Light-emitting diode

LO City of Lake Oswego

MCL Maximum contaminant level

NMDS Nonmetric dimensional scaling ordination

NWQL U.S. Geological Survey National Water-Quality Laboratory

PAC Powdered activated carbon

PCA Principal Component Analysis 

QA Quality assurance

rpm Revolutions per minute S Spectral slope SR Spectral slope ratio

SRM Standard reference material

SHAAFP Specific formation potential for haloacetic acids

STHMFP Specific formation potential for trihalomethanes

SUVA Specific ultraviolet absorbance

THM Trihalomethane

THM4 Total of four trihalomethanes

THMFP Trihalomethane formation potential 

TOC Total organic carbon

TP Total phosphorus

TPC Total particulate carbon

TPN Total particulate nitrogen

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

UVA Ultraviolet absorbance

UVA254 Ultraviolet absorbance at 254 nanometers

VOC Volatile organic compounds
x

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Sources and Characteristics of Organic Matter in the 

Clackamas River, Oregon, Related to the Formation of 

Disinfection By-Products in Treated Drinking Water

By Kurt D. Carpenter1, Tamara E.C. Kraus1, Jami H. Goldman1, John Franco Saraceno1, Bryan D. Downing1, 

Brian A. Bergamaschi1, Gordon McGhee2, and Tracy Triplett2

Executive Summary

This study characterized the amount and quality of 

organic matter in the Clackamas River, Oregon, to gain an 

understanding of sources that contribute to the formation of 

chlorinated and brominated disinfection by-products (DBPs), 

focusing on regulated DBPs in treated drinking water from 

two direct-filtration treatment plants that together serve 

approximately 100,000 customers. The central hypothesis 

guiding this study was that natural organic matter leaching 

out of the forested watershed, in-stream growth of benthic 

algae, and phytoplankton blooms in the reservoirs contribute 

different and varying proportions of organic carbon to the 

river. Differences in the amount and composition of carbon 

derived from each source affects the types and concentrations 

of DBP precursors entering the treatment plants and, as 

a result, yield varying DBP concentrations and species 

in finished water. The two classes of DBPs analyzed in 

this study—trihalomethanes (THMs) and haloacetic acids 

(HAAs)—form from precursors within the dissolved and 

particulate pools of organic matter present in source water.

The five principal objectives of the study were to 

(1) describe the seasonal quantity and character of organic 

matter in the Clackamas River; (2) relate the amount and 

composition of organic matter to the formation of DBPs; 

(3) evaluate sources of DBP precursors in the watershed; 

(4) assess the use of optical measurements, including in-situ 

fluorescence, for estimating dissolved organic carbon (DOC) 

concentrations and DBP formation; and (5) assess the removal 

of DBP precursors during treatment by conducting treatability 

“jar-test” experiments at one of the treatment plants.

Data collection consisted of (1) monthly sampling of 

source and finished water at two drinking-water treatment 

plants; (2) event-based sampling in the mainstem, tributaries, 

and North Fork Reservoir; and (3) in-situ continuous 

monitoring of fluorescent dissolved organic matter 

(FDOM), turbidity, chlorophyll-a, and other constituents to 

continuously track source-water conditions in near real-time. 

Treatability tests were conducted during the four event-based 

surveys to determine the effectiveness of coagulant and 

powdered activated carbon (PAC) on the removal of DBP 

precursors. Sample analyses included DOC, total particulate 

carbon (TPC), total and dissolved nutrients, absorbance 

and fluorescence spectroscopy, and, for regulated DBPs, 

concentrations of THMs and HAAs in finished water and 

laboratory-based THM and HAA formation potentials 

(THMFP and HAAFP, respectively) for source water and 

selected locations throughout the watershed.

The results of this study may not be typical given the 

record and near record amounts of precipitation that occurred 

during spring that produced streamflow much higher than 

average in 2010–11. Although there were algal blooms, lower 

concentrations of chlorophyll-a were observed in the water 

column during the study period compared to historical data.

Concentrations of DBPs in finished (treated) water 

averaged 0.024 milligrams per liter (mg/L) for THMs 

and 0.022 mg/L for HAAs; maximum values were about 

0.040 mg/L for both classes of DBPs. Although DBP 

concentrations were somewhat higher within the distribution 

system, none of the samples collected for this study or 

for the quarterly compliance monitoring by the water 

utilities exceeded levels permissible under existing U.S. 

Environmental Protection Agency (USEPA) regulations: 

0.080 mg/L for THMs and 0.060 mg/L for HAAs.

DOC concentrations were generally low in the Clackamas 

River, typically about 1.0–1.5 mg/L. Concentrations in the 

mainstem occasionally increased to nearly 2.5 mg/L during 

storms; DOC concentrations in tributaries were sometimes 

much higher (up to 7.8 mg/L). The continuous in-situ FDOM 

measurements indicated sharp rises in DOC concentrations 

in the mainstem following rainfall events; concentrations 

were relatively stable during summer base flow. Even though 

the first autumn storm mobilized appreciable quantities of 

carbon, higher concentrations of DBPs in finished water were 

observed 3-weeks later, after the ground was saturated from 

additional rainfall.

1

U.S. Geological Survey.

2

Clackamas River Water.
2 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

The majority of the DOC in the lower Clackamas 

River appears to originate from the upper basin, suggesting 

terrestrial carbon was commonly the dominant source. Lowerbasin tributaries typically contained the highest concentrations 

of DOC and DBP precursors and contributed substantially to 

the overall loads in the mainstem during storms. During lowflow periods, tributaries were not major sources of DOC or 

DBP precursors to the Clackamas River. 

Although the dissolved fraction of organic carbon 

contributed the majority of DBP precursors, at times the 

particulate fraction (inorganic sediment and organic particles 

including detritus and algal material) contributed a substantial 

fraction of DBP precursors. Considering just the main-stem 

sites, on average, 10 percent of THMFP and 32 percent of 

HAAFP were attributed to particulate carbon. This finding 

suggests water-treatment methods that remove particles 

prior to chlorination would reduce finished-water DBP 

concentrations to some degree.

Overall, concentrations of THM and HAA precursors 

were closely linked to DOC concentrations; laboratory DBP 

formation potentials (DBPFPs) clearly showed that THMFP 

and HAAFP were greatest in the downstream tributaries 

that contained elevated carbon concentrations. However, 

carbon-normalized “specific” formation potentials for THMs 

and HAAs (STHMFP and SHAAFP, respectively) revealed 

changes in carbon character over time that affected the two 

types of DBP classes differently. HAA precursors were 

elevated in waters containing aromatic-rich soil-derived 

material arising from forested areas. In contrast, THM 

precursors were associated with carbon having a lower 

aromatic content; highest STHMFP occurred in autumn 2011 

in the mainstem from North Fork Reservoir downstream 

to LO DWTP. This pattern suggests the potential for a link 

between THM precursors and algal-derived carbon. The 

highest STHMFP value was measured within North Fork 

Reservoir, indicating reservoir derived carbon may be 

important for this class of DBPs. Weak correlations between 

STHMFP and SHAAFP emphasize that precursor sources 

for these types of DBPs may be different. This highlights not 

only that different locations within the watershed produce 

carbon with different reactivity (specific DBPFP), but also 

that different management approaches for each class of DBP 

precursors could be required for control.

Treatability tests conducted on source water during 

four basin-wide surveys demonstrated that an average of 

about 40 percent of DOC can be removed by coagulation. 

While the decrease in THMFP following coagulation was 

similar to DOC, the decrease in HAAFP was much greater 

(approximately 70 percent), indicating coagulation is 

particularly effective at removing HAA precursors—likely 

because of the aromatic nature of the carbon associated with 

HAA precursors. 

Several findings from this study have direct implications 

for managing drinking-water resources and for providing 

useful information that may help improve treatment-plant 

operations. For example, the use of in-situ fluorometers 

that measure FDOM provided an excellent proxy for DOC 

concentration in this system and revealed short-term, rapid 

changes in DOC concentration during storm events. In 

addition, the strong correlation between FDOM values 

measured in-situ and HAA5 concentrations in finished water 

may permit estimation of continuous HAA concentrations, 

as was done here. As part of this study, multiple in-situ 

FDOM sensors were deployed continuously and in real-time 

to characterize the composition of dissolved organic matter. 

Although the initial results were promising, additional 

research and engineering developments will be needed to 

demonstrate the full utility of these sensors for this purpose.

In conclusion, although DBPFPs were strongly correlated 

to DOC concentration, some DBPs formed from particulate 

carbon, including terrestrial leaf material and algal material 

such as planktonic species of blue-green algae and sloughed 

filaments, stalks, and cells of benthic algae. Different 

precursor sources in the watershed were evident from the 

data, suggesting specific actions may be available to address 

some of these sources. In-situ measurements of FDOM 

proved to be an excellent proxy for DOC concentration as 

well as HAA formation during treatment, which suggests 

further development and refinement of these sensors have 

the potential to provide real-time information about complex 

watershed processes to operators at the drinking-water 

treatment plants.

Follow-up studies could examine the relative roles that 

terrestrial and algal sources have on the DBP precursor pool to 

better understand how watershed-management activities may 

be affecting the transport of these compounds to Clackamas 

River drinking-water intakes. Given the low concentrations 

of algae in the water column during this study, additional 

surveys during more typical river conditions could provide a 

more complete understanding of how algae contribute DBP 

precursors. Further development of FDOM-sensor technology 

can improve our understanding of carbon dynamics in the 

river and how concentrations may be trending over time.

This study was conducted in collaboration with 

Clackamas River Water and the City of Lake Oswego water 

utilities. Other research partners included Oregon Health and 

Science University in Hillsboro, Oregon, Alexin Laboratory 

in Tigard, Oregon, U.S. Geological Survey National 

Research Program Laboratory in Denver, Colorado, and the 

U.S. Geological Survey Water Science Centers in Portland, 

Oregon, and Sacramento, California. This project was 

supported with funding from Clackamas River Water, City 

of Lake Oswego, the U.S. Geological Survey, and the Water 

Research Foundation. 
Introduction 3

Introduction

The Clackamas River in northwestern Oregon (fig. 1) 

is a valued resource to the region, supporting runs of wild 

steelhead and salmon and providing drinking water for nearly 

400,000 people. From its headwaters near Olallie Butte south 

of Mount Hood, the Cascade River descends from the High 

Cascades flowing northwest for 82 mi to reach its confluence 

with the Willamette River southeast of Portland. Although 

72 percent of the 940-mi2 watershed is contained within the 

Mount Hood National Forest (Metro Regional Services, 1997), 

the lower third of the watershed drains private forests and 

agricultural, urban, and light industrial land that variously 

contribute sediments, nutrients, pesticides, and other organic 

compounds to the Clackamas River (Carpenter, 2003; 

Carpenter and others, 2008; Carpenter and McGhee, 2009). 

Although the river, for the most part, is exceptionally clear, it 

sometimes becomes turbid with sediment and organic matter 

from storm runoff that degrades the quality of source water at 

the drinking-water intakes in the lower river. 

Two of the four drinking-water treatment plants (DWTPs) 

in the lower river—the Clackamas River Water (CRW) and 

the City of Lake Oswego (LO) DWTPs—use direct filtration 

as a means to clarify raw source water (fig. 2). Together, 

these two plants serve about 100,000 people. Both water 

utilities use chlorination simultaneously with coagulation 

as part of the water-treatment process. The use of chlorine 

as a disinfectant, although essential for pathogen control, 

leads to the halogenation of organic matter present in source 

water (Croué and others, 1999). Halogenated (chlorine- and 

bromine-containing) compounds form from dissolved and 

particulate organic carbon during water treatment and are 

collectively referred to as disinfection by-products (DBPs). 

Although only a small fraction of the organic carbon present 

in source water reacts to form DBPs, several DBPs have 

been identified as mutagenic and carcinogenic (Krasner 

and others, 2006; Richardson and others, 2007). For this 

reason, the USEPA currently regulates two classes of DBPs 

commonly found in drinking water—trihalomethanes (THMs) 

and haloacetic acids (HAAs) (U.S. Environmental Protection 

Agency, 2009). In addition to being a source of DBPs, organic 

carbon contributes to biofouling, increases chlorine demand, 

and can affect aesthetic qualities of water such as taste, odor, 

and color (Cooke and Kennedy, 2001). 

Water managers are concerned about DBPs in drinking 

water and are interested in identifying the types of organic 

carbon that contribute DBP precursors in source water to 

better understand the potential for future deterioration in river 

quality resulting from a wide array of possible sources (fig. 3). 

Understanding the timing, sources, and composition of organic 

matter entering drinking-water intakes will help drinkingwater utilities develop source-water-protection programs, 

facilitate successful and cost-effective treatment strategies, 

and help plan for future upgrades to treatment plants (Kraus 

and others, 2010). 

Although much of the carbon in the watershed is 

contained in its forests and soils, the Clackamas River, along 

much of its length, is gravel bedded and provides appreciable 

habitat for benthic algae (periphyton) to attach and grow (see 

photographs 1a-e). This material contains organic carbon 

that may decompose to yield DBP precursors (Jack and 

others, 2002; Huang and others, 2009; Kraus and others, 

2011). Periphyton growths occur in the Clackamas River 

during periods when nutrients, light, and flow conditions are 

favorable. At times, periphyton biomass reaches nuisance 

levels along river margins and causes supersaturated 

concentrations of dissolved oxygen (DO), high (alkaline) 

pH, and large daily fluctuations in pH and DO. The pH in 

the lower river is particularly high, and regularly exceeds 

the State of Oregon water-quality standard during parts of 

the growing season, particularly in spring (U.S. Geological 

Survey, 2012). Periphyton biomass is typically higher in the 

lower river downstream from Estacada, but nuisance biomass 

levels also occur in the upper river, upstream from North Fork 

Reservoir (fig. 1). A previous U.S. Geological Survey (USGS) 

study (Carpenter, 2003) found nuisance levels of benthic 

algae in the main-stem Clackamas River during the summer, 

and continuous monitoring of DO and pH—definitive algalphotosynthesis indicators—shows this problem has continued 

on and off for at least the past decade.

When periphyton detaches from the riverbed and 

becomes entrained in the flow during algal “sloughing” events, 

the cells and stalks of diatoms, filaments of green algae, and 

colonies of blue-green algae (Cyanobacteria) in varying states 

of decomposition enrich the river with organic carbon. This 

algae has the capacity to clog water intakes and negatively 

affect drinking-water quality through the production of tastes 

and odors and algal toxins (Graham and others, 2010). Algae 

also contribute carbon that contains DBP precursors (Graham 

and others, 1998; Jack and others, 2002; Kraus and others, 

2011). Phytoplankton (floating algae) also occasionally form 

blooms during summer in the two primary reservoirs, Timothy 

Lake in the headwaters of the Oak Grove Fork and North Fork 

Reservoir on the mainstem (see photographs 2a-c), which can 

also contribute DBP precursors (Kraus and others, 2011).

Decomposition products of terrestrial plant material, 

including leaves from deciduous trees, conifer needles, and 

other plant material contained in soils are certainly a source 

of organic carbon in the Clackamas River. High rainfall leads 

to saturated conditions and flow of water through organic-rich 

surface soils. High rainfall combined with steep topography 

in much of the basin causes erosion, and landslides are 

particularly common during large storms. The epic February 

1996 rain-on-snow event, for example, produced over 200 

landslides in the Fish Creek Basin alone (DeRoo and others, 

1998), and much of that material deposited in the mainstem Clackamas River near the upstream end of North Fork 

Reservoir. Decomposing vegetation buried in that debris could 

be another source of DBP precursors, along with wastewater 

from three municipal treatment plants, septic-tank effluents, 

and other potential sources.
4 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig01

Milwaukie

Portland

Happy

Valley

West

Linn

Clackamas

Oregon

City

Hot Springs Fk Collawash R

Timothy

Lake

Rock Cr

WILLAMETTE RIVER

N Fk

Doane Cr

Creek

Tickle

Deep Creek

Creek

Clear Creek

Creek

Foster Cr

Clear

Eag

Bear Cr le

N Fk Eagle Cr Currin Cr

Delph Creek

N Fk Clackamas R

Roaring

Three Lynx Cr

Fish Creek

River

CLACKAMAS

RIVER

Collawash

S Fk Clackamas R

Eagle

River

Creek

Clackamette

Cove

Oak Grove Fk Clackamas R

Cow

Cr

Sieben

Cr

North

Fork

Reservoir

Olallie

Butte

MULTNOMAH CO.

CLACKAMAS CO.

CLACKAMAS CO.

MARION CO.

CASCADE RANGE

A C

B

D

1

2

3

4

5

6

7

8

E

9

Carver Sandy Barton

Estacada

City of Lake Oswego drinking-water treatment

plant (14211023)

A

Site name (see table 2 for site details)

Sampling sites

Fixed sites

Additional basin-wide survey sites

Map

identifier

B Clackamas River near Oregon City (14211010)

Clackamas River Water drinking-water treatment

plant (14211005)

C

D Clackamas River at Estacada (14210000)

E Clackamas River at Carter Bridge (14209710)

1 Sieben Creek (14210900)

2 Rock Creek (14210850) 

3 Clackamas River at Carver (452335122294500)

4 Clear Creek (14210750)

5 Deep Creek (14210200)

6 Clackamas River near Barton (452255122244500)

7 Eagle Creek (14210150)

8 North Fork Reservoir (4514381221640)

Clackamas River at Two Rivers Campground

(450156122033100)

9

Clackamas River above Three Lynx Creek

(14209500)

10

Oak Grove Fork above power plant intake

(14209000)

11

Oak Grove Fork near Government Camp

(14208700)

12

EXPLANATION

National Forest

Warm Springs Indian Reservation

Area of private and other public ownership

City

City (small)

Wastewater treatment facility (WWTF)

Sampling site and map identifier

Streamgaging site

Water-quality site

OREGON

Study

area

C

7

10

11

12

0 5 10 15 20 25 MILES

0 5 10 15 20 25 KILOMETERS

B Streamgaging and water-quality site

122° 37' 30" 122° 30' 122° 22' 30" 122° 15' 122° 07' 30" 122° 121° 52' 30" 121° 45' 121° 37' 30"

45°

45°

07'

30"

45°

15'

45°

30'

44°

52'

30"

45°

22'

30"

Base map modified from various digital datasets from Portland, Oregon 

METRO and USGS; scales vary. Projection: State Plane, 

Zone 5076, North American Datum 1983.

Estacada

WWTF

Boring

WWTF

Sandy

WWTF Mt

Hood

Figure 1. Land ownership of the Clackamas River basin, Oregon, and the sampling-site locations.
Introduction 5

tac12-0779_fig02

Diagram courtesy of Clackamas River Water. 

http://crwater.com/images/stories/2011WQReport_web_version_2.pdf

Disinfectant

Filter Aid

Disinfectant

Clackamas River

Filters

Mixers

pH Adjustment

Coagulant

Backwash System

Treated Water

Storage Reservoirs

Fire

Protection

Low-level

Pump

Pump

Station

Water

Intake

Flow

Meter

Natural Pond

(Swale) Settling Ponds

Other

Water Providers

Residential

Customers

Pump

Clearwell Storage

Filtered Water

Traveling

Screens

Contact Basins

fine garnet

coal

sand

gravel

coarse garnet

fine garnet

coal

sand

coarse garnet

gravel

Commercial

Customers

Flow

Meter

Figure 2. The direct-filtration process at the Clackamas River Water drinking-water treatment plant, Clackamas, Oregon.

Photograph 1a. Riffles support many types of 

benthic algae in the upper Clackamas River 

upstream of Carter Bridge. (Photograph by Kurt 

Carpenter, U.S. Geological Survey, June 2010.)

Photograph 1b. Prasiola sp. (green algae) on the 

large cobbles in the Clackamas River downstream 

of River Mill Dam near Estacada. (Photograph by 

Kurt Carpenter, U.S. Geological Survey, April 23, 

2006.)

Photograph 1c. Prasiola sp. (green algae) on a 

cobble from the Clackamas River downstream 

of River Mill Dam near Estacada. (Photograph 

by Kurt Carpenter, U.S. Geological Survey, 

June 2010.)

Photograph 1d. Stalked diatoms (Cymbella sp.) on 

a cobble from the Clackamas River downstream 

of River Mill Dam near Estacada. (Photograph 

by Kurt Carpenter, U.S. Geological Survey, 

June 2010.)

Photograph 1e. Nostoc sp. (blue-green algae) 

colony balls on a cobble from the upper 

Clackamas River upstream of Carter Bridge. 

(Photograph by Kurt Carpenter, U.S. Geological 

Survey, June 2010.)
6 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig03

Land cover satellite imagery from U.S. Geological 

Survey 1992-93, 30-meter resolution.

0 10 20 30 MILES

0 10 20 30 KILOMETERS

EXPLANATION

Urban

Land cover

Agriculture

Regrowth forest

Mature forest

Water

Clackamas River Basin boundary

Other - includes no-data areas

Non-forest upland/harvest areas

A

C

B

D

F E

G

H

122°

122° 15'

122° 30'

121°

45'

45°

45°

15' B

D

F

H

A

C

E

G

Figure 3. Land cover and potential sources of organic carbon that contribute disinfection by-product precursors to the Clackamas 

River, Oregon. (Photographs show (A) irrigated container nursery in the Deep Creek basin, (B) blue-green algae bloom in North Fork 

Reservoir, (C) timber harvest unit in the Eagle Creek basin, (D) regrowth forest in the Oak Grove Fork basin, (E) filamentous green algae 

in the Collawash River, (F) riparian forest in the Collawash River basin, (G) timber harvest unit adjacent to North Fork Reservoir, and 

(H) row crops in the Rock Creek basin.)
Introduction 7

Disinfection By-Products and Historical 

Trends in Disinfection By-Product 

Concentrations

DBPs are known to be carcinogenic and are indicated 

to cause increased risks of reproductive and developmental 

problems in humans (Richardson and others, 2007). For 

these reasons, the USEPA regulates the total concentration 

of four THMs (THM4) and five HAAs (HAA5) in finished 

(treated) drinking water (U.S. Environmental Protection 

Agency, 2006). In 2004–05, as part of the USGS National 

Water-Quality Assessment Program, source and finished 

water from the CRW DWTP were sampled for a variety 

of organic compounds, including THMs. Compounds 

including pesticides and gasoline hydrocarbons were 

commonly detected in finished water; relative to regulatory 

standards, however, THMs, were the compound class 

of most concern (Carpenter and McGhee, 2009). Even 

though THM4 concentrations were always below current 

regulatory thresholds, THM4 concentrations may be higher 

when organic carbon is elevated in source water such as 

during periods of active rainfall runoff, algal blooms, or 

periphyton sloughing events—which were not targeted for 

sampling during that study.

Compliance monitoring data from the CRW and 

LO DWTPs show THMs in finished drinking water have 

increased to some degree over the past 20 years (fig. 4). 

The cause of this increase is not known, and data to 

evaluate potential causes, such as trends in historic total 

organic carbon (TOC) concentrations, are not available. 

Historically, the two highest THM4 concentrations 

were measured in mid-to-late June 1997 and 2000. It is 

possible these high THM4 concentrations were caused by 

sloughed benthic algae because this is a time of year when 

sloughing events have occurred (as in 2004 and 2005) (U.S.

Geological Survey, 2012). 

To better understand this increase in DBP 

concentrations, basin-specific information is needed 

regarding the sources of carbon that form the DBPs and 

factors that contribute to carbon losses from watersheds 

so that management strategies can be developed and 

successfully targeted. Information on the type of carbon 

present can provide insights into the chemical reactions 

that take place during chlorination that might shape 

water-treatment strategies to control DBPs. This could be 

especially important if the types of carbon that contribute to 

DBPs increase in source water or if the USEPA regulations 

become more restrictive.

Photograph 2a-c. Blue-green algae (Anabaena flos-aquae) 

blooms in North Fork Reservoir and Timothy Lake. (Photographs 

by Kurt Carpenter, U.S. Geological Survey, August 31, 2010.)

Photograph 2b. 

Photograph 2c. 
8 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig04

Annual moving average

EXPLANATION

0

0.05

0.10

0.15

0.20

0.25

0.30

Total trihalomethanes concentration,

in milligrams per liter (mg/L)

Total trihalomethanes concentration,

in milligrams per liter (mg/L) (Rescaled)

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

1982 1985 1990 1995 2000 2005 2010

Total haloacetic acids concentration,

 in milligrams per liter (mg/L)

Year

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

6/11/1997 non-storm,

0.26 mg/L

6/28/2000 non-storm,

0.19 mg/L

12/19/1995 storm,

0.13 mg/L

3 post-storm samples

A

C

B

Figure 4. Historical trends (1982–2011) in disinfection by-product concentrations in finished drinking 

water from the Clackamas River Water and City of Lake Oswego direct-filtration treatment plants, 

Clackamas River basin, Oregon. (A) Total trihalomethanes (THM4). (B) Total trihalomethanes (THM4) 

with y-axis rescaled. (C) Total haloacetic acids (HAA5).
Introduction 9

Sources of Organic Matter and Disinfection 

By-Product Precursors

The DBP precursor pool is a subset of the bulk 

organic-matter pool present in source water. Possible sources 

of organic matter in the Clackamas River basin are shown in 

figure 3. Previous studies have shown terrestrial plants and 

soils (allochthonous sources) and algae and macrophytes 

(autochthonous sources) contribute organic matter and DBP 

precursors to surface waters (Aiken and Cotsaris, 1995; 

Reckhow and others, 2004). As described previously, algae 

are a possible source of organic carbon in the Clackamas 

River. While less is known about the propensity for periphyton 

to form DPBs, phytoplankton is a well-known source of 

dissolved organic carbon (DOC) and DBP precursors, 

especially HAAs (Jack and others, 2002; Nguyen and others, 

2005; Kraus and others, 2011).

The amount and reactivity of organic matter entering 

a DWTP is a function of the amount and composition of 

material entering the water throughout the watershed, as 

well as environmental processes such as biodegradation, 

photodegradation, sedimentation, and sorption that may take 

place during transport through the river system. The types 

of DBPs that form are controlled by the physiochemical 

properties of the carbon molecules and the complex 

reactions that occur with disinfectants such as chlorine, 

along with coagulation treatment, pH, temperature, bromide 

concentration, and other factors (Crepeau and others, 2004). 

The amount of organic matter in a water sample is 

typically determined by measuring carbon concentration, 

assuming that half the organic matter pool is made up of 

carbon. TOC is commonly characterized by laboratory 

measurements of whole (unfiltered) water; however, this 

method has a tendency to under-report carbon concentrations 

(Aiken and others, 2002). In this report, TOC was derived 

by summing laboratory measurements of DOC and total 

particulate carbon (TPC). In addition to these concentrationbased constituents, the composition of dissolved organic 

matter (DOM) was characterized using absorbance and 

fluorescence spectrophotometry, and continuous in-situ 

fluorescence was used as an indicator of DOC concentration.

Use of Optical Properties to Characterize 

Dissolved Organic Matter

Spectral optical property measurements such as 

absorbance and fluorescence can be used to determine the 

amount of DOC in water and to broadly characterize dissolved 

organic matter (DOM) composition (Hudson and others, 2007; 

Fellman and others, 2010; Matilainen and others, 2011). Shifts 

in the spectral response of optical properties can help identify 

sources of carbon within watersheds and inform watershed 

management (Kraus and others, 2010; Beggs and others, 2011; 

Bridgeman and others, 2011). Absorbance measures the amount 

of light absorbed by material in a water sample at specified 

excitation (ex) wavelengths, and fluorescence measures the 

light that is re-emitted (emission (em) wavelengths). Depending 

on the type of material present, the spectral properties can 

be diagnostic of certain types of organic matter. If material 

derived from different sources provides a unique “signature”, 

important sources of carbon within a watershed can be 

identified. Studies have demonstrated that continuous, in-situ 

fluorescence measurements can be used as a reliable surrogate 

for DOC concentration, and advances are ongoing to improve 

understanding of how changes in carbon composition affect 

freshwater systems using this approach (Bergamaschi and 

others, 2005, 2012; Spencer and others, 2007; Saraceno and 

others, 2009; Pellerin and others, 2012).

Absorbance and Fluorescence Spectroscopy 

The measurement of ultraviolet absorbance at 254 nm 

(UVA254) has been used by the drinking-water industry as a 

proxy for DOC concentration for several decades (Edzwald and 

others, 1985; Rathbun, 1996; Korshin and others, 1997; Sadiq 

and Rodriguez, 2004). In addition to providing information 

about DOC concentration, absorbance data can provide insight 

into the chemical make-up of the DOM pool (table 1). For 

example, UVA254 normalized by DOC concentration, also 

known as “specific” UVA (SUVA, reported in units of liters 

per milligram-meters, L/mg-m), has been correlated with 

DOM aromatic content (Weishaar and others, 2003). Similarly, 

the spectral slope of the absorbance curve has been shown to 

relate to aromatic content and molecular weight. For example, 

decreasing spectral slope between 275 and 295 nm is associated 

with higher aromatic content and increasing molecular weight. 

Spectral slope has also been shown to change upon irradiation 

(Helms and others, 2008; Spencer and others, 2009). 

As with absorbance, the fluorescence response and 

intensity at a single ex/em wavelength pair can be related to 

DOC concentration; the presence of different peaks, peak 

slopes, changes in the ratios of ex/em pairs, carbon normalized 

values, or shifts in peak maxima have been shown to provide 

information about DOM character and origin (Coble, 2007; 

Hudson and others, 2007; Stedmon and Bro, 2008). The 

fluorescence index (FI), calculated as the ratio of em 470 to 

520 nm and ex 370 nm, has been widely used to indicate relative 

contributions of terrestrial- and algal-derived DOM (table 1). 
10 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water Table 1. Optical properties commonly used as indicators of carbon composition.

[Abbreviations: DOC, dissolved organic cargon; DOM, dissolved organic matter; L, liters; mg, milligrams; m, meters; nm, nanometers; ex, excitation; em, emission; –, not applicable]

Parameter Description Calculated Units

Specific Ultraviolet Absorbance 

(SUVA)1

Positively correlated with aromatic carbon content. Absorbance at 254 nm normalized to DOC 

concentration

(L/mg-m)1

Spectral Slope (SX-Y) 2,3,4,5 Higher values indicate low molecular weight DOM and (or) 

decreasing aromaticity; lower values indicate DOM with a higher molecular weight and (or) higher aromatic content.

Nonlinear fit (exponential function) of the 

absorption spectrum over a specified wavelength range from X to Y nm. 

nm-1

Spectral Slope Ratio (SR) 4,6 Positively correlated to DOM molecular weight; generally increases 

upon irradiation.

Ratio of spectral slopes: S275-295 divided by 

S350-400

–

Fluorescence Index (FI) 7,8 Higher values associated with microbial sources such as extracellular 

release and leachate from bacteria and algae; lower values associated with terrestrially-derived soil and plant organic matter; typical values range from 1.3–1.9.

The ratio of em wavelengths at 470 nm to 

520 nm, obtained at ex 370 nm

–

Humification Index (HIX) 9,10,11 Indicator of humic substance content or extent of humification; based 

on the idea that the emission spectra of fluorescing molecules will shift toward longer wavelengths due to lower H:C ratios as humification of DOM proceeds.

Area under the em spectra 435–480 nm divided 

by the peak area 300–345 nm, at ex 254 nm

–

1Weishaar and others (2003).2Blough and Del Vecchio (2002).3Obernosterer and Benner (2004).4Helms and others (2008).5Twardowski and others (2004).6Spencer and others (2009).7McKnight and others (2001).8Cory and McKnight (2005).9Zsolnay and others (1999).10Ohno (2002).11Fellman and others (2010).
Introduction 11

FI values obtained in the laboratory typically range from about 

1.3 to 1.9; lower FI values are associated with terrestrial soil 

and plant organic matter—highly processed material having 

greater aromatic content and higher molecular weight—

while higher FI values are associated with lower molecular 

weights and lower aromatic content indicative of algal and 

microbial sources (McKnight and others, 2001; Cory and 

others, 2010). Qualitative information can also be derived 

from the identification of fluorescence regions that have been 

linked to different DOM pools such as humic and fulvic 

acids, protein like substances, and phytoplankton-derived 

material (Stedmon and others, 2003; Coble, 2007; Hudson and 

others, 2007). 

In-Situ Fluorometers as Proxies for Dissolved 

Organic Carbon and Disinfection By-Product 

Precursor Concentrations

The use of fluorescence around ex 370/em 460 nm has 

been shown to have similar, possibly better, predictive ability 

for DOC concentration compared to absorbance measurements 

(Nakajima and others, 2002; Coble, 2007; Kraus and others, 

2010). The continuous measurement of fluorescing DOM 

(FDOM) with in-situ fluorometers has been successfully used 

to provide a high-resolution proxy for DOC concentration 

(Downing and others, 2009; Saraceno and others, 2009; 

Pellerin and others, 2012). However, because only a subset 

of the DOM pool fluoresces and this fluorescing pool is a 

function of DOM composition, the relation between DOC 

concentration and FDOM needs to be validated for each 

watershed over the complete range of riverine conditions. The 

effects of optical density, DOC concentration, temperature, 

and turbidity on FDOM also need to be evaluated and 

accounted for (Lakowicz, 2006; Downing and others, 2012). 

Because DBP precursors are a sub-set of the bulk carbon 

pool, FDOM may also serve as a good proxy for THM and 

HAA precursor concentrations and, thus, for finished-water 

THM and HAA concentrations. However, the composition 

of the DOM pool can affect this relation because it affects 

the fraction of the DOC pool that reacts to form DBPs and 

the fraction of the DOM pool that fluoresces. Currently, the 

information regarding the relation between FDOM and DBP 

formation is limited (but see Nakajima and others, 2002; Hua 

and others, 2007, 2010; Beggs and others, 2009; Marhaba 

and others, 2009; Kraus and others, 2010). Given that the 

optically-active aromatic fraction of DOM typically dominates 

the DBP precursor pool in terrestrial environments, there is 

good reason to believe that in most cases there is a strong 

relation between these constituents. Furthermore, data from 

the McKenzie River in Oregon, a similar Cascade Mountain 

drainage, showed FDOM was a better predictor of THMFP 

and HAAFP than DOC concentration, suggesting a strong 

overlap between DOM moieties that are fluorescent and react 

with chlorine to form DBPs (Kraus and others, 2010).

To date, commercially available fluorometers intended 

to assess DOM dynamics are centered near ex 370/em 

460 nm, referred to as “Peak C,” a humic region of the 

excitation‐emission (fluorescence) matrix (EEM) commonly 

identified in surface waters. While measurement of a single 

ex/em pair provides information about DOC concentration, 

information about the composition of the DOM pool is 

currently only available using bench-top fluorometers; sample 

scans using these instruments produce nearly 2,300 ex/em 

pairs, which are depicted in the EEM diagrams (fig. 5). 

Ongoing developments in light-emitting diode (LED) 

manufacturing technology have resulted in the availability 

of light sources with lower excitation wavelengths into the 

deep ultraviolet spectrum. Pairing the nearly monochromatic 

output of these LEDs with a wide array of optical filters 

makes novel development of miniaturized in-situ fluorometers 

with different excitation/emission pairs possible. Instruments 

designed using narrow band-pass filters allow for more 

focused emission spectra and, thus, also a more specific 

emission–fluorescence signal around a narrower excitation/

emission region. Sensors designed to measure different 

regions of EEMs can provide signal ratios that may indicate 

carbon composition changes due to varying proportions 

of fluorescence associated with different pools of organic 

matter (humic peaks compared to amino-acid-like peaks, for 

example). When deployed continuously and in real-time, these 

sensors present new opportunities to gain insights about how 

rivers function and identify what factors affect water quality 

and source-water supplies, especially when a multitude of 

constituents are measured simultaneously. 

Recent work in an agricultural watershed demonstrated 

the in-situ FDOM sensor accurately predicted DOC 

concentrations throughout a precipitation and watershed 

runoff event, where DOC concentration cycled from a 

baseline of 2 mg/L to a peak value of 10 mg/L and back again 

to base‐flow levels (Saraceno and others, 2009). Several 

recent studies from a range of environments including 

wetlands, tidal marshes, and forested watersheds produced 

predictive relations between in-situ FDOM values from 

a WET LabsTM colored DOM (CDOM) fluorometer and 

laboratory-determined DOC concentrations (Downing and 

others, 2008, 2009; Bergamaschi and others, 2012; Pellerin 

and others, 2012). This Clackamas study is, to our knowledge, 

the first to deploy a continuously-operated multi‐channel 

in-situ FDOM sensor within a drinking-water treatment plant 

intake with simultaneous periodic measurement of DBPs in 

finished water. 
12 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Study Objectives and Approach

The five main study objectives were to (1) characterize 

the seasonal quantity and quality of organic carbon in the 

Clackamas River and its primary tributaries; (2) relate the 

amount and composition of organic carbon to the formation 

of DBPs at sites throughout the watershed and in finished 

drinking water; (3) evaluate the major suspected sources 

of DBP precursors in the watershed, including tributaries, 

North Fork Reservoir, litter/soils, and algae; (4) assess 

the use of optical properties, including in-situ FDOM, for 

estimating DOC and DBP precursor concentrations; and 

(5) assess the treatability of DOC and DBP precursors by 

conducting “jar-test” experiments at one of the drinking-water 

treatment plants. 

The approach consisted of (1) continuous in-situ 

fluorescence monitoring of source water at the CRW DWTP 

in the lower river from April 2010 to September 2011; 

(2) monthly water sampling at four main-stem sites (table 2) 

for DOC, TPC and total particulate nitrogen (TPN), total 

and dissolved nutrients, and optical properties including 

absorbance and fluorescence (table 3); (3) four basin-wide 

surveys: spring high flow, summer base-flow period, 

late‐summer reservoir drawdown, and the first “initial” autumn 

storm (table 4); and (4) four treatability tests on source water 

using standard jar-tests procedures to evaluate the potential 

for coagulant and powdered activated carbon (PAC) to reduce 

DOC and DBP precursor concentrations. During each basinwide survey, DBP formation potentials (DBPFP) for THMs 

(THMFPs) and HAAs (HAAFPs) were measured to determine 

DBP precursor concentrations in surface water in the major 

lower-basin tributaries, North Fork Reservoir at the log boom, 

and the Clackamas River mainstem. tac12-0779_fig05

D

C

FDOM

A

M

N

T

B

Fluorescence intensity, in Raman units

0.12

0.10

0.08

0.06

0.04

0.02

600

550

500

450

400

350

300

280 320 340 360 380 400 420 440

Excitation, in nanometers

Emission, in nanometers

240 260 300

Figure 5. Example excitation–emission matrix showing the general locations of selected fluorescence peaks. 

(The contour plot is made up of 2,291 unique excitation-emission (ex/em) pairs. Individual letters indicate the 

location of common peaks associated with different pools of dissolved organic matter, which are described 

in figure 21. FDOM indicates the region measured with standard in-situ sensors. Example shown is from a 

source-water sample collected from the Clackamas River Water DWTP on April 4, 2010.)
Study Objectives and Approach 13

Table 2. Description of continuous monitors and sites sampled during monthly sampling and basin-wide surveys, Clackamas River basin, Oregon.[Sampling sites–short name: Site locations are shown in figure 1. Abbreviations: NWIS, National Water Information System; CRW, Clackamas River Water; LO, City of Lake Oswego; DWTP, drinking-water treatment plant; USFS, U.S. Forest Service; ~, approximately; –, not applicable]

Sampling activity

Sampling sites—

short name

River 

mile NWIS site No. NWIS site name Dominant land cover Monthly Basin-wide surveys Continuous water-quality monitor Streamflow gage

Main-stem Clackamas River / CRW / LO DWTP sites

X Clackamas River at Two Rivers 

Campground

450156122033100 Upper Clackamas River at Two 

Rivers C.G., OR

Forested (fir-hemlock dominant); 

USFS land

X Clackamas River agove Three Lynx 

Creek

47.8 14209500 Clackamas River above Three 

Lynx Creek, OR

Forested (fir-hemlock dominant); 

USFS land

X X X Clackamas River at Carter Bridge 40.8 14209710 Clackamas River at Carter 

Bridge, near Estacada, OR

Forested (fir-hemlock dominant); 

USFS land

X X X X Clackamas River at Estacada 23.1 14210000 Clackamas River at Estacada, 

OR

Mix of forest, rural residential, and 

urban

X Clackamas River at Barton 13.4 452255122244500 Clackamas River near Barton 

(QW site), OR

Mix of forest, rural residential, and 

urban

X Clackamas River at Carver 7.9 452335122294500 Clackamas River at Carver, OR Mix of forest, rural residential, and 

urban

X X X CRW DWTP–Source water 3.1 14211005 Clackamas River at Clackamas, 

OR (source)

Mix of forest, rural residential, 

urban, and industrial

X X CRW DWTP–Finished water – 14211006 Clackamas River at Clackamas, 

OR (finished)

Mix of forest, rural residential, 

urban, and industrial

X X Clackamas River near Oregon City 1.6 14211010 Clackamas River near Oregon 

City, OR

Mix of forest, rural residential, 

urban, and industrial

X X LO DWTP–Source water 0.9 14211023 Clackamas River at Gladstone, 

OR (source)

Mix of forest, rural residential, 

urban, and industrial

X X LO DWTP–Finished water – 14211024 Clackamas River at West Linn, 

OR (finished)

Mix of forest, rural residential, 

urban, and industrial
14 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water Sampling activity Sampling sites— short name River mile NWIS site No. NWIS site name Dominant land cover Monthly Basin-wide surveys Continuous water-quality monitor Streamflow gage Tributary sites

X Oak Grove Fork near Government 

Camp

15.5 14208700 Oak Grove Fork near 

Government Camp, OR

Forested (fir-hemlock dominant); 

USFS land

X Oak Grove Fork above powerplant 

intake

6.1 14209000 Oak Grove Fork above 

powerplant intake, OR

Forested (fir-hemlock dominant); 

USFS land

X Eagle Creek 16.7 14210150 Eagle Creek at Bonnie Lure 

State Park, OR

Forested (alder-fir dominant), 

regrowth forests/Christmas trees, rural residential

X Deep Creek 12.1 452340122251000 Deep Creek at Hwy 224, OR Agricultural (nurseries, Christmas 

trees, berries, other crops); regrowth forests

X Clear Creek 8 14210750 Clear Creek at Carver, OR Forested (alder-fir dominant) and 

rural residential, Christmas trees; regrowth forests

X Rock Creek 6.4 14210850 Rock Creek near Carver, OR Agricultural (nurseries, berries), 

rural residential

X Sieben Creek 5.8 14210900 Sieben Creek at Hwy 224, OR Residential, industrial

North Fork Reservoir sites

X 1 - Surface (0.5 foot depth) 31.2 451438122164001 North Fork Res at Boom 

Station 1 (Depth 1), OR

Forested (fir-hemlock dominant)

X 2 - Metalimnion (mid-depth) 31.2 451438122164002 North Fork Res at Boom 

Station 1 (Depth 2), OR

Forested (fir-hemlock dominant)

X 3 - Release depth (~80 feet) 31.2 451438122164003 North Fork Res at Boom 

Station 1 (Depth 3), OR

Forested (fir-hemlock dominant)

X 4 - Hypolimnion (off bottom) 31.2 451438122164004 North Fork Res at Boom 

Station 1 (Depth 4), OR

Forested (fir-hemlock dominant)

Table 2. Description of continuous monitors and sites sampled during monthly sampling and basin-wide surveys, Clackamas River basin, Oregon, 2010–11.—Continued[Sampling sites–short name: Site locations are shown in figure 1. Abbreviations: NWIS, National Water Information System; CRW, Clackamas River Water; LO, City of Lake Oswego; DWTP, drinkingwater treatment plant; USFS, U.S. Forest Service; ~, approximately; –, not applicable]
Study Objectives and Approach 15

Table 3. Field and laboratory data collected during monthly samplings and basin-wide surveys, Clackamas River basin, Oregon, 

2010–11.

[Sampling site locations are shown in figure 1. Constituents defined in table 6. Abbreviations: TPN, total particulate nitrogen; TPC, total particulate carbon; chla, chlorophyll-a; CRW, Clackamas River Water; DOC, dissolved organic carbon; DWTP; LO, City of Lake Oswego DWTP; DWTP, drinking-water treatment 

plant; DBP, disinfection by-product; FP, formation potential; RM, river mile; ~, approximately]

Sampling activity

Field parameters Total nutrientsTPN / TPCDOCDissolved nutrientsOptical properties 2

Water-column chl-aBenthic chl-aDBPFP unfiltered 3

DBPFP filtered 3DBPs 4

Monthly

Basinwide 

surveys

Targeted 

DBP 

sources1

Sampling sites

Main-stem Clackamas River / DWTP sites5

X X Clackamas River at Carter Bridge X X X X X X X X X X

X X Clackamas River at Estacada X X X X X X X X X X

X Clackamas River at Barton Bridge X X X X X X X X X X

X Clackamas River at Carver Bridge X X X X X X X X X X

X X CRW DWTP–Source water X X X X X X X X X

X X CRW DWTP–Finished water X X X

X X LO DWTP–Source water X X X X X

X X LO DWTP–Finished water X X X

Tributary sites

 X Eagle Creek X X X X X X X X X X

X Deep Creek X X X X X X X X X X

X Clear Creek X X X X X X X X X X

X Rock Creek X X X X X X X X X X

X Sieben Creek X X X X X X X X X X

North Fork Reservoir sites

X 1 - Surface (0.5 foot depth) X X X X X X X X X

X 2 - Mid-depth (mid-depth) X X X X X X X X X

X 3 - Release depth (~80 feet) X X X X X X X X X

X 4 - Near bottom (off bottom) X X X X X X X X X

Upper Clackamas River/Forest

X Clackamas River upstream from Carter Bridge X X X X X X

X Alder Flat Trail X X X X X X

1

 Includes four types of benthic algae (periphyton) and water extracts from two types of forest leaf litter/soils (Douglas fir and Red alder).

2

 Optical properties include absorbance and fluorescence.

3

 DBPFPs include determination of THMs and HAAs.

4

 DBPs, DOC, and optical properties were analyzed in finished water during monthly samplings and basin-wide surveys.

5

 U.S. Geological Survey operates three continuous water-quality monitors in the Clackamas River (fig. 1), including water temperature, specific conductance, 

dissolved oxygen, pH, turbidity (all stations), and water-column chlorophyll-a (at Estacada [RM 23.1] and Oregon City [RM 1.6] only). Data from the Oregon 

City station were used to indicate quality of source water for both DWTPs.
16 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Study Site Descriptions

The study sites (tables 2 and 3) included 7 in the 

mainstem: three sites in the upper basin including North Fork 

Reservoir at the log boom, and 5 sites in the lower basin, 

including two DWTPs that obtain source water from the 

river in the lower 3 miles (fig. 1). Five tributaries that drain 

a range in land cover, from mostly forested to mostly urban, 

also were sampled. There are two major reservoirs in the 

basin—Timothy Lake in the headwaters of the Oak Grove 

Fork and North Fork Reservoir on the middle mainstem. 

Timothy Lake is typically maintained at full pool during 

most of summer, and then drawn down in September to 

accommodate autumn and winter precipitation and to augment 

flows during the low‐flow period. While no sampling occurred 

at Timothy Lake, North Fork Reservoir was sampled each 

year during summer blooms of blue-green algae. In contrast 

to Timothy Lake, North Fork is operated as a run-of-theriver reservoir, along with two other downstream diversions 

and dams that serve Faraday and River Mill hydroelectric 

facilities. Residence times are thus typically short for North 

Fork (on average, hours up to approximately 7 days, possibly 

longer depending on temperature stratification) compared 

to Timothy Lake (on average, approximately 8.5 months). 

More information on these reservoirs and the associated 

hydroelectric project is presented in Carpenter (2003).

Discrete Sampling

Discrete water samples were collected from within the 

watershed and at the DWTPs during each monthly and all 

basin-wide and reservoir samplings (table 3). Seven additional 

discrete water samples were collected at the CRW DWTP 

intake over a range in flow/turbidity conditions and analyzed 

for DOC concentration, absorbance, and fluorescence to 

compare laboratory measurements to the in-situ FDOM 

sensor response.

Watershed Samples

Water samples were collected from wadeable stream 

sites (table 2) using the Equal-Width Increment method, 

where the entire stream cross section was sampled using a 

depth-integrating sampler (Edwards and Glysson, 1999). 

Water samples were collected from unwadable main-stem 

Table 4. Summary of data collection including continuous dissolved organic matter fluorescence deployments and discrete watersample collection in the Clackamas River basin, Oregon, 2010–11.

[Sampling date(s): Range of dates is shown in parentheses. Abbreviation: FDOM, fluorescing dissolved organic matter]

Sampling

date(s)

FDOM sensor Sampling activity

WET LabsTM Flow condition Description

WETStar

Turner 

DesignsTM 

Cyclops

Monthly Basin-wide 

surveys

Reservoir 

sampling

04-14-2010 X X Medium high Springtime melt-runoff

05-(11–12)-2010 X X X Medium high

06-05-2010 X X Very high

07-08-2010 X X Medium low

08-03-2010 X X X Low Algal bloom in North Fork Reservoir

09-(03–07)-2010 X X X Low-medium low

10-10-2010 X X X Medium low Initial autumn storm

11-01-2010 X X Medium Major flush storm event

12-07-2010 X X Medium

01-18-2011 X X Very high

02-16-2011 X X Medium high

03-16-2011 X X X High

04-19-2011 X X X Medium high

05-24-2011 X X X Medium high

06-29-2011 X X X Medium Periphyton sloughing

08-02-2011 X X X Medium low Algal bloom in North Fork Reservoir

09-(08–09)-2011 X X X X X Medium low Summer base flow

09-(16–22)-2011 X X X Medium low Timothy Lake drawdown

October 2011 X Medium

November 2011 X Medium

December 2011 X Medium low
Study Objectives and Approach 17

sites using a D-74 sampler in mid-stream (at bridge sites). 

Samples from non-wadable, bridgeless sites were collected as 

grab samples from the main flow directly into 1-L combusted 

amber glass bottles. Point samples from each site were 

composited and homogenized in a 16-L TeflonTM churn 

splitter and placed on ice prior to dispensing subsamples 

for nutrients, DOC, optical properties, DBPFPs, and 

water-column chlorophyll-a. Samples for DOC and optical 

properties (absorbance and fluorescence) were filtered using 

25-mm, 0.7-μm precombusted WhatmanTM GF/F filters with a 

glass filtration unit. The filtrate was collected into combusted 

amber-glass bottles with TeflonTM-lined caps and stored in the 

dark at 4°C until analyzed. All samples were analyzed within 

5 days of collection.

Dissolved nutrient samples were filtered through a PallTM 

0.45 μm pore-size capsule filter. Total and dissolved nutrient 

samples were immediately frozen and stored at -20°C until 

analysis. Whole-water samples for total phosphorus and total 

nitrogen were obtained from the churn splitter. Samples for 

TPN and TPC were dispensed from the churn splitter into 

125-mL baked amber-glass bottles. The contents were filtered 

using precombusted 25-mm WhatmanTM GF/F filters (0.7-

μm pore size), noting the filtrate volume. Filters were frozen 

and shipped to the USGS National Water-Quality Laboratory 

(NWQL) in Denver, Colorado, for analysis. A known volume 

of sample water was filtered for water-column chlorophyll-a

using the same GF/F filters as above. Filters were wrapped in 

aluminum foil, frozen at -4°C, and analyzed within 30 days at 

the Oregon Graduate Institute in Portland, Oregon. Samples 

for DBPFP were collected into 1-L baked amber glass bottles 

and placed on ice. Formation potentials were evaluated on 

filtered and unfiltered samples. Filtered samples were passed 

through 142-mm GF/F filters (0.7 μm pore size) into 1-L 

baked amber glass bottles. DBPFP samples were acidified 

to a pH of 2.0 units using reagent-grade concentrated HCl 

to prevent possible microbial transformations in the samples 

prior to analysis. Samples were shipped on ice within 24 hours 

to the organic chemistry laboratory at the USGS California 

Water Science Center (CAWSC) in Sacramento, California, 

where they were refrigerated at 4°C until further processing. 

Source-Water and Finished-Water Samples

Raw intake “source” water was collected from both 

DWTPs. At the CRW DWTP, source-water samples were 

collected adjacent to the FDOM sensor inside the gravity-fed 

vault adjacent to the river, where water enters before being 

pumped up to the treatment plant. Samples at the LO DWTP 

were collected from taps inside the treatment plant. Treated or 

“finished” water was collected 90 minutes after source-water 

sampling to account for time-of-travel through each plant. 

Because DBPs commonly continue to form within the 

distribution system, on one occasion, as part of the regular 

monthly sampling, an extra DBP sample was collected from 

within the distribution systems of both water utilities, at the 

location where historical maximum DBP concentrations have 

occurred. These samples were collected on November 2, 2010, 

1 day following the collection of the regular finished-water 

samples to account for time-of-travel.

Finished-water samples for determination of DBPs were 

collected by CRW and LO personnel into baked amber glass 

bottles with a TeflonTM septa to help remove bubbles. Bottles 

contained a quenching agent (65 mg sodium thiosulfate for 

THMs, final concentration 1.5 g/L, and 150 mg ammonium 

chloride for HAAs, final concentration of 1.18 mg/L) to fully 

oxidize the residual chlorine and stop further DBP formation. 

THM samples were collected directly into 40-mL baked amber 

glass vials used for collection of volatile organic compounds, 

using care to slowly and completely fill each vial to prevent 

bubble formation. HAA samples were collected into 125-mL 

baked amber glass bottles. Sample collection and processing 

of other source- and finished-water samples were conducted as 

described above for watershed samples.

Benthic Algae (Periphyton)

Periphyton samples were collected for biomass 

(chlorophyll-a) and dominant species composition at 

four main-stem sites and five tributary sites in July to 

September 2010. For the tributaries, samples were collected 

from 10 representative rocks/cobbles in shallow riffle areas; in 

the mainstem, samples were collected along wadeable portions 

of the channel margin. All samples were collected using USGS 

methods (Moulton and others, 2002) described in detail in 

Carpenter (2003). A known area of periphyton (approximately 

4-5-in. diameter circle) was scraped from the top of each rock 

into a plastic dishpan using a plastic bristle brush. Multiple 

samples were composited and transferred to plastic 1-L bottles 

and placed on ice until processing (described below).

Laboratory Analytical Procedures

Organic Carbon

DOC concentrations were measured in duplicate using 

the platinum catalyzed persulfate wet-oxidation method on 

an O.I. Analytical Model 700 TOC AnalyzerTM (Aiken and 

others, 1992), which produced standard errors of ±0.2 mg/L 

carbon. The standard curve, consisting of a minimum of five 

standards over the range of interest, was repeated for every 

10–12 water samples; reported values are the average of two 

duplicate measurements on each sample. 

TPC samples were collected and analyzed using USEPA 

method 440.0. Particles from a known volume of water were 

filtered onto 25-mm GF/F filters (0.7-μm pore size), which 

were frozen at -4°C and shipped to the USGS NWQL for 

analysis. TOC was calculated as the sum of DOC and TPC.
18 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Absorbance and Fluorescence

The absorption spectra was measured between 200 and 

750 nm on filtered samples at constant 25°C temperature 

with a J&M TIDASTM spectrophotometer, using a 1-cm 

quartz cell and distilled water for the blank. SUVA was 

calculated by dividing UVA254 by DOC concentration and 

is reported in L/mg-m units (Weishaar and others, 2003). 

Spectral slopes were calculated using a non-linear fit of 

an exponential function to the absorption spectrum over 

specified wavelength ranges (275–295 nm and 350-400 nm, 

for example) as described by Twardowski and others (2004). 

The spectral slope ratio (SR) was calculated as the ratio of 

S275–295 to S350–400 (Helms and others, 2008) (table 1). Because 

of the marked decrease in UVA absorbing DOM following 

treatment, an exponential fit could not be applied to the 

absorbance curve; thus, spectral slope was not calculated for 

finished‐water samples.

Fluorescence EEMs were measured on filtered samples 

in a 1-cm cuvette at 20°C with a SPEX Fluoromax–4 

spectrofluorometer (Horiba Jobin Yvon, Edison, New 

Jersey) using a 150W XenonTM lamp, a 5-nm band pass, 

and 0.05-second integration time. Fluorescence intensity 

was measured at excitation wavelengths of 240 to 450 nm at 

10-nm intervals and emission wavelengths of 300 to 600 nm 

at 2-nm intervals on room-temperature samples (25°C) 

in a 1-cm quartz cell. The resulting matrix consisted of 

2,291 individual ex–em pairs that form the basis of the EEM 

diagrams. EEMs were blank corrected, instrument corrected, 

and normalized to the daily water Raman peak area, and the 

Rayleigh scatter lines were removed. The FI was calculated 

as the ratio of emissions at 470 to 520 nm at an excitation of 

370 nm (McKnight and others, 2001; Cory and others, 2010). 

The Humic Index (HIX) was calculated by dividing the sum of 

fluorescence intensities at emission 436–480 nm by emission 

300–346 nm at excitation 254 nm (Zsolnay and others, 1999). 

Disinfection By-Product Formation Potentials

DBPFP was determined on filtered and unfiltered 

samples to determine the relative importance of the dissolved 

and particulate fractions. Both THMFP and HAAFPs 

were determined following a version of USEPA Methods 

502.2, 510.1, and 552.2 as described by Crepeau and others 

(2004). Briefly, the method involved a 7-day reaction time, 

pH buffered at 8.3, temperature held at 25°C, and final, 

residual-free chlorine concentration restricted to between 

2–5 mg/L. This incubation period provides information on the 

total DBP precursor pool and should result in concentrations 

of THM and HAA reflective of potential distribution-system 

concentrations based on residence times within the systems. 

For consistency, the same quenching agents used for the 

finished-water samples were used to quench the chlorination 

reaction—sodium thiosulfate for THMFP and ammonium 

chloride for HAAFP. However, instead of using solid material, 

quenching agents were dissolved in deionized water and added 

to the reaction vials (approximately 3 μL/mL, 0.3 percent 

by volume) to obtain equivalent concentrations to those 

used for the finished-water sample vials. Chlorine dosing 

and quenching were conducted at the USGS Laboratory at 

the CAWSC in Sacramento, California. Determination of 

four THMs and five HAAs was then performed by Alexin 

Laboratories as described below.

For quality-assurance (QA) purposes, one blank and 

seven samples of standard reference material (SRM) were 

submitted to the laboratory for analysis. As described by 

USEPA Method 5710B, a freshly prepared solution of 

anhydrous 3,5-dihydroxy-benzoic acid (DHBA, 0.039 g/L) 

was made to test the precision of laboratory chlorine 

dosing, quenching, storage, and DBP analysis. According 

to this method, a 7-day reaction period should result in 

approximately 0.119 mg/L THM as chloroform with 

essentially no contribution from bromide-containing THMs. 

With the exception of the first batch of samples sent to Alexin 

Laboratories, each batch of samples submitted to the lab 

contained at least one DHBA SRM sample. 

Disinfection By-Product Concentrations in 

Finished Water

DBP analyses were performed on finished-water 

and formation-potential samples at Alexin Laboratories 

(ORELAP Certification ID# OR100013) in Tigard, Oregon. 

Analyses included four THMs (chloroform, Cl3CH; 

bromoform, Br3CH; bromodichloromethane, Cl2BrCH; 

and dibromochloromethane, ClBr2CH) and five HAAs 

(monochloroacetic acid, MCAA; dichloroacetic acid, DCAA; 

trichloroacetic acid, TCAA; bromoacetic acid, BrAA; and 

dibromoacetic acid, Br2AA) following the same methods 

used for DWTP compliance monitoring—USEPA method 

524.2 for THMs and method 6251B for HAAs. For this study, 

samples were analyzed within 2–14 days for THMs and within 

5–7 days for HAAs.

In this report, the sum of individual THMs (THM4) 

and HAAs (HAA5) is commonly used because these metrics 

form the basis of the DBP drinking-water regulations. 

Benchmark quotients (BQs), the ratio of the concentration in 

finished water divided by the maximum contaminant level 

(MCL), were calculated to compare DBP concentrations to 

USEPA standards. For example, BQ values of 0.5 and 1.0 

would indicate concentrations at one-half the MCL and at 

the MCL, respectively. Although BQs provide an indication 

of how close individual concentrations are to the standards, 

the actual standards are based on the annual running 

averages in quarterly sampling at the DWTP and within the 

distribution system. 

“Specific” THMFPs and HAAFPs (STHMFPs and 

SHAAFPs) values were calculated for filtered and unfiltered 

water by dividing formation potentials by sample DOC 

or TOC concentration, respectively, and are reported in 

milligrams of DBP (THMs or HAAs) per milligram of carbon. 
Study Objectives and Approach 19

Some studies refer to these carbon-normalized formation 

potentials as DBP “yields” (Summers and others, 1996). This 

expression indicates the average reactivity of carbon in a water 

sample to form DBPs during chlorination.

Water-Column Chlorophyll-a

Water-column chlorophyll-a measurements were 

conducted at Oregon Health and Science University using a 

Turner 10-AU fluorometer according to the manufacturer’s 

procedure. A known amount of whole water was filtered 

through 0.7-μm GF/F filters. The filters were stored at -20°C 

for no more than 30 days prior to analysis. The filters were 

steeped in 5-mL of 90 percent acetone for 24 hours and stored 

at -20°C prior to analysis. The fluorometer was zeroed with 

90 percent acetone, and standard curves were generated. 

Standard- and regular-sample fluorescence was measured 

before and after addition of 3 drops of 10 percent HCL. 

Values were “blank corrected” by subtracting the background 

fluorescence of the 90 percent acetone solution. Data 

were entered into the manufacturer’s equation to calculate 

chlorophyll-a concentrations.

Benthic Algal Chlorophyll-a and General 

Species Composition

In the laboratory, periphyton samples were homogenized 

in an electric blender and transferred to an 8-L churn splitter; 

subsamples were removed using a 5-mL pipette. Known 

aliquots were transferred onto 0.7-μm 45-mm GF/F filters 

under a mild vacuum. Each filter was folded into quarters, 

placed in a plastic petri dish, wrapped in aluminum foil, and 

frozen at -4°C until analysis. Filters were hand ground in 

90 percent acetone to extract the chlorophyll-a pigment, and 

samples were analyzed using standard methods (fluorometry 

with acid correction) at the Oregon Water Science Center 

(ORWSC). A Certified chlorophyll-a standard solution from 

the USGS NWQL was used to generate standard curves 

to extrapolate sample concentrations. Samples also were 

analyzed for dominant species composition at the ORWSC 

using a Leica microscope and current taxonomic references.

Nutrients

Dissolved nutrient analyses for nitrate, nitrite, soluble 

reactive phosphate, ammonium, and silicate were performed 

at Oregon Health and Science University using a 5-channel 

2008 model Astoria-Pacific Segmented Continuous Flow 

Injection Analyzer designed for spectrophotometric analysis of 

nutrients in freshwater. In order to attain low detection limits, 

the protocols followed the manufacturer’s recommendations 

(detailed in Whitledge and others, 1986), with adjustments 

for low-concentration detection as outlined in Sakamoto and 

others (1990). All measurements were quality controlled 

using commercially purchased standard reference samples 

approved by the USGS. Dissolved organic nitrogen, total 

dissolved nitrogen, total dissolved phosphorus, total nitrogen, 

and total phosphorus were analyzed following an alkaline 

persulfate digestion followed by the colorimetric procedures 

outlined above. TPN samples were collected and analyzed 

using USEPA method 440.0. Particles from a known volume 

of water were filtered onto 25-mm 0.7-μm GF/F filters, 

which were frozen at -4°C and shipped to the USGS NWQL 

for analysis. 

Treatability Experiments Using Jar Tests

During each of the four basin-wide sampling events, jar 

tests were conducted on CRW DWTP source water using a 

Phipps and Bird Stirrer Model 7790-400 following standard 

protocols to assess treatability, defined here as the percentage 

of the DOC and DBP precursor pool removed from solution 

by coagulation. Water was collected from the intake vault and 

composited in an 18-L TeflonTM churn splitter, and subsamples 

were dispensed into test jars. Jar tests were designed to 

simulate the typical treatment at the CRW DWTP to remove 

suspended particles, primarily the addition of aluminum 

sulfate (alum) and aluminum chlorhydrate (ACH). The amount 

of coagulant added was adjusted as needed to an “optimum 

dose,” defined as the amount of coagulant per liter required 

to reach the point of zero charge as determined by an in-line 

streaming current monitor. Thus, when TOC (DOC + TPC) 

increases, higher coagulant doses were typically applied. 

Because CRW chlorinates during coagulation, there is no 

opportunity within the drinking-water treatment train to isolate 

the effects of coagulation alone on DOC and DBP precursor 

removal, making laboratory jar tests a natural choice for 

helping address this question. In addition, up to 5 mg/L PAC 

may be added during treatment to control tastes and odors. 

This practice might also reduce DBPs in finished water, so 

PAC was included as one of the treatments.

Three treatments (two replicates each) were compared: 

(1) coagulants (alum and ACH) applied at optimum dose; 

(2) coagulants (alum and ACH) applied at optimum dose 

along with 5 mg/L PAC; and (3) a control that received no 

coagulation. The amount of coagulant required to remove the 

maximum amount of DOC was determined during the time 

of source-water collection by the in-line streaming current 

monitor at the plant.

The jar-test protocol loosely followed that described 

by Shin and others (2008). Briefly, 2-L samples were placed 

in each of six jars and mixed at 300 revolutions per minute 

(rpm) for 5 seconds. Coagulants and PAC were added, and 

samples were mixed at 150 rpm for 3 minutes to provide 

rapid mixing, followed by mixing at 25 rpm for 15 minutes 

to enable flocculation. Samples were allowed to settle for at 

least 15 minutes and then filtered through 0.7-μm GF/F filters 

into baked amber glass containers to generate subsamples 

for DOC, absorbance, fluorescence, and DBPFP. The pH 

of coagulated sample water was checked to confirm that 

coagulation did not lead to a substantial drop in pH (greater 

than 0.5 standard units). 
20 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

On each of the four sampling dates, to verify the 

coagulation rates used represented the optimal dose, an 

additional series of six jar tests were conducted with dosing 

rates ranging from 25 to 200 percent of the optimal dose. 

Results from those tests verified the coagulant dosages used 

were appropriate and attained maximum DOC removal 

(±0.1 mg/L, data not shown). Jar-test results were compared 

to information obtained by comparing source-water and 

finished‐water DOC concentrations and changes in optical 

properties. 

Continuous Real-Time Measurement of 

Streamflow, Field Parameters, and Fluorescent 

Dissolved Organic Matter

The existing monitoring network of five USGS 

streamgages and three continuous water-quality monitors 

(fig. 1) provided flow, water temperature, specific 

conductance, DO, pH, and turbidity data which provided 

information on river conditions during the study in near 

real-time. In addition, at two main-stem sites—Estacada and 

Oregon City—water-column chlorophyll-a fluorescence was 

also measured continuously. One additional temporary site, 

Clackamas River near Clackamas, USGS station 14211005, 

was established within the CRW DWTP intake vault to 

monitor the quality of the source water (actual pump water) 

every 30 minutes for water temperature, turbidity, and FDOM. 

FDOM measurements were made using two standard 

fluorometers: (1) a Wet LabsTM WETStar flow-through 

sensor deployed from April 4, 2010, to October 20, 2011, and 

(2) a Turner DesignsTM “open-faced” Cyclops-7 sensor (see 

photograph 3) deployed from April 14, 2011, to January 31, 

2012. In addition, three custom-built Cyclops-7 sensors were 

deployed along with the standard Cyclops-7 sensor (table 5). 

One of these sensors was designed to detect fluorescence 

around ex 270/em 340 nm, a signal associated with amino 

acid/protein-like (peak T) fluorescence and, in the Clackamas 

River, expected to indicate the presence of algal-derived 

DOM. Two additional custom sensors with relatively narrow 

band-passes centered around emissions 470 and 520 nm both 

at excitation 370 nm were used to calculate the FI that, based 

on prior studies, can indicate a shift between microbial or 

algal-derived and terrestrial-derived sources of DOM.

All four of the Turner DesignsTM Cyclops-7 fluorometers 

were mounted on a Cyclops 6 data logger equipped with a 

wiper. Water temperature, turbidity, and specific conductance 

also were measured at the CRW DWTP intake using a Yellow 

Springs Instruments, Inc., OMS multi-probe sonde. All sensors 

were housed within the CRW intake vault. The sensors were 

raised and lowered into the water in a non-reflective black 

plastic basket using a pulley system. The basket was adjusted 

to be approximately 3 ft below the water surface, which was 

typically about 10-15 ft off the bottom.

The WET LabsTM WETStar FDOM sensor was deployed 

in an unfiltered flow-through configuration using a 12-volt 

submersible pump (see Saraceno and others, 2009). One 

drawback of this configuration is that fouling of the optics 

is not mitigated through the use of a wiper; therefore, optics 

were cleaned manually every 2-4 weeks with lens paper. 

Prior to each measurement, the pump flushed the sensor with 

approximately 2–3 L of water for 10 seconds. Then while 

pumping continued, the measurements were recorded for 

30 seconds at 1 hertz to yield a set of burst data. The burst data 

were reduced to a single data point using descriptive statistics. 

Typical variation within a burst was less than 1 percent. The 

median of the last 20 samples within the burst was used in the 

final dataset. Unlike the WETStarTM, the Turner DesignsTM 

C6 was outfitted with a wiper that cleaned each sensor before 

each measurement. In addition, as with the WETStarTM, optics 

Table 5. Description of in-situ fluorescing dissolved organic matter (FDOM) fluorometers sensors deployed in the 

Clackamas River basin, Oregon, April 2010–September 2011.

[Excitation-emission wavelengths (± bandpass values) given in nanometers as full width half maximum. Abbreviation: FI, Fluorescence Index]

Sensor Excitation Emission

Primary 

fluorescence 

peak1

Abbreviation

WETLabsTM WETStar Fluorometer2 370 (±10) 460 (±120) Peak C WETStar 

TurnerTM Cyclops-7 Fluorometer3 365 (±60) 470 (±30) Peak C Standard FDOM

TurnerTM Cyclops-7 Custom Fluorometer #13 370 (±20) 470 (±20) Peak C FI-A

TurnerTM Cyclops-7 Custom Fluorometer #23 370 (±20) 520 (±20) Peak C FI-B

TurnerTM Cyclops-7 Custom Fluorometer #33 270 (±12)4 340 (±20) Peak T Peak T

1

Fluorescence peaks as commonly defined in the literature (Coble, 2007, for example).

2

Sensor deployed April 01, 2010, to October 20, 2011.

3

Sensor deployed April 14, 2011, to January 31, 2012.

4

There is no optical filter on the deep ultraviolet light-emitting diode light source.
Study Objectives and Approach 21

Photograph 3. The Turner CyclopsTM “C6”multi-sensor sonde 

deployed in the Clackamas River at the Clackamas River Water 

drinking-water treatment plant intake. (Photograph from Turner 

Designs, http://www.turnerdesigns.com/products/submersible/

c6-multi-sensor.)

Photograph 4. The data and telecommunications system used to operate 

the real-time continuous instrumentation at the Clackamas River Water 

drinking-water treatment plant intake. (Photograph by Kurt Carpenter, U.S. 

Geological Survey, May 5, 2010.)

were cleaned manually every 2-4 weeks with lens paper. 

Data reduction was performed using burst data acquired at 

a rate of one sample every 11 seconds. Real-time data were 

recorded using a Campbell ScientificTM model CR 1000 

data logger (see photograph 4) programmed for the suite of 

sensors deployed at the site. The data logger provided the 

power-distribution controls (turning sensors on and off), time 

stamp, and internal logging of all sensor data at predetermined 

sampling frequencies. The data logger was interfaced with 

a cellular modem (Sierra WirelessTM RAVEN XT-V) to 

allow remote data acquisition and troubleshooting. The data 

logger transmitted data to the USGS server every 2–4 hours 

using a Campbell ScientificTM model COM220 telephone 

modem. This system provided data in near real-time, which 

was examined using the USGS Data Grapher program (U.S. 

Geological Survey, 2011).

Because of the diel demand on water consumption within 

the service district, pumps within the intake typically turn off 

at night, even though the sensors continued to operate. For 

this reason, pump records were used to extract FDOM data 

only when the pumps were operating (a threshold of 7 Mgal/d 

or about 11 ft3/s was used to indicate when the pumps were 

on). This assured the measurements of FDOM within the 

intake vault were representative of the river and not that of 

possible impoundment effects in the intake of the river and not 

influenced by potential impoundment effects in the vault.

Water temperature and suspended particles affect 

fluorescence measurements (Zepp and others, 2004; Lakowicz, 

2006; Saraceno and others, 2009). To correct for these effects, 

experimentally-derived correction factors were applied using 

the approach outlined in Downing and others (2012). When 

available, temperature and turbidity data from the co-located 

YSI instrument were used to make the corrections; otherwise 

data were obtained from the Clackamas River at Oregon 

City site about 1.5 mi downstream. Although temperature 

and turbidity values sometimes differed between these two 

locations, the relations were linear and reasonably wellcorrelated for overlapping data (r=0.94, n=13,112 and 

r=0.99, n=15,929, respectively). Applying temperature and 

turbidity corrections to raw FDOM data resulted in about an 

additional 8–15 percent signal recovery for baseline periods 

and up to an additional 23 percent signal recovery for periods 

characterized by high-flow storm events. High turbidity 

(greater than 300 NTUs) during one January storm required 

the most corrections that increased FDOM values by as much 

as 55 percent. Temperature- and turbidity-corrected FDOM 

data were converted to quinine sulfate dihydrate equivalents 

on the basis of laboratory calibration tests for quality-control 

purposes (Downing and others, 2012; Pellerin and others, 

2012). These data were converted to DOC concentration in 

milligrams per liter using the near-linear relation between 

discrete DOC concentrations and in-situ FDOM values. 
22 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Because the effects of temperature and turbidity on 

the three custom sensors were not evaluated, corrections 

could not be applied to these data. Data from the two Peak C 

custom sensors were converted into quinine sulfate dihydrate 

equivalents using laboratory-based calibration data for each 

sensor, and the ratio of these sensors was calculated as Sensor 

FI-A/Sensor FI-B and is referred to as FIin-situ. Data from the 

Cyclops-7 Peak T sensor was also not adjusted for turbidity 

or temperature, but data were used in terms of changes 

in intensity relative to other measurements such as DOC 

and FDOM.

Statistical Analyses

Correlations and Exploratory Data Analyses

Spearman rank correlations were used to evaluate 

nonparametric relations among key constituents for various 

site groupings (mainstem, tributaries, source/finished water) 

using PRIMER-E, version 6 (Clarke and Gorley, 2006). 

Correlations were considered significant at probabilities less 

than 5 percent (p<0.05). To better understand patterns within 

the data and among groups of selected sites/samples for 

exploratory data analyses, multivariate analyses (principal 

component analysis [PCA] and non-metric dimensional 

scaling ordination [NMDS]) were performed using 

PRIMER-E. For brevity, these results are not shown.

Confidence and prediction intervals for relations between 

concentrations of HAA5 and FDOMin-situ were generated 

using SAS (SAS Institute, Inc., 2003). The regression-line 

parameters (slope and intercept) for the linear relation between 

the FDOMin-situ and laboratory-derived HAA5 concentrations 

in finished water were applied to the time‐series 

FDOMin-situ values to generate a predicted time series for 

HAA5 concentrations.

Carbon and Disinfection By-Product Formation 

Potential Loads and Yields

To quantify and better understand the carbon and DBP 

precursor sources during each of the basin-wide synoptic 

samplings, instantaneous loads were calculated by multiplying 

DOC, TOC, and DBPFP concentrations in milligrams per liter 

by streamflow in cubic feet per second, then multiplying by 

2.447 to convert the units to kilograms per day. The carbon 

and DBPFP loads were compared to those at the CRW DWTP 

to evaluate the contribution from a particular site to that 

found in source water. The instantaneous yields (in loads 

per day per square kilometer) were calculated by dividing 

the instantaneous load by the basin area, thus providing an 

estimate of the loading “intensity” at each of the sampling 

sites at one point in time. It should be stressed that this is an 

exploratory analysis to help identify watershed DBP precursor 

sources; it is not intended to convey the magnitude of DBP 

formation under actual treatment conditions, and may not 

necessarily be representative of the full range of carbon 

concentrations or DBP yields at a site.

Parallel Factor Analysis (PARAFAC)

PARAFAC was used to decompose the fluorescence 

signatures in the corrected EEMs into unique fluorescent 

groups and provide more information about the character 

of the DOM pool (Bro, 1997). PARAFAC analysis is a type 

of three-way PCA that resolves absorption and emission 

spectra of orthogonal fluorophore groups (components) and 

determines loadings (proportional to concentrations) of each 

component. The component percentages were calculated by 

dividing the component loading of individual components 

by the sum of the component loadings to reveal qualitative 

differences between samples (Andersson and Bro, 2000). 

The Stedmon and Bro (2008) PARAFAC tutorial was used 

to develop the model with Matlab 2009A using the N-way 

toolbox, version 6.1 (Bro, 1997; Andersson and Bro, 2000). 

Goodness of fit was determined by visual inspection of 

the measured, modeled, and residual (measured minus 

modeled) EEM spectra, as well as by good agreement 

between duplicates. 

PARAFAC models were validated using a combination of 

(1) outlier identification, (2) residual analysis, (3) component 

validation, and (4) replication by split-half analysis (Stedmon 

and Bro, 2008). The model suffered from difficulties in 

distinguishing weak fluorescent components from instrument 

noises in the low-excitation wavelengths between 240 and 

250 nm, which was remedied by censoring all the EEMs at 

250 nm; signals below this cut-off value were not considered. 

The model used a non-negative constraint to help alleviate 

the instrument noise and detection in samples having low 

fluorescence. Several model iterations were performed 

with different subsets of samples, mostly to examine the 

effects of the finished-water samples on the model results. 

Results indicated the finished-water samples did not contain 

significantly different components compared to the untreated 

source-water samples; thus, they were included in the model. 

This is consistent with a previous study that examined 

oxidation effects of chlorine on fluorescence (Beggs and 

others, 2009).
Overview of River Conditions During 2010–11 23

Overview of River Conditions 

During 2010–11

The study period spanned two exceptionally wet years 

with higher-than-average streamflow compared to the 

103-year period of record (fig. 6). Water temperatures in the 

lower Clackamas River at Oregon City were about 2–3°C 

lower during the first half of both summers and, in 2011, 

about 4–6°C lower in mid-July compared with the previous 

8 years (data not shown). These conditions may have limited 

the degree to which algae affected source-water quality during 

this study. Although the continuous FDOM monitor indicated 

sharp rises in DOC concentrations resulting from rainfall 

events, DOC concentrations in the mainstem were generally 

low, typically about 1.0–1.5 mg/L and occasionally up to 

about 2.5 mg/L during early-season storms. Summary statistics 

for selected water-quality, optical, and DBP data are presented 

in table 6.

The first monthly sampling was conducted in midApril 2010 (table 4), during the last snowmelt period of the 

season. The first basin-wide survey was conducted in May 

when flows were still moderately high, about 3,500 ft3/s 

at Oregon City (fig. 7). The last significant storm event 

of the water year, in early June, produced nearly 3 in. of 

rain. Samples were collected near the peak in flow when 

streamflow was about 13,000 ft3/s at Oregon City (fig. 7). 

Later that summer, a blue-green algae bloom in North Fork 

Reservoir (Anabaena flos-aquae and Microcystis aeruginosa) 

was sampled in August and again in September (table 4). 

This bloom prompted the Oregon Health Authority to issue 

a human-health recreational advisory for the reservoir 

on September 2, 2010. The second basin-wide survey on 

October 10, 2010, followed the first initial runoff-producing 

rain event in autumn. About an inch of rain fell over a 2-day 

period that produced slightly higher turbidity (about 5 FNU) in 

the lower mainstem (fig. 7). A series of larger autumn storms 

in late October delivered 3.5 in. of rain that produced the 

major “flush” of the season, mobilizing material accumulated 

in the soil profile during the dry season; this event was 

sampled on November 1, 2010 (table 4).

0

2,000

4,000

6,000

8,000

10,000

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

Streamflow, in cubic feet per second

90th percentile

75th percentile

50th percentile (median)

25th percentile

10th percentile

2010 monthly average

2011 monthly average

Period of record 1908-2011

EXPLANATION

Month

Figure 6. Relation between the average monthly streamflow in the Clackamas River at Estacada, 

Oregon, in 2010 and 2011 and the 103-year period of record (1908–2011). (Streamflow data from the USGS 

gaging station 14210000 located at RM 23.1).
24 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water Table 6. Summary statistics for select streamflow, water quality, optical properties, and disinfection by-products for the upper and lower main-stem Clackamas River, tributaries, and source water, Clackamas River basin, Oregon. [Source water includes samples from the Clackamas River Water and City of Lake Oswego drinking-water treatment plants. Abbreviations: n, number of samples; Min, minimum; Max, maximum; nm, namometers; UV, ultraviolet; DBP, disinfection by-product; FP, formation potential; F, filtered; U, unfiltered] Abbreviation Constituent definition, units Upper main stem Tributaries Lower main stem and source water Source water

n Min Median Max n Min Median Max n Min Median Max n Min Median Max

Flow Streamflow, in cubic feet per 

second

20 343 1,720 11,421 15 0 25 422 65 800 2,247 18,142 38 800 2,269 18,142

Temp Temperature, in degrees 

Celsius

20 3.8 9.3 13.4 15 8.0 14.8 19.3 62 4.7 11.2 18.4 38 5.4 9.6 18.4

SC Specifc conductance, in 

microsemens per centimeter

20 30.0 50.0 82.0 15 32.0 73.0 177.0 62 30.0 50.0 69.0 38 30.0 47.0 69.0

DO Dissolved oxygen, in 

milligrams per liter

20 10.5 11.7 13.3 15 9.6 10.3 12.1 62 8.9 11.1 13.0 38 8.9 11.4 12.7

DO% Percent saturation dissolved 

oxygen

20 101 104 106 15 96 101 111 62 94 100 112 38 95 100 107

pH pH, in standard units 20 7.2 7.7 8.0 15 6.8 7.6 8.1 62 7.3 7.6 8.6 38 7.3 7.6 7.8

Turbidity Turbidity, in formazin 

nephelometric units

19 0.1 1.7 43 15 0.2 4.1 76.5 62 0.1 1.5 68 38 0.1 1.3 68

Chla (water 

column)

Chlorophyll-a, in micrograms 

per liter

20 0.2 0.4 1.0 15 0.4 2.2 7.7 64 0.1 1.1 6.1 37 0.2 1.4 6.1

Chla (benthic) Chlorophyll-a, in milligrams 

per square meter

2 52 -- 58 10 38 157 304 8 93 201 759 -- -- -- --

SRP Dissolved phosphate, in 

milligrams per liter

19 0.010 0.018 0.021 15 0.004 0.031 0.106 64 0.004 0.013 0.030 37 0.004 0.013 0.030

TP Total phosphorus, in milligrams 

per liter

18 0.006 0.010 0.033 15 0.009 0.037 0.252 60 0.009 0.010 0.062 36 0.009 0.010 0.062

Si Dissolved silicate, in 

milligrams per liter

19 0.7 6.4 9.9 15 2.8 8.5 13.1 64 0.8 6.7 8.9 37 0.9 6.2 8.9

NO3 Dissolved nitrate plus nitrite, in 

milligrams per liter

19 0.003 0.009 0.025 15 0.059 0.587 1.728 64 0.003 0.026 0.351 37 0.008 0.054 0.351

TPN Total particulate nitrogen, 

in milligrams per liter

20 0.009 0.017 0.036 15 0.005 0.029 0.596 65 0.003 0.029 0.177 38 0.006 0.037 0.177

DOC Dissolved organic carbon, 

in milligrams per liter

20 0.6 1.1 1.9 15 1.2 2.0 7.8 65 0.8 1.2 2.3 38 0.9 1.3 2.2

TPC Total particulate carbon, 

in milligrams per liter

20 0.1 0.2 0.9 15 0.1 0.5 6.3 65 0.1 0.3 2.0 38 0.1 0.3 2.0

TOC Total organic carbon, 

in milligrams per liter

20 0.8 1.3 2.8 15 1.3 2.5 13.5 65 0.9 1.5 3.6 38 1.0 1.5 3.6

% C as POC Particulate carbon, in percent 20 7 17 33 15 5 20 47 65 5 20 55 38 10 23 55
Overview of River Conditions During 2010–11 25

Abbreviation Constituent definition, units Upper main stem Tributaries Lower main stem and source water Source water n Min Median Max n Min Median Max n Min Median Max n Min Median Max

FDOM Fluorescence FDOM, 

in water Raman units

20 0.02 0.04 0.09 15 0.05 0.14 0.49 65 0.02 0.04 0.12 38 0.03 0.05 0.11

PEAK A Fluorescence Peak A, 

in water Raman units

20 0.03 0.09 0.21 15 0.10 0.33 1.23 65 0.05 0.10 0.25 38 0.07 0.10 0.25

PEAK C Fluorescence Peak C, 

in water Raman units

20 0.02 0.05 0.11 15 0.06 0.19 0.62 65 0.03 0.05 0.14 38 0.04 0.06 0.14

PEAK M Fluorescence Peak M, 

in water Raman units

20 0.01 0.04 0.08 15 0.05 0.15 0.60 65 0.03 0.04 0.12 38 0.03 0.05 0.12

PEAK D Fluorescence Peak D, 

in water Raman units

20 0.01 0.03 0.06 15 0.03 0.08 0.30 65 0.01 0.03 0.08 38 0.02 0.03 0.07

PEAK B Fluorescence Peak B, 

in water Raman units

20 0.02 0.03 0.12 15 0.03 0.09 0.52 65 0.02 0.04 0.10 38 0.02 0.04 0.10

PEAK T Fluorescence Peak T, 

in water Raman units

20 0.02 0.04 0.10 15 0.04 0.10 0.57 65 0.02 0.04 0.14 38 0.03 0.04 0.14

PEAK N Fluorescence Peak N, 

in water Raman units

20 0.01 0.03 0.07 15 0.04 0.14 0.56 65 0.02 0.04 0.13 38 0.03 0.04 0.13

FI Fluorescence Index, unitless 20 1.33 1.38 1.46 15 1.39 1.44 1.50 65 1.31 1.38 1.45 38 1.34 1.38 1.45

HIX Humic Index, unitless 20 2.13 3.49 6.08 15 2.10 4.71 7.52 65 2.36 3.68 5.32 38 2.36 3.69 4.75

C1 Loading Component 1, in fluorescence 

maximum values

20 0.01 0.04 0.09 15 0.04 0.14 0.94 65 0.02 0.04 0.17 38 0.03 0.05 0.17

C2 Loading Component 2, in fluorescence 

maximum values

20 0.01 0.03 0.08 15 0.04 0.09 0.35 65 0.02 0.03 0.10 38 0.02 0.04 0.10

C3 Loading Component 3, in fluorescence 

maximum values

20 0.02 0.05 0.12 15 0.06 0.18 0.59 65 0.03 0.05 0.14 38 0.04 0.06 0.14

C4 Loading Component 4, in fluorescence 

maximum values

20 0.02 0.04 0.10 15 0.05 0.16 0.52 65 0.03 0.05 0.12 38 0.03 0.05 0.12

C5 Loading Component 5, in fluorescence 

maximum values

20 0.01 0.03 0.09 15 0.02 0.08 0.56 65 0.02 0.03 0.10 38 0.02 0.03 0.10

Total C Loading Total component, in 

fluorescence maximum values

20 0.08 0.18 0.45 15 0.24 0.72 2.81 65 0.13 0.21 0.57 38 0.15 0.22 0.57

Table 6. Summary statistics for select streamflow, water quality, optical properties, and disinfection by-products for the upper and lower main-stem Clackamas River, 

tributaries, and source water, Clackamas River basin, Oregon.—Continued

[Source water includes samples from the Clackamas River Water and City of Lake Oswego drinking-water treatment plants. Abbreviations: n, number of samples; Min, minimum; Max, maximum; nm, namometers; UV, ultraviolet; DBP, disinfection by-product; FP, formation potential; F, filtered; U, unfiltered]
26 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water Abbreviation Constituent definition, units Upper main stem Tributaries Lower main stem and source water Source water

n Min Median Max n Min Median Max n Min Median Max n Min Median Max

% C1 Percentage of component 1 20 13 19 22 15 16 22 41 65 17 21 33 38 18 21 33

% C2 Percentage of component 2 20 13 16 20 15 11 14 18 65 11 16 21 38 11 17 19

% C3 Percentage of component 3 20 20 26 29 15 19 25 29 65 16 25 30 38 19 26 28

% C4 Percentage of component 4 20 18 22 25 15 17 22 26 65 18 22 24 38 18 23 24

% C5 Percentage of component 5 20 8.3 17 31 15 5.4 12 30 65 5.7 16 27 38 8.2 15 23

S275-295 Spectral Slope for 

275-295 nm, in per nm

20 0.01 0.01 0.02 15 0.01 0.01 0.01 65 0.01 0.01 0.02 38 0.01 0.01 0.02

S290-350 Spectral Slope for

 290-350 nm, in per nm

20 0.01 0.01 0.02 15 0.01 0.01 0.02 65 0.01 0.01 0.03 38 0.01 0.01 0.03

S350-400 Spectral Slope for 

350-400 nm, in per nm

19 0.01 0.01 0.02 15 0.01 0.01 0.01 63 0.01 0.01 0.02 37 0.01 0.01 0.02

SR Spectral Slope Ratio, unitless 19 0.67 0.96 1.50 15 0.78 0.92 1.23 63 0.71 0.98 2.05 37 0.71 0.98 2.05

A254 Absorbance at 254 nm, 

in per centimeter

20 0.01 0.03 0.07 15 0.03 0.05 0.39 65 0.01 0.03 0.10 38 0.01 0.03 0.10

SUVA Specific UV absorbance at 

254 liters per milligramper meter 

20 1.43 2.60 4.06 15 2.00 2.88 4.03 65 1.64 2.64 4.41 38 2.00 2.73 4.41

THMFP-F DBPFP for total 

trihalomethanes, in milligrams per liter (filtered)

5 0.055 0.096 0.130 15 0.113 0.198 0.595 20 0.091 0.134 0.158 8 0.119 0.145 0.156

HAAFP-F DBPFP for total haloacetic 

acids, in milligrams per liter (filtered)

5 0.010 0.015 0.059 15 0.030 0.071 0.637 20 0.016 0.026 0.076 8 0.020 0.031 0.064

THMFP-U DBPFP for total 

trihalomethanes, in milligrams per liter (unfiltered)

5 0.063 0.083 0.123 15 0.116 0.194 0.956 20 0.096 0.148 0.237 8 0.140 0.166 0.223

HAAFP-U DBPFP for total haloacetic 

acids, in milligrams per liter (unfiltered)

5 0.013 0.038 0.096 15 0.036 0.087 0.929 20 0.021 0.040 0.154 8 0.025 0.044 0.154

Table 6. Summary statistics for select streamflow, water quality, optical properties, and disinfection by-products for the upper and lower main-stem Clackamas River, 

tributaries, and source water, Clackamas River basin, Oregon.—Continued

[Source water includes samples from the Clackamas River Water and City of Lake Oswego drinking-water treatment plants. Abbreviations: n, number of samples; Min, minimum; Max, maximum; nm, namometers; UV, ultraviolet; DBP, disinfection by-product; FP, formation potential; F, filtered; U, unfiltered]
Overview of River Conditions During 2010–11 27

tac12-0779_fig07

0

5

10

15

20

25

30

35

40

0

20

40

60

80

100

Chlorophyll-a, in micrograms per liter

Turbidity, in nephelometric units

Turbidity (30-minute)

Chlorophyll-a (30-minute)

0

0.5

1.0

1.5

2.0

2.5

3.0

Dissolved organic carbon concentation,

in milligrams per liter

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

90

95

100

105

110

115

120

125

130

pH, in standard units

Dissolved oxygen, in percent saturation

State of Oregon

pH standard

0

10,000

20,000

30,000

40,000

50,000

60,000

Streamflow, in cubic feet per second

May June July Aug. Sept.

2010 2011 2012

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

Dissolved oxygen, daily maximum

pH, daily maximum

Water samples collected

A

C

B

Dissolved organic carbon estimated

from dissolved organic matter

fluorescence (30-minute)

Dissolved organic carbon - 

laboratory measurement

Streamflow (Oregon City streamflow

gage - 14211010)

EXPLANATION

EXPLANATION

EXPLANATION

Figure 7. Time series of (A) streamflow and dissolved organic carbon, (B) turbidity and chlorophyll-a, 

and (C) dissolved oxygen and pH in the lower Clackamas River, Oregon, 2010–11.
28 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

The 2011 water year and growing season were similarly 

wet and cool; higher-than-average streamflow resulted in 

relatively low water-column concentrations of chlorophyll-a

(fig. 8). Anabaena bloomed again in September 2011, leading 

to another advisory for North Fork Reservoir and Timothy 

Lake (fig. 1) on September 8, 2011. The bloom in North Fork 

Reservoir was not as severe as in years past (Carpenter, 2003), 

although the bloom did produce enough biomass to form 

a surface scum on the reservoir and may have contributed 

to reports of tastes and odors in treated drinking water. In 

addition to the depth-profile sampling at the log boom in 

North Fork Reservoir, one surface sample containing a high 

abundance of Anabaena was collected at Promontory Park 

during the bloom (see photograph 2b). This sample represents 

DOM enriched in algal-derived carbon and served as an “end 

member” in terms of carbon characterization.

The third and fourth basin-wide surveys were conducted 

near base-flow conditions in early and mid-September, 

respectively, a low-carbon period for the river. The final 

sampling was during the annual drawdown of Timothy Lake 

(table 4). In addition to the typical suite of main-stem sites, the 

fourth survey included the main-stem site at the Two Rivers 

Campground (fig. 1), a site not influenced by the Timothy 

Lake release. Although in years past the Timothy Lake 

drawdown contributed higher concentrations of DOC, TOC, 

chlorophyll-a, and Anabaena cells to the upper mainstem 

(Carpenter, 2003), in 2011, the observed effect was limited to 

a three-fold increase in TOC at Carter Bridge that continued 

downstream to the DWTP intakes. Although discrete sampling 

ended with this last synoptic, the in-situ FDOM sensors 

continued to operate through January 2012.

Algal growth in the Clackamas River and its impact on 

diel changes in pH and DO are affected by factors such as 

streamflow, water temperature, nutrients, and availability of 

solar radiation for photosynthesis. The higher flows during 

this study, especially during springtime (fig. 6), produced 

lower-than-average water temperatures and probably also 

delayed colonization by benthic algae in faster velocity zones. 

These conditions, exacerbated by clouds and scant sunshine, 

delayed and (or) truncated the algal growing season both 

years. While much higher-than-average rainfall and prolonged 

cloudy weather in 2010 might have resulted in less solar 

radiation available for growing periphyton, field surveys 

in June 2010 revealed areas of high algal biomass, especially 

in the lower mainstem where periphyton chlorophyll-a

levels exceeded the commonly applied nuisance threshold 

of 100–150 mg/m2 (Welch and others, 1988) at all four 

main-stem sites—Estacada, Barton, Carver, and Highway 

99E (table 7). By early September, periphyton biomass was 

even higher at the tributary sites but lower at all lower-basin 

main-stem sites, possibly from slow, metered losses due to the 

scouring high flows, grazing by benthic invertebrates, or some 

other factor.

Despite the apparent reduction in benthic algal biomass 

in the lower mainstem, conditions either precluded or 

obscured the occurrence of any substantial sloughing of 

viable chlorophyll-a into the water column, or the material 

was not particularly prone to fluoresce. In fact, concentrations 

of chlorophyll-a in the water column remained low and, on 

average, were much lower compared with previous years 

(fig. 8). It is possible that higher flow, as well as higher 

water velocity, may have produced a more steady (and less 

punctuated) losses of periphyton algal particles into the 

water column.

Jan. Feb. Mar. Apr. May June July

Month

Aug. Sept. Oct. Nov. Dec.

Water-column chlorophyll-a concentration,

in micrograms per liter

0

1

2

3

4

5

6

90th percentile

75th percentile

50th percentile (median)

25th percentile

10th percentile

2010 monthly average

2011 monthly average

Period of record 2001-2011

EXPLANATION

Figure 8. Relation between the average monthly water-column chlorophyll-a concentrations in the 

Clackamas River at Oregon City, Oregon, in 2010 and 2011, and the 9-year period of record (2001–11).
Overview of River Conditions During 2010–11 29

Table 7. Benthic algal conditions in the Clackamas River and select tributaries, Oregon.

[Sampling site locations are shown in figure 1. Algal biomass given as concentration of chlorophyll-a in milligrams per square meter. Abbreviation: sp., 

species]

Sampling sites Benthic algae description

Benthic 

algal 

biomass

July–August 2010

Clackamas River at Carter Bridge Green algae (Prasiola sp., Ulothrix sp., Zygnema sp.), stalked diatoms (Cymbella 

mexicana), benthic diatoms (Epithemia sp.), and blue-green algae (Nostoc sp.)

52

Clackamas River at Estacada Stalked diatoms (Cymbella mexicana), benthic diatoms (Melosira sp. and 

Epithemia sp.)

421

Clackamas River at Barton Bridge Filamentous green algae (Cladophora sp.), stalked diatoms (Cymbella mexicana), 

benthic diatoms (Melosira, Gomphonema, Synedra, and Nitzschia sp., Epithemia

sp.), and lesser amounts of filamentous blue-green algae (Oscillatoria sp.)

251

Clackamas River at Carver Stalked diatoms (Cymbella mexicana), benthic diatoms (Melosira sp., Gomphonema 

sp.), with lesser amounts of filamentous green algae (Cladophora sp.)

759

Clackamas River upstream of 

Highway 99E

Filamentous green algae (Cladophora sp.), stalked diatoms (Cymbella mexicana), 

benthic diatoms (Melosira, Gomphonema, Synedra, and Nitzschia sp.), and red 

algae (Lemanea sp.)

342

Eagle Creek Stalked diatoms (Gomphoneis sp.), and blue-green algae (Oscillatoria sp. and 

Rivulariceaen heterocystous filaments)

47

Deep Creek Benthic diatoms (Melosira sp., Synedra sp., Navicula sp., and Cocconeis sp.) 120

Clear Creek Benthic diatoms (Melosira sp., Gomphonema sp., Synedra sp., Navicula sp., and 

Cocconeis sp.)

38

Rock Creek Benthic diatoms (Melosira sp., Rhoicosphenia sp., Synedra sp., Navicula sp., 

Nitzschia sp., and Cocconeis sp.)

181

Sieben Creek Benthic diatoms (Melosira sp., Synedra sp., Achnanthidium sp., Navicula sp., 

Nitzschia sp., and Cocconeis sp.) and green algae (Closterium sp.)

155

September 2010

Clackamas River at Carter Bridge Filamentous green algae (Ulothrix sp.), blue-green algae (Nostoc sp.), stalked 

diatoms (Cymbella mexicana), and benthic diatoms (Epithemia sp. and 

Rhopalodia sp.)

58

Clackamas River at Estacada Stalked diatoms (Cymbella mexicana), benthic diatoms (Epithemia sp., 

Synedra sp., Gompohonema sp.)

152

Clackamas River at Barton Bridge Filamentous green algae (Cladophora sp.), stalked diatoms (Cymbella mexicana), 

benthic diatoms (Melosira, Gomphonema, Synedra, and Rhopalodia sp.)

144

Clackamas River at Carver Filamentous green algae (Cladophora sp.), benthic diatoms (Melosira, 

Gomphonema, Synedra, and Rhopalodia sp.), and filamentous blue-green 

algae (Oscillatoria sp.)

125

Clackamas River upstream of 

Highway 99E

Filamentous green algae (Cladophora sp.), benthic diatoms (Melosira, 

Gomphonema, Synedra, and Cocconeis sp.), and red algae (Lemanea sp.)

93

Eagle Creek Blue-green algae (Oscillatoria sp.) and benthic diatoms (Gomphonema sp., 

Cymbella minuta, Epithemia sp.)

84

Deep Creek Benthic diatoms (Melosira sp., Synedra sp., Navicula sp.) and green algae 

(Closterium sp.)

193

Clear Creek Benthic diatoms (Melosira sp., Synedra sp., Navicula sp., and Cymbella sp.) 159

Rock Creek Benthic diatoms (Melosira sp., Synedra sp., Navicula sp., Nitzschia sp., and 

Rhoicosphenia sp.)

304

Sieben Creek Benthic diatoms (Melosira sp. and Bacillaria paradoxa) and filamentous blue-green 

algae (Oscillatoria sp.)

301
30 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Dissolved and Particulate 

Carbon Concentrations

DOC concentrations were generally low in the 

Clackamas River, typically about 1.0–1.5 mg/L; during 

storms, concentrations occasionally increased up to about 

2.5 mg/L (fig. 9). DOC concentrations were much higher 

in the tributaries during the one storm sampling in October 

2010; Rock Creek had the highest concentration (7.8 mg/L). 

Within the mainstem, carbon concentrations (DOC and 

TOC) increased downstream to the drinking-water intakes 

(fig. 10). Longitudinal increases between the Carter Bridge 

and Estacada sites, although not large, may be attributed to an 

effect of the hydroelectric project reservoirs, including North 

Fork Reservoir, but also could be caused by input from Wade 

Creek (fig. 1), which drains the city of Estacada and receives 

treated effluent from the wastewater treatment facility that 

discharges to the Clackamas River upstream from River Mill 

dam (fig. 3).

There was a strong correlation between concentrations 

of DOC and TOC (r=0.98, p<0.001). In general, the TOC 

pool was dominated by the dissolved fraction (greater than 

70 percent); there were, however, times when the particulate 

fraction (TPC) was significant, making up 30 to 40 percent 

of the TOC (fig. 10). In the tributaries, the highest percentage 

of carbon as TPC was during the October 10, 2010, initial 

storm event when turbidity also was relatively high. The 

TPC fraction of the TOC pool was also high (approximately 

40 percent) in several of the samples from North Fork 

Reservoir in 2010. In mid-March 2011, an unusually high TPC 

value in the source-water sample from the CRW DWTP intake 

(2.0 mg/L) resulted in a relatively high TOC value (3.6 mg/L) 

that was largely made up of TPC (55 percent). Given that 

just 2.2 mi downstream at the LO DWTP, TPC was 0.5 mg/L 

and TOC was 2.0 mg/L that day, this relatively high TPC 

value at CRW DWTP was likely the result of resuspension of 

particles accumulated within the intake vault rather than being 

reflective of river conditions.

Seasonal variations in streamflow had a pronounced 

effect on turbidity and carbon concentrations in the mainstem 

and tributaries and on DBPs in finished water. The DOC 

concentrations in source water were highest (2.0 to 2.6 mg/L) 

in June and November 2010 and January 2011—all samples 

affected by storm runoff (fig. 10). Carbon concentrations were 

commonly closely tied to streamflow (fig. 7) and resulted 

in a significant positive correlation between streamflow and 

DOC (r=0.82, p<0.001). The effect was, however, complex 

because higher streamflow also occasionally diluted carbon 

concentrations. The net effect of this situation is that over the 

course of a storm, or longer periods of time with successive 

storms, there is a “hysteresis effect.” Relative to streamflow, 

carbon concentrations are initially high as the storms mobilize 

organic matter from the watershed, but as the storm continues, 

concentrations are lower, relative to streamflow, because less 

carbon is flushed later, leading to lower concentrations from 

the effect of greater dilution (Pellerin and others, 2012).

Disinfection By-Product 

Concentrations in Finished 

Drinking Water

DBP concentrations in finished water from the CRW and 

LO DWTPs were similar, about 0.024±0.006 mg/L for THM4 

and 0.022±0.008 mg/L for HAA5 (fig. 11). Maximum DBP 

concentrations in finished water also were similar for the two 

classes of DBPs, about 0.04 mg/L for both THM4 and HAA5. 

Concentrations of THM4 and HAA5 for the two samples 

collected from within the distribution systems were, however, 

higher (fig. 11).

Following the winter high-flow period, streamflow in the 

Clackamas River declined through summer as the snow packs 

diminished and groundwater made up a progressively greater 

contribution to the flow in the river. This seasonal transition 

for the Clackamas River and other rivers draining the Cascade 

Range was first described by Piper (1942). The lower 

turbidity and carbon levels that resulted (fig. 7) produced 

lower concentrations of DBPs in finished water (fig. 11). 

Lower concentrations of DBPs during a time when (regional) 

groundwater dominates flow is consistent with findings from 

a USEPA study (U.S. Environmental Protection Agency, 

2005) that reported generally lower concentrations of THMs 

in finished water when source water was from groundwater 

rather than from surface-water sources.

The seasonal patterns in concentrations of THM4 and 

HAA5 and overall composition of DBPs in finished water 

also were similar at the two DWTPs. Chloroform was the 

dominant THM in finished water, making up 86–97 percent 

of the THM4 for both DWTPs; bromodichloromethane made 

up the remaining 3–14 percent. For HAAs, DCAA and TCAA 

were the primary DBPs, making up 40–60 percent each. 

MBAA was detected in finished water from both DWTPs at 

concentrations equal to the detection limit of 0.001 mg/L; no 

other DBPs were detected.

An examination of individual concentrations and 

Benchmark Quotient (BQ) values can reveal times when 

concentrations are most elevated, and how close they may 

be to existing standards. BQ values indicated no drinkingwater standards were exceeded (fig. 12). The maximum BQ 

values were somewhat higher for HAA5, which has a lower 

MCL (0.060 mg/L) than THM4 (0.080 mg/L). Drinking-water 

standards for DBPs are based on the annual running average 

of the maximum concentrations from within the distribution 

system, not the “time zero” finished-water samples collected 

from within the treatment plant. The quarterly monitoring data 

for the CRW and LO DWTPs showed similar maximum BQ 

values—about 0.35 for THM4 and 0.46 for HAA5 (Oregon 

Health Authority, 2012) as those reported here. The two 

samples collected from within the distribution system (one 

from each DWTP) had the highest BQs of 0.76 and 0.93, 

which indicate that HAA5 concentrations approached but did 

not exceed USEPA standards.
Disinfection By-Product Concentrations in Finished Drinking Water 31

tac12-0779_fig09

Dissolved organic carbon

Particulate organic carbon

EXPLANATION

Streamflow (instantaneous)

EXPLANATION

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

Streamflow, in cubic feet per second

M M T M T T M T T S S 0.1

1

10

100

1,000

10,000

Carbon concentration,

in milligrams per liter

M M T M T T M T T S S 0

1

2

3

Streamflow, in cubic feet per second

M M T M T T M T T S S

1

10

100

1,000

10,000

Carbon concentration,

in milligrams per liter

M M T M T T M T T S S

0

2

4

6

8

Streamflow, in cubic feet per second

0

1

10

100

1,000

CR at Carter Bridge

CR at Carver

CRW DWTP–

source water

LO DWTP–

CR at Estacada

CR near Barton

source water

Eagle Creek

Deep Creek

Sieben Creek

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

Carbon concentration,

in milligrams per liter

0

0.5

1.0

1.5

2.0

Streamflow, in cubic feet per second

0

200

400

600

800

1,000

1,200

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

Carbon concentration,

in milligrams per liter

0

0.2

0.4

0.6

0.8

1.0

1.2

Figure 9. Streamflow and concentrations of dissolved and particulate organic carbon in the tributaries and main-stem Clackamas 

River, Oregon, 2010–11. (Sites are listed in downstream order. Note variable y-axis scales, and log scale for y-axis in streamflow plots. 

X-axis labels for Summer base flow also apply to the Spring high flow and Initial autumn storm event. Abbreviations: M, main-stem site; 

T, tributary; S, source-water intake; mg/L, milligram per liter; CR, Clackamas River; CRW, Clackamas River Water DWTP; LO, City of Lake 

Oswego DWTP; DWTP, drinking-water treatment plant; C.G., campground.) 
32 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig10

0.5

0

1.0

1.5

2.0

2.5

3.0

Dissolved organic carbon concentation,

in milligrams per liter

May June July Aug. Sept.

2010 2011

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

A

C

B

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Total carbon, in milligrams per liter

0

10

20

30

40

50

60

Percentage of total carbon in particulate fraction

Clackamas River at Carter Bridge (RM 40.8)

North Fork Reservoir (RM 33.4)

Clackamas River at Estacada (RM 23.1)

Clackamas River near Barton (RM 13.4)

Clackamas River at Carver (RM 7.9)

CRW DWTP–source water (RM 3.1)

LO DWTP–source water (RM 0.9)

Note: RM, River Mile; CRW DWTP, Clackamas 

River Water drinking water treatment plant; LO, 

Lake Oswego. 

EXPLANATION

Figure 10. Seasonal patterns in (A) dissolved organic carbon, (B) total carbon, and (C) percentage of total 

carbon in particulate fraction in the main-stem Clackamas River, Oregon, 2010–11.
Disinfection By-Product Concentrations in Finished Drinking Water 33

Figure 11. Seasonal patterns in (A) total trihalomethanes (THM4) and (B) total haloacetic acids (HAA5) 

in finished water from the Clackamas River Water (CRW) and City of Lake Oswego (LO) drinking-water 

treatment plants, Clackamas River basin, Oregon, 2010–11.

tac12-0779_fig11 

Note: CHCl3, chloroform; CHBrCl2, bromodichloromethane; TCAA, trichloroacetic acid; DCAA, dichloroacetic acid; MBAA, monobromoacetic acid; 

CRW, Clackamas River Water; LO, City of Lake Oswego; D, distribution sample; DOC, dissolved organic carbon; THM4, trihalomethanes; HAA5, 

haloacetic acids.

A. Trihalomethanes (THM4)

B. Haloacetic acids (HAA5)

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

0.01

0.02

0.03

0.04

0.05

0.06

Dissolved organic carbon, in milligrams per liter Dissolved organic carbon, in milligrams per liter

THM4 concentration, in milligrams per liter

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0

0.01

0.02

0.03

0.04

0.05

0.06

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW-D

LO-D

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

CRW

LO

HAA5 concentration, in milligrams per liter

4–14 5–12 6–5 7–8 8–3

Date 2010

9–7 10–5 10–10 11–1-2 12–7 1–18 2–16 3–16 4–19 5–24

Date 2011

6–29 8–2 9–229–9

CHCl3

CHBrCl2

TCAA Source-water DOC

DCAA

MBAA

EXPLANATION
34 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Storm Effects on Organic Carbon Concentrations 

and Disinfection By-Products

Several large storm events during the study caused 

surface runoff (see photographs 5a-b), including one high-flow 

event in June 2010 that resulted in a peak DOC concentration 

of about 2.5 mg/L (fig. 7). In October, with the onset of 

autumn precipitation, DOC concentration increased about 

50 percent in the lower Clackamas River during the initial 

storm (fig. 13). This storm, the first significant precipitation 

event in months, caused moderate runoff that increased 

turbidity in the mainstem from 0.5 to 4.5 FNUs, more than 

doubling the TPC and TPN at the CRW DWTP intake 

(table 8). Although a shift in DOM composition was observed 

at the CRW intake during this storm, it did not produce any 

notable change in finished-water DBPs (fig. 11). 

tac12-0779_fig12 

A. Trihalomethanes (THM4)

B. Haloacetic acids (HAA5)

0

0.25

0.50

0.75

1.00

0

0.25

0.50

0.75

1.00

2010 2011

A M J J A S O DN J F M M J J A S A

Benchmark quotient Benchmark quotient

EXPLANATION

Clackamas River Water DWTP

City of Lake Oswego DWTP

CRW distribution-system sample

LO distribution-system sample

Figure 12. Seasonal patterns in disinfection by-product 

benchmark quotients (concentrations relative to 

drinking-water standards) for (A) four trihalomethanes 

and (B), five haloacetic acids in finished water from 

the Clackamas River Water and City of Lake Oswego 

drinking-water treatment plants, Clackamas River basin, 

Oregon, 2010–11.

Photograph 5a-b. Clear Creek upstream from Carver Park 

running turbid after a storm showing its effect on the 

main-stem Clackamas River below. (Photographs by Kurt 

Carpenter, U.S. Geological Survey, November 14, 2011 [top] 

and May 25, 2010 [bottom].)

Photograph 5b.
Disinfection By-Product Concentrations in Finished Drinking Water 35

tac12-0779_fig13 

A

B

0

0

5

10

15

20

25

0

0.5

1.0

1.5

2.0

2.5

3.0

Turbidity, in nephelometric units

Dissolved organic carbon concentration, in milligrams per liter

2,000

4,000

6,000

8,000

0

0.5

1.0

1.5

2.0

2.5

3.0

Streamflow, in cubic feet per second

Dissolved organic carbon concentration, milligrams per liter 

Major flush

Dissolved organic carbon concentration - laboratory measurement

Turbidity (30-minute)

Dissolved organic carbon concentration estimated from dissolved

organic matter fluorescence (30-minute)

Dissolved organic carbon concentration estimated from dissolved

organic matter fluorescence (30-minute)

Dissolved organic carbon concentration - laboratory measurement

Streamflow, in cubic feet per second

October 2010 November 2010

1 5 10 15 20 25 30 1 5 10 15

Initial rise

EXPLANATION

EXPLANATION

Figure 13. Streamflow, turbidity, and dissolved organic carbon in the lower Clackamas River at the 

Clackamas River Water drinking-water treatment plant intake, October–November 2010, highlighting two 

different responses to storms.

The second, larger storm at the end of October 2010 

(sampled on November 1st) was more characteristic of a true 

soil “flushing” that mobilized carbon from the watershed into 

the hydrologic system. Although turbidity was not as high 

during this second storm, the DOC increased to 2 mg/L at the 

DWTPs, or about 2.5 times higher than during the summer 

base-flow period (table 8 and fig. 7). More importantly, the 

DOC was reactive, producing the highest concentrations of 

DBPs in finished water during this study: 0.04–0.05 mg/L 

THM4 and 0.03–0.04 mg/L HAA5. The two samples collected 

from within the distribution system had higher concentrations 

(fig. 11). In addition to increased DBPs, moderate taste and 

odor issues began at this time, along with increased demands 

for chlorine and coagulant doses (Kari Duncan, City of Lake 

Oswego, written commun., 2011). Even though the first storm 

mobilized quantities of carbon, concentrations of DBPs were 

higher in finished water later in the autumn after the ground 

was saturated.
36 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Table 8. Trends in source-water and finished-water quality during two storms in the Clackamas River basin, Oregon, October–

November 2010 at the Clackamas River Water drinking-water treatment plant.

[For ease in comparision among sampling dates, red and black arrows indicate trends in constituent values. Abbreviations: mg/L, milligram per liter; μg/L, 

microgram per liter; nm, nanometers]

Parameter Pre-storm 

(October 5, 2010) Trend

“Initial” autumn

storm event

(October 10, 2010)

Trend “Major flush” event

(November 1, 2010)

Clackamas River Water—source water

Streamflow, in cubic feet per second 951 ↑ 1,548 ↑ 2,247

Turbidity, in nephelometric units 0.5 ↑ 4.5 ↓ 2.1

Dissolved organic carbon, in mg/L 1.3 ↑ 1.6 ↑ 2.0

Total particulate carbon, in mg/L 0.41 ↑ 0.98 ↓ 0.23

Total particulate nitrogen, in mg/L 0.06 ↑ 0.11 ↓ 0.04

Chlorophyll-a (water column), in μg/L 1.7 ↑ 3.8 ↓ 2.3

Specific ultraviolet absorbance at 254 liters per milligram 

per meter

2.08 ↑ 2.75 ↑ 3.25

Fluorescence index 1.41 ↑ 1.44 ↓ 1.40

Humic index 3.52 ↑ 3.61 ↑ 4.29

Percentage of component C1 19 ↑ 29 ↓ 18

Percentage of component C2 13 ↓ 12 ↑ 18

Percentage of component C3 24 ↓ 21 ↑ 28

Percentage of component C4 22 ↓ 20 ↑ 24

Percentage of component C5 22 ↓ 17 ↓ 12

Clackamas River Water—finished water

Total trihalomethanes (THM4) 0.033 ↓ 0.025 ↑ 0.026

Total haloacetic acids (HAA5) 0.026 ↓ 0.020 ↑ 0.029

Disinfection By-Product 

Formation Potentials

During each basin-wide survey (table 4), laboratory 

measurements of DBPFP were made to compare the 

propensity for waters from a variety of locations within 

the watershed to form DBPs upon chlorination. These 

instantaneous concentrations, and the load and yield 

calculations based on these concentrations, reflect stream 

conditions at the time of sampling; because the discrete 

sampling only captured one point in time, there likely are 

considerable variations in these estimates. These variations 

were probably limited to periods of dynamic river conditions, 

especially during the October 2010 storm; data from the 

three other basin-wide surveys, conducted during stable 

streamflow, were less affected by such temporal variability. 

Additionally, the DBPFP values represent DBPs formed from 

non-coagulated water under controlled laboratory conditions 

and do not necessarily equate with DBPs formed during actual 

water treatment.

By far, the highest DBPFP values were from the 

tributaries (fig. 14), particularly Deep, Rock, and Sieben 

Creeks, where DOC concentrations were also highest. 

Concentrations were particularly high—up to almost 1.0 mg/L 

in Deep Creek—during the October 10, 2010, storm event, 

when longitudinal increases in main-stem DBPFP values were 

observed. These high concentrations are likely the result of 

these tributary inputs.

DBPFPs were measured on filtered and unfiltered 

water, allowing for an evaluation of the relative importance 

of particles (greater than 0.7 μm) to total DBP formation. 

Particulate carbon occurs in the form of sediment, detritus, 

and plant cells including floating and detached benthic 

algae. Although the majority of the DBPs that formed 

(60–100 percent of the THMFP and 40–100 percent of the 

HAAFP) were attributed to the dissolved fraction, particulate 

carbon also contributed DBPs. Considering just main‐stem 

sites, on average, 10 percent of the unfiltered THMFP 

and 32 percent of the unfiltered HAAFP were attributed 

to particles. A comparison of the formation potentials for 

filtered and unfiltered samples also generally showed higher 

values in unfiltered samples for the three primary DBPs in 

finished water—chloroform, DCAA, and TCAA (fig. 15). 

This finding suggests there would be some reduction in DBPs 

by introducing a coagulation step prior to chlorination during 

water treatment.
Disinfection By-Product Formation Potentials 37

tac12 0779_fig14 

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

M M T M T T M T T S S M M T M T T M T T S S

THM formation potential,

in milligrams per liter

HAA formation potential,

in milligrams per liter

0

0.05

0.10

0.15

0.20

0.25

0

0.05

0.10

0.15

M M T M T T M T T S S M M T M T T M T T S S

THM formation potential,

in milligrams per liter

HAA formation potential,

in milligrams per liter

0.2

0

0.4

0.6

0.8

1.0

1.2

0

0.2

0.4

0.6

0.8

1.0

1.2

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

No

data

No

data

THM formation potential,

in milligrams per liter 

HAA formation potential,

in milligrams per liter

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

0

0.1

0.2

0.3

0

0.01

0.02

0.03

0.04

0.05

0.06

THM formation potential,

in milligrams per liter

HAA formation potential,

in milligrams per liter

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

0

0.10

0.20

0.30

0.40

0

0.05

0.10

0.15

THM formation potential

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

HAA formation potential

Filtered

Error bars, 15 percent variation

EXPLANATION

Unfiltered

Figure 14. Disinfection by-product formation potentials for total trihalomethanes (THM4) and total haloacetic acids (HAA5) for samples 

collected in the Clackamas River basin, Oregon, 2010–11. (Sites are listed in downstream order. Note variable Y-axis scales. X-axis 

labels for Summer base flow also apply to the Spring high flow and Initial autumn storm event. Abbreviations: M, main-stem site; T, 

tributary; S, source-water; mg/L, milligram per liter; CR, Clackamas River; CRW, Clackamas River Water DWTP; LO, City of Lake Oswego 

DWTP; DWTP, drinking-water treatment plant; C.G., campground.)
38 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig15

Tributaries

North Fork Reservoir

Disinfection by-product

Source water for drinking-water treatment plants

Main-stem Clackamas River

0.20

0.40

0.60

Chloroform Total THMs DCAA TCAA Total HAAs

0

0.10

0.20

0.30

0

0.10

0.20

0.30

0

0.10

0.20

0.30

0

0.80

Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered

90th percentile

Years 2010-2011

75th percentile

50th percentile (median)

25th percentile

10th percentile

EXPLANATION

 Disinfection by-product formation potential, in milligrams per liter

Figure 15. Range of disinfection by-product formation potentials for filtered and unfiltered samples from the Clackamas 

River basin, Oregon, 2010–11. (Abbreviations: THMs, trihalomethanes; DCAA, dichloroacetic acid; TCAA, trichloroacetic 

acid; HAA, haloacetic acid.)
Disinfection By-Product Formation Potentials 39

It should be noted that although they contribute a relatively 

small percentage to the total DBPs in treated Clackamas River 

water, brominated DBPs are of potential concern because of 

their high molecular weights and potentially more problematic 

human-health effects compared with chlorinated DBPs. 

Brominated DBPs form if bromide is present in the source 

water because it reacts with chlorine and organic matter to form 

DBPs more quickly than chlorine alone. Although bromide in 

the Clackamas Basin could, potentially, originate from marine 

influences (air currents that deliver salts in sea spray), the major 

source is probably Austin Hot Springs, a hydrothermal suite of 

springs located along the upper Clackamas River about 3 mi 

upstream from the Two Rivers Campground sampling site 

(fig. 1). Chemical analyses conducted 40 years ago showed 

a bromide concentration of 1.2 mg/L in the spring, which 

discharged 275 gal/min (U.S. Geological Survey, 2006). The 

upper Clackamas River at Two Rivers campground had the 

highest proportion of brominated DBPs during this study, likely 

reflecting the higher bromide levels at this site compared with 

downstream locations.

Specific Disinfection By-Product 

Formation Potentials

STHMFP and SHAAFP values provide information 

about the degree to which carbon from various locations in the 

basin reacts with chlorine to form DBPs. Shifts in these values 

reflect changes in DOM composition and, thus, its source 

and processing. Surprisingly, there was a significant negative 

correlation between STHMFP and SHAAFP for both filtered 

and unfiltered samples (appendixes F5 and F6). This suggests 

precursor sources for these two classes of DBPs can differ from 

each other in the Clackamas Basin, as has been reported in other 

studies (Krasner and others, 2006; Kraus and others, 2008, 

2010).

The STHMFP values for filtered samples were highest in 

September 2011 (fig. 16 and table 9). The highest STHMFP 

value was associated with the 80-ft depth release point within 

North Fork Reservoir during a seemingly small blue-green 

algae bloom. STHMFP values at downstream main-stem sites 

were also elevated at this time, including source water for the 

CRW and LO DWTPs. The two basin-wide sampling events 

at this time represented the summer low-flow period and 

the seasonal drawdown of Timothy Lake, which contributed 

about 20 percent of the flow in the lower mainstem. The 

two STHMFP measurements at Carter Bridge and Estacada 

in September 2011 (table 9) were higher than the stormwater samples collected during the October 10, 2010, storm. 

These measurements suggest DOM with a high content of 

THM-forming carbon entered the river at that time, although 

water-column chlorophyll-a levels were not that high: 1 μg/L or 

less in the mainstem and less than or equal to 2.5 μg/L in North 

Fork Reservoir. While these specific DBP values were elevated, 

the overall carbon concentrations were low at this time, which 

resulted in relatively low absolute DBPFP values (fig. 14). 

Table 9. Top ten highest carbon-normalized (specific) 

disinfection by-product formation potentials for filtered and 

unfiltered samples.

[Sampling site locations are shown in figure 1. Specific disinfection 

by-product formation potential concentrations calculated by dividing the 

filtered/unfiltered formation potentials by the dissolved organic carbon (DOC) 

and total organic carbon (TOC) concentrations, respectively. Abbreviations:

THM4/HAA5, total trihalomethanes/total haloacetic acids] 

Sampling site Date

Concentration

(milligrams 

of DBP per 

milligram of 

carbon)

Specific trihalomethane formation potential

Filtered

North Fork Reservoir (release depth) 09-21-11 0.208

Clackamas River at Carver 09-09-11 0.176

Clackamas River at Estacada 09-22-11 0.169

Lake Oswego–Source water 09-22-11 0.169

Clackamas River at Estacada 09-09-11 0.160

Clackamas River at Barton Bridge 09-22-11 0.158

Clackamas River Water–Source water 09-22-11 0.156

Clackamas River at Barton Bridge 09-09-11 0.149

Clackamas River at Carver 09-22-11 0.142

Clackamas River at Carter Bridge 09-08-11 0.137

Unfiltered

Lake Oswego–Source water 09-22-11 0.201

Clackamas River at Barton Bridge 09-22-11 0.196

Clackamas River at Carver 09-22-11 0.194

Clackamas River at Barton Bridge 09-09-11 0.189

Clackamas River Water–Source water 09-22-11 0.176

Clackamas River Water–Source water 09-09-11 0.146

Clackamas River at Estacada 09-22-11 0.140

Clackamas River at Carver 09-09-11 0.135

Eagle Creek 09-08-11 0.131

Clackamas River at Estacada 09-09-11 0.126

Specific haloacetic acid formation potential

Filtered

Deep Creek 10-10-10 0.089

Eagle Creek 10-10-10 0.081

Clear Creek 10-10-10 0.081

Rock Creek 10-10-10 0.070

Sieben Creek 10-10-10 0.070

Sieben Creek 09-18-10 0.068

North Fork Reservoir (hypolimnion) 08-04-10 0.049

Clackamas River at Carter Bridge 10-10-10 0.049

North Fork Reservoir (metalimnion) 09-03-10 0.049

North Fork Reservoir (release depth) 08-04-10 0.048

Unfltered

Eagle Creek 10-10-10 0.094

Clear Creek 10-10-10 0.080

Deep Creek 10-10-10 0.069

Clackamas River Water–Source water 04-19-11 0.067

Sieben Creek 10-10-10 0.066

Rock Creek 10-10-10 0.065

Clackamas River at Carter Bridge 10-10-10 0.062

Clackamas River Water–Source water 10-10-10 0.060

Lake Oswego–Source water 10-10-10 0.055

Clear Creek 09-08-11 0.053
40 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Figure 16. Specific disinfection by-product formation potential for total trihalomethanes (THM4) and total haloacetic acids (HAA5) for 

samples collected in the Clackamas River basin, Oregon, 2010–11. (Sites are listed in downstream order. Note variable y-axis scales. 

X-axis labels for Summer base flow also apply to the Spring high flow and Initial autumn storm event. Abbreviations: M, main-stem 

site; T, tributary; S, source-water; mg/L, milligrams per liter; CR, Clackamas River; CRW, Clackamas River Water DWTP; LO, City of Lake 

Oswego DWTP; DWTP, drinking-water treatment plant.)

tac12-0779_fig16 

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

M M T M T T M T T S S M M T M T T M T T S S

Specific THM formation potential, in

 milligrams DBP per milligram of carbon

Specific HAA formation potential, in

 milligrams DBP per milligram of carbon

No

data

0

0.05

0.10

0.15

0.20

0.25

0

0.02

0.04

0.06

0.08

0.10

0.12

M M T M T T M T T S S M M T M T T M T T S S

Specific THM formation potential, in

 milligrams DBP per milligram of carbon

Specific HAA formation potential, in

 milligrams DBP per milligram of carbon

0

0.05

0.10

0.15

0.20

0.25

0

0.02

0.04

0.06

0.08

0.10

0.12

Specific THM formation potential, in

 milligrams DBP per milligram of carbon

Specific HAA formation potential, in

 milligrams DBP per milligram of carbon

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

0

0.05

0.10

0.15

0.20

0.25

0

0.02

0.04

0.06

0.08

0.10

0.12

Specific THM formation potential, in

 milligrams DBP per milligram of carbon 

Specific HAA formation potential, in

 milligrams DBP per milligram of carbon

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

0

0.05

0.10

0.15

0.20

0.25

0

0.02

0.04

0.06

0.08

0.10

0.12

No

data

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

Specific THM formation potential

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

Specific HAA formation potential

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION
Disinfection By-Product Formation Potentials 41

Patterns in the STHMFP measurements for unfiltered 

samples were similar to filtered samples (fig. 16). The 

values were highest in samples collected September 

2011 from main-stem and source-water sites (table 9), in 

samples from lower-basin tributaries (Eagle, Clear, and 

Rock Creeks), and from the 80-ft release depth within 

North Fork Reservoir.

The highest SHAAFP measurements for filtered 

samples were from Deep, Eagle, and Clear Creeks 

during the initial October 10, 2010 storm. Three samples 

collected from North Fork Reservoir (mid-depth, release 

point, and near the bottom) also had relatively high 

SHAAFP values during an algae bloom in August–

September 2010. As with STHMFP, patterns in SHAAFP 

were similar for unfiltered and filtered samples; the values 

were highest in lower-basin tributaries during the October 

2010 storm. At this time, SHAAFP values were also high 

in the mainstem from Carter Bridge downstream to the 

CRW and LO DWTP intakes. Prior studies found that 

HAA precursors can be linked to soil-derived, degraded 

DOM as well as to DOM more recently added by algal 

production (Kraus and others, 2008, 2011).

The correlations between STHMFP and SHAAFP 

were weak, which again emphasize that sources of 

these two classes of DBPs may differ substantially in 

the Clackamas Basin. Prior studies have suggested that 

aliphatic structures play a more important role in THM 

formation and aromatic structures play a more important 

role in HAA formation (Croué and others, 2000; Liang 

and Singer, 2003). A strong positive correlation was 

found between SHAAFP and SUVA, which suggests 

HAA precursors are associated with ultraviolet absorbing, 

aromatic compounds. This type of DOM is strongly 

associated with soil-derived, humified organic matter.

Higher HAA5 formation per unit carbon seen at 

downstream tributaries during the initial October 2010 

storm, when DOC concentrations and SUVA values were 

elevated, agree with prior studies in this region linking 

HAA precursors to the flushing of organic material from 

soils (Kraus and others, 2010). Higher THM4 formation 

per unit carbon seen in the main-stem samples during 

September and October when SUVA values were low 

suggest a potential link between THM precursors and 

contributions of DOM from algae.

Loads and Yields of Organic Carbon and 

Disinfection By-Product Precursors

The highest measured concentrations of DOC and TOC were 

from the tributaries (fig. 9), especially Deep, Rock, and Sieben 

Creeks, but the limited amount of streamflow from all these 

tributaries resulted in relatively low DBP loads to the main-stem 

Clackamas River during three of four samplings (fig. 17). During 

storm periods, however, loads from the tributaries can become 

important. During the October 2010 storm, for example, the loads 

of DOC and TPC from Deep Creek alone accounted for about 

50 and 70 percent, respectively, of that measured at the CRW 

DWTP intake (table 10). This carbon reacted to produce DBPs, 

accounting for 56 and 65 percent of the unfiltered THMFP and 

HAAFP loads, respectively, at the CRW DWTP intake. At this 

time, about half (47 percent) the total carbon was in particulate 

form, including duckweed fragments (see photograph 6) possibly 

associated with over-spillage from ponds within the Deep Creek 

Basin. Other streams including Eagle, Clear, and Rock Creeks 

contributed 28, 9, and 9 percent of the DOC load at the CRW 

DWTP intake, respectively, during the initial autumn storm. 

Eagle Creek contributed 28 and 37 percent of the unfiltered 

THMFP and HAAFP load, respectively, at the CRW DWTP 

intake (fig. 17).

THMFP in the lower mainstem for unfiltered samples 

increased 44 percent between Estacada and Barton on all four 

dates, including times when the tributaries were seemingly 

unimportant. This suggests a particulate source—such as 

sloughed benthic algae, for example—might be contributing to 

higher downstream THMFP. Although Eagle Creek did produce 

higher THMFP values compared with the mainstem at Estacada, 

concentrations were even higher downstream at Barton (fig. 14). 

The carbon and DBP precursor yields also varied according 

to hydrologic condition. Forested areas with relatively higher 

amount of flow (upper basin, Eagle Creek, and main-stem 

Clackamas River downstream to Barton) had the highest DOC 

yields during the May 2010 spring high-flow period; Sieben, 

Rock, and Deep Creeks had the highest yields during the October 

2010 storm (fig. 18). During the summer base-flow period, 

however, the lowest DBP yields were from the tributaries, and 

the highest yields were from the upper and middle basin. The 

THMFP for unfiltered samples increased from Carter Bridge 

downstream to Estacada and to Barton at this time (fig. 14), 

which could be from both phytoplankton within North Fork 

Reservoir and sloughed benthic algae in the reach upstream 

from Barton.

Photograph 6. Deep Creek running turbid 

during the initial autumn storm (inset shows 

presence of duckweed, a possible indicator of 

overflowing ponds in the basin). (Photographs 

by Kurt Carpenter, U.S. Geological Survey, 

October 10, 2010.)
42 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

M M T M T T M T T S S M M T M T T M T T S S

Streamflow, in cubic feet per second

1 0

10

100

1,000

10,000

2,000

4,000

6,000

8,000

Carbon load, in kilograms per day

M M T M T T M T T S S M M T M T T M T T S S

Streamflow, in cubic feet per second

0

2,000

4,000

6,000

8,000

10,000

12,000

0.1

1

10

100

1,000

10,000

Carbon load, in kilograms per day

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

tac12-0779_fig17 page 1 Streamflow, in cubic feet per second

0

400

800

1,200

1,600

Carbon load, in kilograms per day

0

500

1,000

1,500

2,000

2,500

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

Streamflow, in cubic feet per second

0 0

1

10

100

1,000

Carbon load, in kilograms per day

500

1,000

1,500

2,000

2,500

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

No

data ** **

* * * * ** *

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

EXPLANATION

Dissolved organic carbon

Particulate carbon

Values ≤ 0.02-1.9 kilograms per day

EXPLANATION

Streamflow, instantaneous

*

Figure 17. Streamflow and instantaneous loads of dissolved organic carbon, particulate carbon, and associated disinfection by-product formation 

potentials for total trihalomethanes (THM4) and total haloacetic acids (HAA5) for samples collected in the Clackamas River basin, Oregon, 2010–11. 

(Sites are listed in downstream order. Note log scale for y-axis in streamflow plots. X-axis labels for Summer base flow also apply to the Spring high 

flow and Initial autumn storm event. Loads, in kilograms per day were calculated by multiplying the concentration in mg/L by flow (x 2.447 for units 

conversion). Abbreviations: M, main-stem site; T, tributary; S, source-water; mg/L, milligrams per liter; CR, Clackamas River; CRW, Clackamas River 

Water DWTP; LO, City of Lake Oswego DWTP; DWTP, drinking-water treatment plant.)
Disinfection By-Product Formation Potentials 43

tac12-0779_fig17 page 2 

No

data

No

data

Sieben Creek

**   ** 

**   ** 

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

M M T M T T M T T S S M M T M T T M T T S S

0

200

400

600

800

1,000

0

200

400

600

800

THM formation potential load,

in kilograms per day

HAA formation potential load,

in kilograms per day

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Clear Creek

Rock Creek

0

100

200

300

400

500

600

0

20

40

60

80

100

THM formation potential load,

in kilograms per day

HAA formation potential load,

in kilograms per day

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

0

100

200

300

400

500

600

700

0

20

40

60

80

100

120

140

THM formation potential load,

in kilograms per day

HAA formation potential load,

in kilograms per day

M M T M T T M T T S S M M T M T T M T T S S 0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0

200

400

600

THM formation potential load,

in kilograms per day

HAA formation potential load,

in kilograms per day

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

THM formation potential load

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

HAA formation potential load

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

Values ≤ 0.05-5.0 kilograms per day * Values too low to show *

Figure 17.—Continued
44 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig18 page 1

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

0.0

0.5

1.0

1.5

2.0

2.5

0

2

4

6

8

M M T M T T M T T S S M M T M T T M T T S S

Streamflow yield, in cubic feet

per second per kilometer

Carbon yield, in kilograms per day

per square kilometer

0.0

1.0

2.0

3.0

4.0

5.0

0

10

20

30

40

50

60

M M T M T T M T T S S M M T M T T M T T S S

Streamflow yield, in cubic feet

per second per kilometer

Carbon yield, in kilograms per day

per square kilometer

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.5

1.0

1.5

September 22, 2011 (Timothy Lake drawdown)

Streamflow yield, in cubic feet

per second per kilometer

Carbon yield, in kilograms per day

per square kilometer

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.4

0.8

1.2

Streamflow yield, in cubic feet

per second per kilometer

Carbon yield, in kilograms per day

per square kilometer

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CRW DWTP–

CR at Carver

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

No

data

EXPLANATION

Dissolved organic carbon

Particulate carbon

EXPLANATION

Streamflow yield

Figure 18. Instantaneous yields of streamflow, dissolved organic carbon, particulate carbon, and associated disinfection by-product formation 

potentials for total trihalomethanes (THM4) and total haloacetic acids (HAA5) for samples collected in the Clackamas River basin, Oregon, 2010–11. 

(Sites are listed in downstream order. Note the log scale for y-axis in streamflow plots. Loads, in kilograms per day were calculated by multiplying 

the concentration in milligrams per liter (mg/L) by flow (x 2.447 for units conversion). Yields were derived by dividing the load by the area, in square 

kilometers. Abbreviations: M, main-stem site; T, tributary; S, source water intake; CR, Clackamas River; CRW, Clackamas River Water DWTP; LO, City 

of Lake Oswego DWTP; DWTP, drinking water treatment plant.)
Disinfection By-Product Formation Potentials 45

tac12-0779_fig18 page 2 

May 11-12, 2010 (Spring high flow)

October 10, 2010 ("Initial" autumn storm event)

September 8-9, 2011 (Summer base flow)

September 22, 2011 (Timothy Lake drawdown)

0

0.25

0.50

0.75

1.00

0

0.1

0.2

0.3

0.4

M M T M T T M T T S S M M T M T T M T T S S

THM formation potential yield,

in kilograms per day

HAA formation potential yield,

in kilograms per day

0

2

4

6

8

0

2

4

6

8

M M T M T T M T T S S M M T M T T M T T S S

THM formation potential yield,

in kilograms per day

HAA formation potential yield,

in kilograms per day

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0

0.01

0.02

0.03

0.04

0.05

0.06

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

CR at Two Rivers C.G.

CR at Barton

CR at Carver

CR at Carter Bridge

CR at Estacada

THM formation potential yield,

in kilograms per day

HAA formation potential yield,

in kilograms per day

0

0.10

0.20

0.30

0

0.02

0.04

0.06

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

CR at Carter Bridge

CR at Carver

CR at Estacada

CR near Barton

Eagle Creek

Deep Creek

Sieben Creek

Clear Creek

Rock Creek

HAA formation potential yield,

in kilograms per day

THM formation potential yield,

in kilograms per day

No

data

No

data

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

CRW DWTP–

source water

LO DWTP–

source water

Specific THM formation potential yield

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

Specific HAA formation potential yield

Filtered Unfiltered

Error bars, 15 percent variation

EXPLANATION

Figure 18.—Continued
46 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Table 10. Percentage of dissolved organic carbon, total particulate carbon, and disinfection by-product formation potentials for 

individual sites relative to instantaneous loads at the Clackamas River Water drinking-water treatment plant intake during basin-wide 

sampling events, Clackamas River basin, Oregon.

[Sampling site locations are shown in figure 1. Streamflow in cubic feet per second. Abbreviations: DWTP, drinking-water treatment plant; DOC, dissolved 

organic carbon; TPC, total particulate carbon; THM, trihalomethane; HAA, haloacetic acid; FP, formation potential; F, filtered; U, unfiltered; –, no data]

Sampling site River

mile Date Streamflow

Percent of load at the Clackamas River Water DWTP intake

DOC TPC THMFP-F THMFP-U HAAFP-F HAAFP-U

May 11–13, 2010 (Spring high flow)

Clackamas River at Carter Bridge 40.8 05-11-10 2,665 67 54 55 61 71 99

Clackamas River at Estacada 23.1 05-11-10 3,579 91 57 81 77 117 121

Eagle Creek 16.7 05-11-10 422 12 10 10 10 13 18

Clackamas River at Barton Bridge 13.4 05-12-10 3,700 94 78 121 99 78 99

Deep Creek 12.1 05-11-10 58 2.4 2.2 1.7 1.5 3.2 3.0

Clear Creek 8 05-11-10 162 4.4 6.1 3.8 3.7 5.2 6.9

Clackamas River at Carver 7.9 05-12-10 3,760 95 104 103 107 108 101

Rock Creek 6.4 05-11-10 4.6 0.2 0.05 0.16 0.14 0.24 0.26

Sieben Creek 5.8 05-11-10 0.8 0.04 0.01 0.03 0.03 0.05 0.05

CRW DWTP–Source water 3.1 05-12-10 3,646 100 100 100 100 100 100

LO DWTP–Source water 0.9 05-13-10 3,549 – – – – – –

October 10, 2010 (“Initial” autumn storm event)

Clackamas River at Carter Bridge 40.8 10-10-10 938 45 22 52 41 56 38

Clackamas River at Estacada 23.1 10-10-10 1,338 54 22 52 45 47 31

Eagle Creek 16.7 10-10-10 197 28 16 33 28 57 37

Clackamas River at Barton Bridge 13.4 10-10-10 1,535 68 39 65 62 77 47

Deep Creek 12.1 10-10-10 167 49 70 42 56 108 65

Clear Creek 8 10-10-10 60 9 3.2 8.6 8 19 10

Clackamas River at Carver 7.9 10-10-10 1,650 113 114 109 105 127 95

Rock Creek 6.4 10-10-10 29 9 8 7.3 8.1 16 9.5

Sieben Creek 5.8 10-10-10 21 4.5 3.5 4.0 4.0 7.9 4.5

CRW DWTP–Source water 3.1 10-10-10 1,548 100 100 100 100 100 100

LO DWTP–Source water 0.9 10-10-10 1,639 112 58 109 101 104 85

September 8–9, 2011 (Summer base flow)

Clackamas River at Carter Bridge 40.8 09-08-11 811 73 38 76 50 68 138

Clackamas River at Estacada 23.1 09-09-11 957 98 76 119 82 88 138

Eagle Creek 16.7 09-08-11 25 4 2 4 3 6 6

Clackamas River at Barton Bridge 13.4 09-09-11 980 101 148 113 140 98 134

Deep Creek 12.1 09-08-11 9 2.0 1.2 1.7 1.3 3.3 3.4

Clear Creek 8 09-08-11 18 3.4 1.5 3.0 2.4 7.1 6.8

Clackamas River at Carver 7.9 09-09-11 1,000 103 78 137 92 94 105

Rock Creek 6.4 09-08-11 1.0 0.2 0.05 0.13 0.12 0.18 0.17

Sieben Creek 5.8 09-08-11 0.2 0.0 0.14 0.04 0.03 0.05 0.07

CRW DWTP–Source water 3.1 09-09-11 864 100 100 100 100 100 100

LO DWTP–Source water 0.9 09-09-11 897 115 123 116 92 102 121

September 22, 2011 (Timothy Lake drawdown)

Clackamas River at Two Rivers campground 57 09-16-11 343 22 30 13 10 17 10

Clackamas River at Carter Bridge 40.8 09-16-11 858 64 79 46 34 57 33

Clackamas River at Estacada 23.1 09-22-11 943 103 92 87 66 79 44

Clackamas River at Barton Bridge 13.4 09-22-11 969 95 115 104 112 84 71

Clackamas River at Carver 7.9 09-22-11 994 80 97 86 109 96 53

CRW DWTP–Source water 3.1 09-22-11 944 100 100 100 100 100 100

LO DWTP–Source water 0.9 09-22-11 930 89 88 96 101 109 52
Carbon Characterization Using Optical Properties 47

tac12-0779_fig 19 

A

B

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Specific ultraviolet absorbance at 254 nanometers,

in liters per milligram-meter 

1.25

1.30

1.35

1.40

1.45

1.50

1.55

A M J J

2010

A S O N D J F M A M

2011

J J A S

Fluorescence index, unitless

Sampling sites

EXPLANATION

Clackamas River at Two Rivers Campground

Clackamas River at Carter Bridge

Clackamas River at Estacada

Clackamas River near Barton

Clackamas River at Carver

North Fork Reservoir

Clear Creek

Eagle Creek

Deep Creek

Rock Creek

Sieben Creek

CRW DWTP–source water

LO DWTP–source water

Carbon Characterization Using 

Optical Properties

Optical measurements, including UVA254 and FDOM 

(ex 370/em 460 nm), were strongly correlated with 

concentrations of DOC (table 11) and TOC in the Clackamas 

River. Although DOC concentration remained relatively low 

in the mainstem, there were large changes in SUVA values 

(1.5 to 4.4 L/mg-m) over the study period, indicating a shift 

in DOM composition (fig. 19). SUVA values were highest 

during high-flow periods, indicative of contributions of higher 

aromatic carbon associated with humic-like material. During 

low-flow periods, SUVA values decreased, suggesting the 

DOM pool contained a greater proportion of non-aromatic, 

lower molecular-weight carbon derived from less-degraded 

organic material (Weishaar and others, 2003). SUVA increased 

slightly at the downstream sites in the mainstem. SUVA 

values were also higher in the tributary sites, particularly 

during the October 2010 storm event (3.2–4.3 L/g-m). Like 

SUVA, spectral slope values are also used to indicate changes 

in DOM composition. As was seen with SUVA, seasonal 

changes in spectral slope suggest the DOM contains higher 

molecular-weight, aromatic material during storm events 

compared to base-flow periods (appendix G1).

Contour plots of selected EEMs obtained from the 

fluorescence analysis are shown in fig. 20 to highlight 

observed changes in DOM amount and composition. The 

highest intensity EEMs were from Deep, Rock, and Sieben 

Creeks where DOC concentrations were highest. The first 

set of EEMs for the upper, middle, and lower mainstem 

during low streamflow contain a Peak T-like carbon signal 

near 270–340 ex/em associated with algal-derived organic 

matter high in protein and (or) freshly leached plant material 

high in polyphenols (Coble, 2007; Hernes and others, 2009; 

Beggs and others, 2011). The presence of this peak is best 

demonstrated in the last EEM from North Fork Reservoir 

that represents the algal grab sample collected at the surface 

during the September 2011 phytoplankton bloom. It should be 

noted that the Peak T signal was only apparent, however, in 

the contour plots when overall fluorescence intensities were 

lower, reflecting periods when DOC concentration was low. 

This likely results because the Peak T region is over-shadowed 

by the stronger humic-like signal (for example, Peaks A and 

C) that appeared during high-flow events (fig. 20). In order 

to more accurately quantify the presence of different pools 

of organic matter contributing to the overall EEMs spectra, 

PARAFAC modeling was used; this approach enables the 

more subtle underlying signals that are not necessarily visible 

in the EEM color contours to be detected and the relative 

contribution of each of the underlying fluorophores to 

be quantified. 

Figure 19. Seasonal patterns in (A) specific ultraviolet 

absorbance and (B) fluorescence index values for 

tributary, reservoir, and main-stem sites in the Clackamas 

River basin, Oregon, 2010–11. (Abbreviations: CRW, 

Clackamas River Water DWTP; LO, City of Lake Oswego 

DWTP; DWTP, drinking water treatment plant.)
48 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water Table 11. Spearman rank correlations between carbon concentrations, optical properties, and disinfection by-product concentrations in finished water and DBP formation potentials for filtered and unfiltered samples from the Clackamas River basin, Oregon, 2010–11. [Spearman rank correlations (rho values), with P values indicated by asterisks: * (P<0.05); ** (P<0.01); ***(P<0.001). See table 6 for constituent abbreviations and units. Abbreviations: DOC, dissolved organic carbon; THM4, total trihalomethanes; HAA5, total haloacetic acids; DBP, disinfection by-product; STHMFP, carbon-specific total trihalomethane formation potential; SHAAFP, carbon-specific total haloacetic acid formation poential; F, filtered; U, unfiltered; –, not applicable or no data] Constituent All sites Finished water All sites (basin-wide synoptic sampling events) DOC THM4 HAA5 THMFP-F THMFP-U HAAFP-F HAAFP-U STHMFP-F STHMFP-U SHAAFP-F SHAAFP-U Quantitative constituents Streamflow – **0.47 ***0.64 – – – – – – – – pH ***-0.46 -0.33 **-0.50 **-0.41 **-0.40 ***-0.50 ***-0.51 0.27 *0.29 ***-0.52 ***-0.46 Turbidity ***0.72 **0.46 ***0.55 ***0.88 ***0.93 ***0.94 ***0.95 *-0.29 -0.24 ***0.71 ***0.51 Chlorophyll-a ***0.53 -0.20 -0.16 ***0.71 ***0.71 ***0.70 ***0.72 *-0.29 *-0.31 ***0.60 ***0.47 Total organic carbon ***0.98 ***0.63 ***0.71 ***0.95 ***0.96 ***0.98 ***0.98 *-0.32 *-0.28 ***0.75 ***0.58 Dissolved organic carbon – ***0.72 ***0.83 ***0.96 ***0.95 ***0.98 ***0.97 *-0.34 *-0.28 ***0.77 ***0.62 Total particulate carbon ***0.87 0.32 ***0.71 ***0.87 ***0.93 ***0.93 ***0.94 *-0.28 -0.26 ***0.69 ***0.49 UVA254 ***0.96 ***0.67 ***0.79 ***0.94 ***0.96 ***0.98 ***0.98 *-0.33 -0.27 ***0.78 ***0.60 Dissolved organic matter fluorescence ***0.98 ***0.68 ***0.81 ***0.95 ***0.93 ***0.95 ***0.94 *-0.34 *-0.28 ***0.73 ***0.58 PEAK A ***0.96 ***0.65 ***0.77 ***0.93 ***0.91 ***0.92 ***0.91 **-0.34 -0.28 ***0.69 ***0.53 PEAK C ***0.98 ***0.67 ***0.80 ***0.94 ***0.92 ***0.94 ***0.93 **-0.34 -0.28 ***0.72 ***0.56 PEAK M ***0.96 ***0.64 ***0.71 ***0.93 ***0.91 ***0.92 ***0.92 **-0.34 -0.28 ***0.69 ***0.53 PEAK D ***0.98 ***0.68 ***0.81 ***0.96 ***0.93 ***0.95 ***0.95 **-0.34 -0.28 ***0.74 ***0.59 PEAK B ***0.89 ***0.60 ***0.54 ***0.85 ***0.90 ***0.93 ***0.93 **-0.37 -0.30 ***0.72 ***0.51 PEAK T ***0.92 **0.46 **0.41 ***0.88 ***0.90 ***0.92 ***0.92 **-0.39 -0.32 ***0.69 ***0.51 PEAK N ***0.94 ***0.53 ***0.53 ***0.91 ***0.91 ***0.92 ***0.91 **-0.35 -0.29 ***0.68 ***0.51 C1 Loading ***0.91 ***0.51 ***0.54 ***0.88 ***0.87 ***0.87 ***0.87 *-0.33 -0.27 ***0.63 ***0.47 C2 Loading ***0.98 ***0.67 ***0.80 ***0.96 ***0.94 ***0.96 ***0.95 *-0.34 *-0.28 ***0.74 ***0.59 C3 Loading ***0.97 ***0.68 ***0.81 ***0.95 ***0.93 ***0.94 ***0.93 *-0.34 *-0.28 ***0.72 ***0.56 C4 Loading ***0.98 ***0.68 ***0.80 ***0.95 ***0.93 ***0.95 ***0.94 *-0.34 *-0.29 ***0.73 ***0.58 C5 Loading ***0.89 **0.50 **0.44 ***0.85 ***0.90 ***0.92 ***0.92 **-0.37 *-0.29 ***0.69 ***0.49 Qualitative constituents SUVA ***0.49 **0.44 ***0.59 ***0.72 ***0.67 ***0.73 ***0.73 **-0.39 **-0.37 ***0.78 ***0.70 Spectral slope (S275–295) ***-0.46 **-0.40 -0.27 ***-0.45 **-0.42 ***-0.45 ***-0.45 0.23 0.12 ***-0.52 ***-0.56 Spectral slope (S290–350) -0.18 -0.30 -0.19 -0.18 -0.23 -0.26 -0.26 -0.04 -0.10 -0.26 -0.28 Spectral slope (S350–400) 0.24 0.05 0.23 0.26 0.23 0.19 0.18 -0.03 0.04 0.18 0.09 Spectral slope ratio **-0.43 -0.30 **-0.37 **-0.44 -0.41 **-0.4 **-0.39 0.19 0.08 **-0.45 **-0.39 Fluorescence Index 0.18 -0.25 **-0.44 *0.28 0.23 0.17 0.17 -0.01 0.09 0.13 0.20 Humic Index 0.07 -0.12 -0.03 0.21 0.10 0.04 0.03 0.10 0.08 0.07 0.15 Percent component 1 ***0.55 -0.04 -0.11 ***0.57 ***0.52 ***0.48 ***0.48 -0.23 -0.19 *0.29 *0.28 Percent component 2 -0.02 *0.37 ***0.55 -0.03 -0.09 -0.08 -0.08 0.16 0.07 0.03 0.15 Percent component 3 -0.16 0.02 0.23 -0.14 -0.18 -0.24 -0.25 **0.38 *0.32 -0.18 -0.10 Percent component 4 -0.23 0.03 0.20 -0.16 -0.23 -0.24 -0.25 **0.36 0.26 -0.06 0.04 Percent component 5 -0.24 -0.21 **-0.45 **-0.41 *-0.30 -0.24 -0.23 -0.10 -0.06 -0.18 -0.27
Carbon Characterization Using Optical Properties 49

August 3, 2010, low flow

January 18, 2011, high flow

September 8, 2011, tributaries and September 12, 2011, reservoir samples

300

350

400

450

500

550

600

0.05

0.1

0.15

0.2

0.25

Emission, in nanometers

300

350

400

450

500

550

600

0.01

0.02

0.03

0.04

0.05

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

300

350

400

450

500

550

600

0.05

0.1

0.15

0.2

0.25

0.3

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.05

0.1

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0.2

0.25

0.3

0.01

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0.03

0.04

0.05

0.06

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0.02

0.03

0.04

0.05

0.05

0.1

0.15

0.2

0.25

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

300

350

400

450

500

550

600

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

250 300 350 400 450

250 300 350 400 450

250 300 350 400 450

250 300 350 400 450

250 300 350 400 450 250 300 350 400 450

250 300 350 400 450

250 300 350 400 450

250 300 350 400 450

Excitation, in nanometers

250 300 350 400 450

250 300 350 400 450

250 300 350 400 450

Fluorescence intensity, in Raman units

CR at Carter Bridge: DOC - 0.7 mg/L

CR at Carter Bridge: DOC - 1.9 mg/L

Deep Creek: DOC - 1.7 mg/L

Sieben Creek: DOC - 1.9 mg/L

CR at Estacada: DOC - 0.9 mg/L

CR at Estacada: DOC - 2.2 mg/L

Clear Creek: DOC - 1.5 mg/L

Rock Creek: DOC - 1.3 mg/L

CRW DWTP-source water: DOC - 1.0 mg/L

CRW DWTP-source water: DOC - 2.2 mg/L

Eagle Creek: DOC - 1.2 mg/L

North Fork Reservoir: DOC - 0.8 mg/L

Figure 20. Excitation–emission matrices (EEMs) for water samples from selected main-stem, tributary, and reservoir sites 

across a range of stream conditions, Clackamas River basin, Oregon, 2010–11. (Abbreviations: DOC, dissolved organic carbon; 

CR, Clackamas River; CRW, Clackamas River Water drinking-water treatment plant; mg/L, milligrams per liter.)
50 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

The PARAFAC model developed and validated with 

167 samples from the mainstem, tributary, North Fork 

Reservoir, and source and finished water produced five 

components: C1–C5 (fig. 21). Model components C2, C3, 

and C4 are associated with terrestrial humic-like substances 

commonly associated with fluorescence Peaks A and C 

(Stedmon and Markager, 2005; Coble, 2007; Murphy and 

others, 2008; Yamashita and others, 2008). There are, 

however, subtle differences among these three components. 

Fluorescence associated with C1 has been associated with 

several different sources of DOM, including marine and 

terrestrial humic acids that have been microbially processed, 

freshly produced (phytoplankton derived, for example) labile 

material identified as Peak N, and material exported from 

agricultural and wastewater-impacted catchments identified as 

Peak M (Stedmon and Markager, 2005; Coble, 2007; Fellman 

and others, 2010). C2 is typical of terrestrial organic matter 

composed of high molecular-weight and aromatic compounds 

(McKnight and others, 2001; Stedmon and others, 2003). 

C5, located in a region frequently referred to as “proteinlike” because tryptophan and tyrosine fluoresce in this 

region, is associated with less-processed carbon derived from 

fresh terrestrial plants, algae, and (or) wastewater (Murphy 

and others, 2008; Hernes and others, 2009; Beggs and 

others, 2011).

Examination of the relative contributions of the different 

PARAFAC components shows that C1, C3, and C4 represent 

the bulk of DOM fluorescence (fig. 22). Component C5 

generally represented a smaller and more highly variable 

fraction. Although there were slight seasonal shifts in the 

relative proportions of the different PARAFAC components 

in CRW source water (fig. 22B), the overall trend shows a 

consistent pattern. Average percentages were 26 percent for 

C3, 21–22 percent for C1 and C4, and 15–16 percent for C2 

and C5, reflecting dominance by terrestrial types of carbon in 

the lower mainstem.

The fluorescence spectra of Eagle and Clear Creeks, two 

streams draining predominantly forested basins (table 2), were 

similar to the main-stem sites (fig. 22), reflecting dominance 

by terrestrial humic substances. The other three tributaries—

Deep, Rock, and Sieben Creeks—have less forested area and 

are variously influenced by agriculture and urban development 

(table 2). These streams contained the highest proportion of 

component C1 (24–39 percent; fig. 22), suggesting the DOM 

contributed from these tributaries differed in composition 

compared to the other sites. Rock and Sieben Creeks also 

showed the lowest proportion of C5.

The fluorescence spectra of samples from North Fork 

Reservoir had the highest average percentage of component 

C5 (fig. 22), likely reflecting the presence of recently added 

DOM from phytoplankton algae that were prevalent in the 

reservoir during all samplings. Organic matter recently 

contributed by algae is expected to contain a greater 

protein-like signal. 

Although there were only minor changes in the 

FI values (1.3–1.5 across the watershed samples), seasonal 

trends suggest DOM in the mainstem is more dominated 

by microbial-derived carbon between August and October 

(fig. 19), when more algal contributions would be expected. 

This trend is in accordance with the SUVA values. FI values 

were also higher in the tributary sites compared to main-stem 

sites for the May 2010 and September 2011 basin-wide 

samplings. The FI value was highest (2.3) for the algal grab 

sample collected from the surface of North Fork Reservoir in 

September 2011 (not shown in fig. 19), which contained a high 

abundance of blue-green algae (Anabaena sp.). Although the 

FI did not vary a lot, these results suggest this measure could 

be indicative of algal-derived carbon despite the relatively low 

levels of chlorophyll-a observed during the study.

The HIX (table 1) ranged from about 2 to 8; the 

most notable trends were higher values in some of the 

tributaries, particularly in September 2011, possibly from 

highly-processed humified material, and lower values for 

reservoir samples, indicative of more recently added “fresh” 

material (appendix G1).

Carbon characterization based on optical properties 

provided complementary evidence to the carbon 

concentrations during the two autumn storms in 2010 that 

offers insights into the different effects of these storms on 

watershed processes. The second, larger storm (fig. 13), for 

example, produced higher SUVA and HIX values and lower 

FI values generally associated with greater aromatic content 

and higher molecular-weight material. This was accompanied 

by declines in the percentages of carbon components C1 

and C5 and increases in components C2, C3, and C4—all 

indicative of the carbon quality shifting from a more labile 

microbial/algal source with lower aromaticity during the 

initial storm to a higher molecular-weight, aromatic source 

of terrestrial origin after the larger storm (table 8). These 

results point to the success of fluorescence technologies in 

detecting these shifts in DOM quality (and quantity) and 

provide a means by which the quality of source water can be 

closely monitored for treatment-plant operations, for gaining 

a deeper understanding of river conditions and processes, 

and for evaluating trends over short and long time scales. 

Through implementation of these instruments in studies 

like this, new technologies are being developed to advance 

these capabilities.
Carbon Characterization Using Optical Properties 51

tac12-0779_fig 21

Excitation–Emission Matrix (EEM) contour map

Emission, in nanometers (nm) Fluorescence intensity, in Raman units 300 0.7 0.6 0.5 0.4 0.3 0.2 0.1

0

250 350 450 550 250 350 450 550 250 350 450 550 250 350 450 550 250 350 450 550

600550500450400350

250 300 350 400 450

Excitation, in nanometersExcitation, in nanometers

250 300 350 400 450 250 300 350 400 450 250 300 350 400 450 250 300 350 400 450

Excitation

Emission

Excitation-Emission line graph

Component C5 Component C2 Component C4 Component C3 Component C1

Wavelength maxima1

250 (290) / 392 nm 285 (390) / 510 (336) nm 255 (370) / 450 (312) nm 315 (250) / 425 nm 280 / 324 (600) nm

Peak associations2

Peaks A, M, and N Peaks A, C, and D Peaks A, C, and D Peaks A, C, and M Peaks B and T

Descriptions2

Humic-like materials, possible alteration by microbial reprocessing, possible phytoplankton-derived, 

labile material

Humic-like and soil fulvic acid substances

Humic-like and soil fulvic acid substances

Humic-like and soil fulvic acid substances

Protein-like, autochthonous organic matter associated with algae, also polyphenolic compounds

1Secondary excitation–emission maximas shown in parentheses. 2Peak names and descriptions that are commonly used in the literature (Stedmon and others, 2003, Stedmon and Markager, 2005a, Coble, 2007; Murphy and others, 2008; Hernes and others, 2009; Fellman and others, 2010)

Component

Figure 21. Principal fluorescence components identified in the PARAFAC model for the Clackamas River basin, Oregon, 2010–11.
52 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Figure 22. Relative percentages of PARAFAC components in water samples collected from (A) all sites, 

and (B) source water at the Clackamas River Water drinking-water treatment plant, Clackamas River 

basin, Oregon, 2010–11.

tac12-0779_fig 22

A. All sites

B. CRW–source water

0

5

10

15

20

25

30

35

40

45

50

Average percent value

19 12 19 3 4 3 3 4 3 4 26 19

0

20

40

60

80

100

Percent

4-14 5-12 5-24 5-24 5-25 5-25 6-4 6-4

Date 2010 Date 2011

6-5 7-8 8-3 9-7 9-20 10-5 10-10 11-1 12-7 1-18 2-16 3-16 4-19 5-24 6-29 8-2 9-9 9-22

CR at

Carter

Bridge

North

Fork

Reservoir

CR at

Estacada

Eagle

Creek

CR near

Barton

Deep

Creek

Clear

Creek

CR at

Carver

Rock

Creek

Sieben

Creek

CRW

DWTPsource

water

LO

DWTPsource

water

Site name

Number of

samples

EXPLANATION

Plus or minus one standard deviation

C1

C2

C3

C4

C5

Carbon components
Proxies for Carbon and Disinfection By-Product Precursor Concentrations 53

Proxies for Carbon and Disinfection 

By-Product Precursor Concentrations

With growing concerns about the effects of DBPs 

on human health and associated changes in the regulatory 

standards, there is much interest in identifying the sources 

of DBP precursors and developing tools to better monitor 

and predict carbon and DBP concentrations. To identify 

source‐water attributes that are good predictors or “surrogates” 

for DBP formation, relations between concentrations of THM 

and HAA (in finished water and DBPFP samples) and organic 

carbon concentrations and optical properties were examined 

using Spearman rank correlations (table 11). While many of 

the variables were significantly positively correlated with 

concentrations of THM4 and HAA5 in finished water as well 

as with THMFP and HAAFP, the best DBP predictors were 

DOC concentration, FDOM and several fluorescence peaks, 

and total component loadings from components C2, C3, and 

C4. In general, the correlations between concentrations of 

TOC and DOC and DBPs in drinking-water samples were 

somewhat higher for HAA5 compared with THM4, although 

all were significant (p<0.001).

Laboratory DBPFPs were highly correlated with 

concentrations of DOC and TOC, turbidity, UVA, fluorescence 

peak intensities, PARAFAC component loadings, and various 

other indicators (r >0.9, p<0.001; table 11); r values for 

THMFP and HAAFP were similar. Correlations between 

STHMFP, filtered and unfiltered, and other constituents 

were not that high (r <0.4), although some correlations 

were significant (table 11). In contrast, SHAAFP was highly 

correlated with a host of other indicators; for example, 

SHAAFP in filtered-water was significantly correlated 

(r=0.78, p<0.001) with SUVA (table 11). 

Given that DBP precursors are strongly correlated with 

DOC concentrations, there is much interest in identifying 

DOC proxies, especially for water systems where DBPs 

are approaching drinking-water standards. Laboratory 

bench-top measurements of FDOM were highly correlated 

(r=0.98; p<0.001) with DOC concentration (fig. 23) as 

well as laboratory THMFP and HAAFP (table 11). In 

addition, continuous in-situ FDOM measurements were 

highly correlated with laboratory FDOM (r=0.99; p<0.001; 

appendix F2) and provided similarly high correlations 

with DOC concentration (r=0.96; p<0.001) as did the 

laboratory based optical measurements (fig. 23; appendix F2). 

These strong correlations provide convincing evidence that 

in-situ FDOM measurements can be effective at tracking 

concentrations of DOC and DBP precursors in source water. 

In addition to providing high-frequency, real-time data, the 

in-situ measurements do not require the sample collection, 

processing, and analyses needed for UVA measurement.

The ability to monitor DWTP inflow water quality to 

predict finished-water DBPFP continuously, in real-time, 

would be of great benefit for DWTP operations. In addition 

to source-water quality, however, the amount of DBPs that 

form during treatment is also influenced by DWTP operations, 

including coagulation, disinfection type and dose, contact 

times, pH, temperature, and other factors. Much can be 

learned by using this information along with monitoring 

feedback to adaptively manage treatment plants and optimize 

for DBP precursor removal.

During this study, finished-water DBPs were determined 

approximately monthly, resulting in 18 data points to 

compare in-situ FDOM measurements to finished-water 

THM4 and HAA5 concentrations. The correlation between 

in-situ FDOM and finished-water HAA5 concentration was 

significant (r=0.83; p<0.001; appendix F3); applying the 

equation derived from this relation, the high-frequency in-situ 

FDOM data were used to estimate finished-water HAA5 

concentrations (fig. 24). If accurate, these estimates would 

suggest HAA5 concentrations did not exceed the 0.06 mg/L 

MCL during this study. While the correlation between in-situ 

FDOM and chloroform was significant (r=0.64; p<0.01), the 

relation between FDOM and finished-water THM4 was not, 

so estimates of continuous finished-water THM concentrations 

were not generated for this study. Further research on this 

topic is warranted given that fluorescence was a good 

predictor of laboratory THM formation potential, here and 

elsewhere (Hua and others, 2007; Marhaba and others, 2009; 

Kraus and others, 2010). It should also be emphasized that 

the relation between source-water quality and DBP formation 

potentials conducted on untreated water in the laboratory 

under uniform conditions is expected to be stronger than the 

correlation with treated (finished) water that has undergone 

coagulation (table 11). 

The significant, positive correlation between SHAAFP 

and SUVA (table 11) suggests the HAA precursor pool is 

made up of, if not linked to, chromophoric DOM, which may 

explain why there is a stronger correlation between FDOM 

and HAA5 concentrations compared to THM4. Prior studies 

indicate HAA precursors are more aromatic compared to 

THM precursors (Croué and others, 2000; Liang and Singer, 

2003; Hong and others, 2008), and HAA precursors have 

been associated with terrestrially derived fulvic and humic 

acids (Kraus and others, 2008, 2010). However, other studies 

have also observed a link between HAA precursors and 

algal-derived DOM (Chen and others, 2008; Hong and others, 

2008; Kraus and others, 2011). 
54 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig 23

A. All watershed samples

E. CRW DWTP source-water samples

B. All watershed samples

C. Main-stem samples D. Main-stem samples

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10

Laboratory dissolved organic matter fluorescence, in Raman units

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0 2 4 6 8 10

Absorbance at 254 nanometers, per centimeter

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 1 2 3

Laboratory dissolved organic matter fluorescence, in Raman units

0

5

10

15

20

25

30

35

40

0 1 2 3

Dissolved organic carbon concentration, in milligrams per liter

Dissolved organic carbon concentration, in milligrams per liter

0

0.02

0.04

0.06

0.08

0.10

0.12

0 1 2 3

Absorbance at 254 nanometers, per centimeter

In-situ dissolved organic matter fluorescence,

in micrograms per liter quinine sulfate

y = 0.072x - 0.035

r = 0.98

y = 0.044x - 0.020

r = 0.96

y = 21.829x - 12.513

r = 0.96

y = 0.062x - 0.025

r = 0.97

y = 0.0516x - 0.0284

r = 0.95

Clackamas River at Two Rivers Campground

Sampling sites

EXPLANATION

Clackamas River at Carter Bridge

Clackamas River at Estacada

Clackamas River near Barton

Clackamas River at Carver

CRW DWTP–source water

LO DWTP–source water

North Fork Reservoir

Clear Creek

Eagle Creek

Deep Creek

Rock Creek

Sieben Creek

Figure 23. Relation between concentrations of dissolved organic carbon and (A) laboratory measurement of fluorescent 

dissolved organic matter (FDOM), (B) laboratory measurement of UVA254, (C) laboratory measurement of FDOM for main-stem 

samples, (D) laboratory measurement of UVA254, and (E) in-situ FDOM, from the Clackamas River basin, Oregon, 2010–11. (Note 

variable axes values.)
Proxies for Carbon and Disinfection By-Product Precursor Concentrations 55

tac12-0779_fig 24

EXPLANATION

A

B

0

0.01

0.02

0.03

0.04

0.05

0 5 10 15 20 25 30 35 40

In-situ dissolved organic matter fluorescence (FDOM), in quinine sulfate units

0

0.01

0.02

0.03

0.04

0.05

0.06

HAA5 concentration, in milligrams per liter (mg/L) HAA5 concentration, in milligrams per liter (mg/L) 

2010 2011

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

r = 0.83, p < 0.001

95th-percent

confidence

interval

Upper and lower 95th-percent prediction interval

HAA5 Maximum

Contaminant 

Level (MCL) 

at 0.06 mg/L

Predicted HAA5 concentration from in-situ FDOM

HAA5-measured in finished water at Clackamas

River Water drinking-water treatment plant

Figure 24. (A) Relation between in-situ dissolved organic matter fluorescence (FDOM) and total 

haloacetic acid (HAA5) concentrations in finished water, and (B) predicted HAA5 concentrations in 

finished water from in-situ FDOM at the Clackamas River Water drinking-water treatment plant, 2010–11.
56 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Water Treatment and Removal 

Efficiencies for Dissolved 

Organic Carbon and Disinfection 

By-Product Precursors

Water-treatment processes are designed to remove 

organic carbon, among other constituents, to minimize the 

formation of DBPs. The CRW and LO DWTPs coagulate and 

chlorinate raw source water simultaneously, which reduces 

DOC concentrations in finished water to about 0.7 mg/L 

(fig. 25). The loss of DOC during treatment was partially 

dependent on the source-water DOC concentration entering 

the treatment plant such that when DOC concentrations 

were elevated, the percentage of DOC removed increased 

(up to about 50 percent). Although these samples with high 

concentrations of DOC resulted in the highest percentage of 

DOC removed, they also had the highest DBP concentrations 

in finished water (fig. 11).

A. Clackamas River Water drinking-water treatment plant

B. City of Lake Oswego drinking-water treatment plant

0

0.5

1.0

1.5

2.0

2.5

Dissolved organic carbon concentration, in milligrams per liter

0

0.5

1.0

1.5

2.0

2.5

Dissolved organic carbon concentration, in milligrams per liter

4-14 5-12 6-5 7-8 8-3

Date 2010

9-7 10-5 10-10 11-1 12-7 1-18 2-16 3-16 4-19 5-24 6-29 8-2

Date 2011

9-9 9-22

4-14 5-12 6-5 7-8 8-3

Date 2010

9-7 10-5 10-10 11-1 12-7 1-18 2-16 3-16 4-19 5-24 6-29 8-2

Date 2011

9-9 9-22

Percentage of dissolved organic

carbon removed

Source water

Finished water

15% 31%

45%

20% 20% 20%

15%

31%

40%

29%

41%

33%

44%

38%

36%

22%

30%

0%

20%

23%

25%

48%

27%

10% 20%

17%

29%

40%

29%

50%

40% 47%

38%

30% 20%

0%

20%

11%

EXPLANATION

Figure 25. The seasonal pattern in source- and finished-water dissolved organic carbon concentrations 

and percent DOC removal at (A) Clackamas River Water and (B) City of Lake Oswego drinking-water 

treatment plants, Clackamas River basin, Oregon, 2010–11.
Water Treatment and Removal Efficiencies for Dissolved Organic Carbon and Disinfection By-Product Precursors 57

During the water treatment process, there was a 

60–85 percent reduction in fluorescing material between 

source and finished water; 3 representative sample pairs 

are shown in fig. 26. There was also a marked decrease 

in SUVA and an increase in the FI values, suggesting the 

DOM remaining in finished water following coagulation and 

chlorination contained lower aromatic content and lower 

molecular-weight substances, as is commonly reported (Kitis 

and others, 2001; Sharp and others, 2006; Beggs and others, 

2009). Compared with source water, finished water had 

consistently higher percentages of C1 relative to the other 

components, although the total fluorescence loading from 

C1 was lower indicating some removal (fig. 26). Because C1 

was highest in samples from the tributary sites most heavily 

impacted by anthropogenic activities (fig. 20), there may be a 

connection between these land uses and the presence of DOM 

that is less amenable to removal by standard coagulation. 

Additional jar test experiments with similar fluorescence 

measurement might identify treatment methods that target 

removal of such “C1 carbon” that might lead to reduced DBP 

concentrations in finished water.

Results from the treatability jar-test experiments 

conducted on CRW DWTP source water during the four 

basin-wide surveys provided information on the amount 

of DOC and DBP precursors that could be removed by 

coagulation itself and in combination with PAC. Source-water 

DOC concentrations during the experiments ranged from 

0.9 to 1.7 mg/L (fig. 27). Coagulation with optimum doses 

of alum and ACH coagulant reduced DOC concentrations 

30 to 39 percent; the removal rate was highest when the DOC 

concentrations were highest during the October 2010 storm 

(table 12). Following coagulation, SUVA decreased and 

FI values increased, indicating preferential removal of the 

higher molecular-weight, aromatic carbon. 

Co-addition of PAC and coagulants led to slightly 

higher removal of DOC (4–10 percent higher) and lower 

DBPFP values compared to coagulant alone for all jar tests 

except the one conducted on November 9, 2011, which 

showed no difference (fig. 27). These decreases, however, 

only represented a further reduction in DOC concentration 

of about 0.1 to 0.2 mg/L. The amount of DOM removed 

by the laboratory jar tests was higher than that measured in 

finished water by about 20 percent, which was equivalent to 

about 0.2 mg/L DOC. The reduction in DOC concentration 

following coagulation led to a similar decrease in THM 

precursor concentrations; however, the reduction in HAA 

precursor concentrations was much greater. HAAFPs 

decreased about 70 percent during all four jar tests (table 12). 

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Component loadings

0

10

20

30

40

50

60

70

80

90

100

Source

water

Finished

water

Percent component

July 8, 2010 October 10, 2010 November 1, 2010

Sample type and date

Source

water

Finished

water

-77%

-71% -67%

+18%

Source

water

Finished

water

-8%

+19%

+12%

-10%

A

B

tac12-0779_fig 26

-10%

C1

C2

C3

C4

C5

PARAFAC

components

EXPLANATION

Figure 26. Effect of water treatment on (A) component 

loading, and (B) percentage of each component in source 

and finished water during July 2010 low flow, October 2010 

initial storm, and November 2010 major-flush event at the 

Clackamas River Water drinking-water treatment plant, 

Clackamas River basin, Oregon. (Numbers show percent 

reduction in total fluorescence in (A), and the largest 

changes in selected component percentages in (B). See 

figure 21 for explanation of carbon components.)
58 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig 27

A

B

0

0.04

0.08

0.12

0.16

May 12, 2010 October 10, 2010 September 9, 2011 September 22, 2011

Dissolved organic carbon concentration, in miligrams per liter

Disinfection by-product formation potential

for filtered water, in milligrams per liter

Source

CoagulationCoagulation

and PAC

Finished

Source

CoagulationCoagulation

and PAC

Finished

Source

CoagulationCoagulation

and PAC

Finished

Source

CoagulationCoagulation

and PAC

Finished

No

data

No

data

No

data

No

data

0

0.5

1.0

1.5

2.0

Source

Coagulation

Coagulation

and PAC

Finished

Source

Coagulation

Coagulation

and PAC

Finished

Source

Coagulation

Coagulation

and PAC

Finished

Finished

Source

Coagulation

Coagulation

and PAC

PAC

Dissolved organic carbon (DOC)

Powdered activated carbon

EXPLANATION

Trihalomethane formation potential

Haloacetic acid formation potential

PAC Powdered activated carbon

EXPLANATION

Figure 27. Results of treatability jar-test experiments showing the effect of coagulation (aluminum 

sulfate and aluminum chlorhydrate) and powdered activated carbon (PAC) on concentrations of 

(A) dissolved organic carbon and (B) disinfection by-product precursors at the Clackamas River Water 

drinking-water treatment plant, Clackamas River basin, Oregon, 2010–11.
Custom In-Situ Fluorescence Sensors 59

The use of coagulation during drinking-water treatment 

removes TPC and DOC. Because DOC concentrations in the 

Clackamas River have been historically low, the CRW and 

LO DWTPs were designed primarily to target the removal 

of particles during treatment; thus, the coagulation and 

chlorination steps take place simultaneously. Removal of 

TPC and DOC prior to chlorination would, however, likely 

further reduce the formation of THMs and HAAs. Results 

from the jar tests showed coagulation with alum and ACH 

is particularly effective at removing HAA precursors. The 

preferential removal of HAA precursors over the bulk DOM 

pool and THM precursors agrees with the finding that the 

HAA precursor pool is associated with more aromatic, high 

molecular-weight material and that the pool of DOM is more 

amenable to removal by coagulation (Croué and others, 2000; 

Liang and Singer, 2003; Hong and others, 2008). Because 

the BQ values for HAAs in finished water are occasionally 

elevated, upgrades to treatment plants that allow for 

coagulation and filtration prior to chlorination may, therefore, 

be a design improvement. 

The CRW and LO DWTPs occasionally use PAC during 

water treatment (up to 5 mg/L at CRW and up to 50 mg/L 

at LO) to control for tastes and odors. In contrast to studies 

that have found substantial removal of HAAs and THMs 

with commercial charcoal-filtration water pitchers (Levesque 

and others, 2006), the addition of PAC provided only minor 

improvement in DOC and DBP precursor removal, based 

on the four jar tests conducted here. A full evaluation of this 

method to reduce DBP precursors may be warranted because 

these data only reflect conditions during the study period. The 

benefits of PAC may be more pronounced during periods of 

higher algal contributions when the DOM pool contains higher 

fractions of low-SUVA, high-FI material.

Custom In-Situ Fluorescence Sensors

The three custom fluorometers designed for this study 

were deployed at the CRW DWTP intake, but only for a 

limited time, and late in the study period (fig. 28 and table 4). 

The majority of this period covered the prolonged recession 

to the summer base-flow period, through autumn, and into 

the January rainy season. Individually, the two sensors 

centered near Peak C designed to measure the FI tracked DOC 

concentration and were highly correlated to FDOM from the 

standard Cyclops-7 fluorometer, which is also focused on this 

Peak C region but has a wider band-pass (table 5).

While both of the Peak C custom sensors tracked DOC 

concentration, there was a change in the ratio of these two 

sensors, referred to here as FIin-situ (fig. 28). Prior to this study, 

the FI has only been calculated using benchtop measurements 

of fluorescence; the ratio of emission at 470 to 520 nm at 

an excitation wavelength of 370 nm effectively provides 

information about the slope of the fluorescence response in 

this region of EEMs space (McKnight and others, 2001). As 

described above, the FI provides information about DOM 

source and composition. The two in-situ sensors developed 

for this study, however, have a broader band pass than 

benchtop instruments. Thus, while the ratio of these two 

sensors is expected to be correlated to the benchtop FI ratio, 

the absolute value of the FIin-situ is expected to differ from 

what is commonly reported for benchtop measurements of 

surface-water samples—a range of 1.2 to 1.8 (Cory and others, 

2010). The FIin-situ values for this study ranged from 0.9 to 

about 1.2 (fig. 28). 

Over the 10-month deployment of these sensors, 

the FIin-situ values first decreased as streamflow and DOC 

concentration decreased and then increased in September 

through December 2011 when streamflow and DOC 

concentrations remained low and relatively stable (fig. 28). 

Table 12. Percent removal of dissolved organic carbon and 

disinfection by-product formation precursors during treatability 

jar-test experiments conducted at the Clackamas River Water 

drinking-water treatment plant, Clackamas River basin, Oregon, 

2010–11.

[Average values ± one standard devation. Removal of dissolved organic 

carbon (DOC) from source to finished water is shown for comparison. 

Abbreviations: DBP, disinfection by-product; THMs, trihalomethanes; 

HAAs, haloacetic acids; PAC, powdered activated carbon; –, not applicable or 

no data]

Date Treatment

Percent removal

DOC DBP precursors

THMs HAAs

05-12-10 Coagulation 36 – 42 ± 5.3 76 ± 1.6

Coagulation + PAC 40 ± 5.7 52 ± 5.4 79 ± 1.2

Source-finished water1 28 – – – – –

10-10-10 Coagulation 39 ± 0.0 45 ± 1.8 69 –

Coagulation + PAC 48 ± 4.3 53 ± 1.1 76 ± 2.2

Source-finished water1 33 – – – – –

09-09-11 Coagulation 33 ± 0.0 13 ± 2.9 69 ± 1.4

Coagulation + PAC 33 ± 0.0 15 ± 10.0 74 ± 5.4

Source-finished water1 0 – – – – –

09-22-11 Coagulation 30 ± 0.0 27 – 65 –

Coagulation + PAC 40 ± 0.0 38 ± 4.1 73 ± 1.0

Source-finished water1 20 – – – – –

1

Percent reductions in DOC during actual treatment at the Clackamas River 

Water drinking-water treatment plant.
60 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

tac12-0779_fig 28

A

B

C

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

In-situ Fluorescence Index

Streamflow, in cubic feet per second

Total chlorophyll-a, in micrograms per liter

0

2

4

6

8

10

12

14

16

18

20

0

100

200

300

400

500

600

Standard, FI-A, FI-B Cyclops fluorometers,

 in relative fluorescence units

Peak T Custom C7 (ex270/em340),

in relative fluorescence units

0

2

4

6

8

10

12

14

16

18

20

22

2011 2012

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

Daily median

Daily maximum

Standard C7 (ex365/em470)

FI-A Custom C7 (ex370/em470)

FI-B Custom C7 (ex370/em520)

Peak T Custom C7 (ex270/em340)

EXPLANATION

Streamflow (Oregon City streamflow 

gage - 14211010)

In-situ Fluorescence Index (FIIn-situ)

EXPLANATION

EXPLANATION

Figure 28. (A) Continuous fluorescence from the standard and three custom fluorescent dissolved 

organic matter sensors, (B) streamflow and in-situ Fluorescence Index, and (C) daily median and maximum 

water-column chlorophyll-a in the lower Clackamas River, Oregon, 2011–12.
Sources of Organic Carbon that Contribute Disinfection By-Product Precursors 61

Because FI values are assumed to be independent of 

concentration, these trends suggest a change in carbon 

composition. The initial decrease in FIin-situ suggests a trend 

towards a DOM pool increasingly dominated by terrestrial, 

high molecular-weight material. The increase during the later 

period of deployment in September–November coincided with 

the drawdown of Timothy Lake, and a period when benthic 

algal populations may have started to senesce at the end of 

the growing season. Concentrations of chlorophyll-a at the 

Oregon City monitor produced occasional spikes up to about 

10 μg/L (fig. 28); these periodic peaks suggest moderate 

sloughing of benthic algae. With the onset of rain, FIin-situ

values increased, perhaps because of suspension of decaying 

algae in the river or some other factor. Although data collected 

for this study are not conclusive, these initial results show a 

dynamic response in the FIin-situ to changes in DOM character 

and quantity, and show promise for future water-quality 

monitoring applications.

The design of the experimental Peak T custom sensor 

(ex 270/em 340 nm) required the use of a low-ultraviolet LED 

(table 5). The signal from this sensor was low, likely because 

of the low amount of fluorescence response from DOM in this 

region of the EEMs. Furthermore, the loss of signal from the 

sensor in mid-July was determined to be caused by loss of 

signal output from the lamp. Testing of LEDs in this region 

has shown that photon output from these deep ultraviolet 

LEDs are short-lived. As LED manufacturers continue to 

improve the efficiency of deep ultraviolet LEDs—the ability 

of the device to convert electrons to photons or the external 

quantum efficiency—these deep ultraviolet LED-based field 

sensors are expected to improve concurrently. Testing of these 

low-ultraviolet fluorometers in waters that are known to have 

higher fluorescence in this region (wastewater impacted rivers, 

for example; Hudson and others, 2008; Goldman and others, 

2012), may also prove to be a more suitable application for 

these sensors. 

Sources of Organic Carbon that 

Contribute Disinfection By-Product 

Precursors

Many lines of evidence were considered to evaluate 

possible sources of DOM that form regulated DBPs in the 

Clackamas River basin. The individual samples provided 

some indication as to where and when samples had elevated 

DBPFP values, and longitudinal and temporal variations in 

the data were useful for identifying patterns in the mainstem, 

especially when combined with Data Grapher analyses of the 

continuous water-quality data from the four-station monitoring 

network (fig. 1).

Considering that STHMFP and SHAAFP values were 

not correlated (appendixes F5 and F6), these two classes of 

DBPs seem to have different fundamental sources, which 

may require different watershed-management strategies 

for control. The filtered STHMFP value from North Fork 

Reservoir during an algal bloom (see photograph 2a-b) was 

the single highest value, which points to this source as worthy 

of future monitoring should THM concentrations increase 

in the future. The highest specific, or carbon-normalized, 

DBPFP values (table 9 and fig. 16) suggest that carbon most 

prone to producing THMs came from the reservoir and lower 

main-stem Clackamas River in September 2011; carbon 

contributing to HAA formation was highest in the three largest 

lower-basin tributaries, Eagle, Clear, and Deep Creeks during 

the initial October 2010 storm (fig. 14). These streams drain 

basins containing large proportions of private timberland, rural 

residential, and some agricultural land, primarily Christmas 

tree plantations, nurseries, cane berries, and some row crops 

(fig. 3). The greatest amount of agricultural land in the basin, 

about 16 mi2, is contained within the Deep Creek Basin where 

nurseries and Christmas tree farms are abundant (Carpenter, 

2003). It is, however, unclear to what degree each of these 

potential sources contributes DBP precursors to the mainstem 

and downstream drinking-water intakes.

While lower-basin tributaries had the highest 

concentrations of organic carbon (DOC and TOC) and DBP 

precursors (fig. 17), because of the relatively low flows from 

these streams most of the time, the primary sources of carbon 

are located in the upper forested basin where most of the flow 

also originates. This finding is consistent with a study of the 

nearby McKenzie River that also found DBP precursors to be 

primarily derived from upper-basin terrestrial carbon (Kraus 

and others, 2010). Our results also show elevated THM- and 

HAA-precursor concentrations when DOC concentrations 

also were elevated (fig. 11), and DBPs were formed primarily 

from chlorination of dissolved, terrestrially-derived organic 

compounds such as humic and fulvic acids. This is consistent 

with results for the DBPFPs that showed the dissolved 

fraction dominating the precursor pool (fig. 14); although 

particulate carbon did contribute some DBP precursors, most 

were dissolved. 

Terrestrial Inputs

Multiple lines of evidence indicate terrestrial inputs are 

the dominant sources of carbon to the lower Clackamas River. 

First, most of the DBPFP loads in the lower Clackamas River 

at the CRW DWTP are already accounted for at Estacada 

(table 10), which drains a mostly forested basin. Second, 

the fluorescence data and PARAFAC model identified 

five dominant components, four of which are soil-related 

(fig. 21). Third, the co-dominant HAA in finished water was 

TCAA, an organic compound widely found in forest soils 

(McCulloch, 2002). 
62 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

TCAA is a chlorinated hydrocarbon with many sources. 

It is used for many industrial purposes, including the synthesis 

of other organic compounds, and as an herbicide, for 

example, but it is also a breakdown product of TCE and other 

solvents. TCAA is also formed during various chlorination 

processes, in wood-pulp processing and drinking-water 

treatment (McCulloch, 2002). TCAA is measureable in the 

atmosphere at concentrations ranging from less than 0.02 

μg/L in Switzerland up to 20 μg/L in urban Berlin, Germany; 

global average concentrations are about 0.5 μg/L (McCulloch, 

2002). Although TCAA is not volatile, it is highly soluble in 

water and can precipitate in rain; this explains its prevalence 

in forest soils, especially coniferous soils (McCulloch, 2002). 

While it is possible there is some background level of TCAA 

in the Clackamas River derived through this process, this has 

not been investigated.

Although unstudied here, the erosion of TCAAcontaining soils might be a factor in the prevalence of 

TCAA in drinking water from the Clackamas River. In a 

previous study, Carpenter (2003) found that levels of total 

phosphorus (TP) in the forested tributaries of the Clackamas 

Basin were highly correlated with percentage of “non-forest 

upland”—mostly timber harvest areas (r = 0.96, p <0.001). 

TP also was correlated with silica concentrations (r = 0.84, 

p <0.001), which suggests soil erosion could be a source of 

phosphorus to the river. It is possible the loss of particulate 

and dissolved carbon from the forested watershed areas are 

lost in a similar fashion through erosion, although leaching of 

DOM from organic soil horizons also certainly contributes to 

riverine carbon. 

The flushing of decomposed organic matter by autumn 

and winter rains is a complex process governed by the types of 

organic matter and microbes present, thickness of the vadose 

zone and local water-table dynamics, temperature, and other 

factors. Decomposition of plant materials is a key process that 

successively transforms solid organic matter into fine particles, 

colloids, and DOM solutes that can leach into surface waters 

and form DBP precursors.

It is hypothesized here that increased deposition of 

nitrogen, from atmospheric and other sources, has accelerated 

decomposition rates of organic matter in portions of the 

watershed including deforested and previously harvested 

regrowth forests. Because of the large store of bulk carbon 

in the forest and the extensive timber harvesting in the basin 

(see numerous U.S. Forest Service “watershed analyses” 

reports referenced in Carpenter [2003]), decomposition of 

organic matter and burned slash left over from previous 

harvest operations might be an important source of carbon 

to the river that may have increased over time and may help 

explain the increase in DBPs observed over the last three 

decades (fig. 4). Although Federal forestland in the Pacific 

Northwest, including most of the Clackamas Basin, has been 

recovering since the Northwest Forest Plan was enacted in 

1993, timber harvesting was extensive in the late 1980s and 

early 1990s, which produced a large reservoir of decaying 

organic matter that may have subsequently leached DOM. 

Previous studies cited in Turner and others (2011) suggest 

that estimates of forest carbon stocks in the region covered by 

the Northwest Forest Plan had declined substantially during 

1953–87 from high rates of timber harvesting. In recent years, 

however, regrowth of many of these forests has increased net 

ecosystem productivity, thereby possibly shifting the balance 

toward storing carbon through sequestration (Turner and 

others, 2011). Whether or how this may change export of DBP 

precursors is, however, not known but could be examined with 

further study.

Algae

Even though algae fix carbon through photosynthesis and 

contribute to organic-matter pools, the degree to which benthic 

and phytoplankton algae contribute DBP precursors in the 

Clackamas River remains an important unanswered question. 

Although it is well-established that algae can be a source of 

DOM-containing DBP precursors, there is conflicting evidence 

regarding whether DOM produced by algae is more or less 

reactive per unit carbon compared with terrestrial sources 

(Jack and others, 2002; Nguyen and others, 2005; Kraus 

and others, 2011). Differences in DBP-precursor content of 

algal-derived DOM likely arise from a combination of factors, 

including algal species, growth stage, release of extracellular 

material, and environmental processing of algal-derived 

material and compounds (Jack and others, 2002; Nguyen and 

others, 2005). 

To initially address this question, correlations were 

examined between chlorophyll-a and DBPFP (fig. 29). This 

approach focuses on the viable particulate algal cells that 

fluoresce and did not include non-fluorescing particles, such 

as dead or decaying algal filaments. Although the correlations 

between chlorophyll-a and THMFP were weak, the relation 

with HAAFP was significant (p <0.001) for filtered (r = 0.56) 

and unfiltered (r = 0.49) samples (fig. 29). 

Other lines of evidence suggest algae are having an 

influence over carbon amount and quality in the Clackamas 

River. While water-column chlorophyll-a concentrations 

were atypically low in the lower mainstem (fig. 8), there were 

several observations from the data that not only demonstrate 

how algae can affect carbon amounts and DOM quality in the 

river, but also provide some evidence algae is contributing 

THM precursors. 
Sources of Organic Carbon that Contribute Disinfection By-Product Precursors 63

The relatively high STHMFP in the September 2011 

sample from the release depth within North Fork Reservoir 

(table 9), for example, indicated the DOM present during 

the blue-green algae bloom was reactive and formed THMs. 

The amount of reactive organic matter produced by any 

individual bloom (and the production of algal toxins or taste 

and odor compounds) could depend on a number of factors, 

including the overall size and health of the bloom. Future 

studies of Timothy Lake or North Fork Reservoir could begin 

to characterize these processes. The concentrated “algae 

grab” sample with a high abundance of Anabaena flos‐aquae

(fig. 20) shows a strong signal in the lower ultraviolet 

“protein-like” region, which was interpreted in this system 

to be indicative of the more labile, freshly produced organic 

matter expected in such a sample (fig. 21). This signal, 

represented by PARAFAC component C5, was also apparent 

in the full suite of reservoir samples (fig. 22). Organic matter 

recently contributed by these blooms is expected to contain 

a greater protein-like signal, particularly because Anabaena 

flos-aquae is a nitrogen-fixer and a common member of the 

phytoplankton assemblage in North Fork and Timothy Lake 

during summer. 

As discussed above, algae may also contribute HAA 

precursors, as the positive correlations with chlorophyll-a

suggest (fig. 29). Further, the highest HAAFP values occurred 

during the October 2010 storm (fig. 16), at the end of the 

growing season, when concentrations of chlorophyll-a due to 

algal sloughing were highest.

tac12-0779_fig 29 

Clackamas River at Two Rivers

Campground

Sampling sites

EXPLANATION

Clackamas River at Carter Bridge

Clackamas River at Estacada

Clackamas River near Barton

Clackamas River at Carver

CRW DWTP-source water

LO DWTP-source water

r2 = 0.08

0 0.05 0.10 0.15 0.20 0.25

r2 = 0.18

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0 7.0

0 0.05 0.10 0.15 0.20

r2 = 0.49

0 0.05 0.10 0.15 0.20

r2 = 0.56

0 0.02 0.04 0.06 0.08 0.10

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0

1.0

2.0

3.0

4.0

5.0

6.0

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Filtered total haloacetic acid formation potential, in milligrams per liter Unfiltered total haloacetic acid formation potential, in milligrams per liter

Filtered total trihalomethane formation potential, in milligrams per liter Unfiltered total trihalomethane formation potential, in milligrams per liter

Chlorohpyll-a concentration, in micrograms per liter

Figure 29. Relations between water-column chlorophyll-a and filtered and unfiltered disinfection by-product formation 

potentials for main-stem Clackamas River sites, Oregon, 2010–11. (Abbreviations: CRW, Clackamas River Water; LO, City of 

Lake Oswego.)
64 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

Phytoplankton Blooms

Blooms of blue-green algae occurred in Timothy Lake 

and North Fork Reservoir during this study, prompting the 

Oregon Health Authority to issue a human-health recreational 

advisory for both water bodies. Although these blooms were 

not as severe as in years past, elevated STHMFP values in 

North Fork Reservoir in September 2011 (table 9) indicate 

a high degree of reactivity for this type of carbon, a finding 

that is consistent with other studies (Jack and others, 2002; 

Kraus and others, 2011). Because the phytoplankton blooms 

in 2010 and 2011 were seemingly small, the high degree of 

reactivity shown by the high STHMFP suggest phytoplankton 

populations in the reservoirs could become important sources 

of THM precursors, especially if larger blooms occur in 

the future.

As described above, phytoplankton in the two Clackamas 

reservoirs could be an important source of DBP precursors 

for the Clackamas River. Blooms of blue-green algae and 

diatoms regularly occur in these reservoirs (see Carpenter, 

2003), although the severity and duration vary widely, likely 

responding to the specific growing conditions and other 

factors that affect bloom development. Although sustained 

flow during summer in 2010 and 2011 may have limited the 

residence time in North Fork Reservoir, blooms were observed 

in the reservoir both years, but biomass was relatively 

low—less than 3 μg/L chlorophyll-a at the log boom. The 

horizontal and vertical mobility of these blooms makes it 

challenging to accurately characterize conditions with just one 

sampling. Considering longitudinal patterns, the STHMFP 

measurement in North Fork Reservoir was nearly two times 

higher compared with Carter Bridge in late summer, possibly 

reflecting the importance of the reservoir-derived carbon 

in forming THMs and possibly also contributing to taste 

and odor issues. The DBPFP measurements also suggested 

phytoplankton or their exudates produce DBP precursors. 

The highest STHMFP value for filtered samples, again, was 

from the release depth within North Fork Reservoir (table 9) 

and suggests the reservoir was a source of THM precursors to 

downstream sites.

Previous studies have examined fluxes of DBP precursors 

in reservoirs and lakes and found them to act as either a 

source or sink for DBP precursors (Stepczuk and others, 

1998; Nguyen and others, 2002; Bukaveckas and others, 

2007). In a recent study examining changes in DOM amount 

and composition in San Luis Reservoir, California, it was 

found that DBP precursors (particularly HAAs) increased 

during the summer months because of high phytoplankton 

activity; however, the reservoir was a sink for DBP precursors 

during the winter months when decomposition processes 

predominated (Kraus and others, 2011). Results from the 

Clackamas River study also suggest that information about the 

composition of organic matter within reservoirs, as well as its 

reactivity with respect to THM and HAA formation, can help 

explain downstream changes in source-water quality.

Benthic Algae and Periphyton Sloughing Events

Although conditions were not particularly favorable 

for algal growth in 2010–11, nuisance levels of benthic 

algae (greater than 100–150 mg/m2; Welch and others, 

1988) developed in some main-stem and tributary locations 

during this study (table 7). Large masses of stalked diatoms, 

filaments of green algae, and ears of blue-green algae (see 

photographs 1a-e) were common among other types of algae 

in the mainstem.

On the basis of water-column chlorophyll-a

concentrations in the lower mainstem, which were nearly 

always relatively low (less than 2 μg/L), it appears on initial 

inspection that benthic algae were not important contributors 

to the water column during the study. While large algal 

“sloughing” events did not occur, higher-than-normal 

streamflow and other possible factors such as high rates of 

benthic invertebrate grazing or other factors may have limited 

or moderated water-column concentrations of chlorophyll-a

during the study, as discussed above. Regardless of this 

finding, recognizing the unusually high streamflow, the high 

algal biomass observed in the river (table 7), along with the 

knowledge of past algal sloughing events (U.S. Geological 

Survey, 2012), it is suspected that sloughed benthic algae 

might, at times, be a significant source of DBP precursors to 

the lower river.

As described previously, sloughing of benthic algae is 

a common feature of the Clackamas River and some other 

Cascade Range rivers such as the North Umpqua and Rogue 

Rivers. Such events do not necessarily take place every year, 

but in some years (fig. 30), concentrations can be high for 

a system dominated by periphyton. Such sloughed algae 

impacts water clarity and fishing conditions in the Clackamas 

River and has clogged DWTP intake screens in the past. This 

demonstrates that, at times, benthic algae are a source of TOC 

in the river and, thus, likely a source of DBP precursors.

These events are sometimes associated with increased 

streamflow resulting from storms, the annual drawdown of 

Timothy Lake (which increases flow in the mainstem by 

100 ft3/s or more), and (or) changes in reservoir releases. 

For example, chlorophyll-a concentrations in the tributaries 

reached moderate levels (5–8 μg/L) during the October 2010 

storm, which contributed to observed increases in the 

mainstem. The algal biomass in the mainstem more than 

doubled between Barton and Carver from tributary inputs—

especially Deep Creek—and possibly also from benthic algae 

sloughed from main-stem locations downstream from Barton. 

Although DBP concentrations were not particularly high in 

finished water at the time, it raises the question about the 

degree to which algae may be contributing DBP precursors 

because chlorophyll-a levels can become elevated in the river, 

as this storm demonstrated. 
Sources of Organic Carbon that Contribute Disinfection By-Product Precursors 65

Hydroelectric project operations, including flow ramping 

for power production and the annual drawdown of Timothy 

Lake near the end of summer starting around Labor Day, can 

increase the amount of algal cells, DOC, and TOC in the upper 

river (Carpenter, 2003). The drawdown releases phytoplankton 

produced through the summer growing season, and increased 

flows—about 10 percent higher flow in the mainstem—can 

scour and suspend benthic algae into the water column. Even 

though, as a condition of the new Federal Energy Regulatory 

Commission operating license, flow ramping in the upper river 

at the Three Lynx powerhouse, upstream from Three Lynx 

Creek (fig. 1), is not as great as in years past, hydroelectric 

project operations still cause abrupt changes in flow that 

can scour algae from the riverbed and increase carbon 

concentrations in the river.

Although not explicitly measured here, the Timothy Lake 

water also may contribute DBP precursors, although it did not 

appear to increase DBPFP substantially at Carter Bridge in 

this study during the drawdown (fig. 14). A comparison of the 

longitudinal conditions in the mainstem near the beginning of 

the September 2011 drawdown period to those later during the 

drawdown, near the peak in flow, shows the TPC and THMFP 

did increase in samples from Estacada downstream to the LO 

DWTP intake (fig. 31A). The unfiltered formation potentials 

were 22–81 percent higher than the filtered THMFP (not 

shown), suggesting a particulate source.

40

25

30

35

20

15

10

5

0

Daily maximum chlorophyll-a concentration, in micrograms per liter

Jan. Feb. Mar. Apr. May June

Month

July Aug. Sept. Oct. Nov. Dec.

tac12-0779_fig 30

2005

2006

2007

2008

2009

2012

2003

2002

Year

EXPLANATION

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

Figure 30. Daily maximum concentrations of chlorophyll-a in the lower Clackamas River at Oregon City, Oregon, 

2002–12, showing the higher concentrations that can result from sloughed benthic algae. (Scale is truncated at 

40 micrograms per liter.)
66 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

A. Main-stem sites

Concentration, in milligrams per liter

0

0.05

0.10

0.15

0.20

0.25

0.30

Sept.

8

CR at

Carter

Bridge

Sept.

16

Sept.

9

Sept.

22

Sept.

9

Sept.

22

Sept.

9

Sept.

22

Sept.

9

Sept.

22

Sept.

9

Sept.

22

CR at

Estacada

CR at

Barton

CR at

Carver

CRW DWTP

sourcewater

LO DWTP

sourcewater

Year

2011

D. Clackamas River near Oregon City, Oregon (14211010)

Daily median turbidity, in

formazin nephelometric units

Daily median chlorophyll-a concentration,

in micrograms per liter

0.8

0.9

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

2011

30

August September October

2 5 8 11 14 17 20 23 26 29 2

C. Clackamas River near Oregon City, Oregon (14211010)

Daily median streamflow, in cubic feet per second

Daily median chlorophyll-a concentration,

in micrograms per liter

1,020

1,000

980

960

940

920

900

880

860

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

2011

30

August September October

2 5 8 11 14 17 20 23 26 29 2

2011

B. Clackamas River near Oregon City, Oregon (14211010)

Streamflow, in cubic feet per second

Chlorophyll-a concentration,

in micrograms per liter

0

2

3

1

4

5

6

7

8

9

750

900

800

1,000

1,100

1,200

Basin-wide

sampling

Basin-wide

sampling

30

August September October

2 5 8 11 14 17 20 23 26 29 2

EXPLANATION

Total particulate carbon

THMFP for unfiltered samples

EXPLANATION

Streamflow

Chlorophyll-a

Streamflow

Chlorophyll-a

EXPLANATION

EXPLANATION

Turbidity

Chlorophyll-a

Figure 31. September 2011 data showing (A) concentrations of total particulate carbon and trihalomethane formation 

potential for unfiltered samples in the Clackamas River at the beginning and near the peak of the Timothy Lake drawdown, 

(B) time series of streamflow and chlorophyll-a, (C) daily median values of streamflow and chlorophyll-a, and (D) daily 

median turbidity and chlorophyll-a in the lower main-stem Clackamas River at Oregon City.
Sources of Organic Carbon that Contribute Disinfection By-Product Precursors 67

Whether these September 2011 increases were due to 

sloughing of periphyton, or from displacement of bottom 

water from North Fork Reservoir (fig. 32) is not known. This 

“displacement” mechanism was first proposed by the USGS 

during the August 2003 taste and odor event at the CRW 

DWTP, when decreases in water temperature and increases 

in turbidity were noted at the Estacada water-quality monitor 

downstream from the North Fork Reservoir at the time of the 

drawdown. Taste and odor problems eventually developed 

in early October 2011, prompting the CRW DWTP to begin 

using powdered activated carbon (PAC). The cause of the 

event was not identified, but in August 2003, taste and odor 

problems were traced to geosmin, a known taste and odor 

compound (Graham and others, 2010), which was detected 

at concentrations between 0.024 and 0.044 μg/L in North 

Fork Reservoir at depths of 80 and 30 ft, respectively (data 

furnished by CRW). Given that a bloom of blue-green 

algae did occur in 2011, it is certainly possible that this 

contributed to the observed taste and odor problem. While 

water displacement from North Fork Reservoir may have 

contributed to the 2011 taste and odor event, the cooler 

temperatures and higher flows in 2010–11 probably minimized 

this process because residence times were likely shortened and 

thermal temperature stratification weaker compared with 2003. 

Drawdown also may have enhanced the export of blue-green 

algae cells to the outflow in 2011 by disrupting population 

“layering” at depth within the reservoir, which has been noted 

previously for this reservoir (Carpenter, 2003).

Water displacement hypothesis

Before Timothy Lake release

During Timothy Lake release

In late summer, warmer water input 

flows through the reservoir to the 

outflow, leaving behind cooler 

bottom water.

Colder water inflows sink and move 

along the bottom, displacing the bottom 

water upward and out of the reservoir, 

increasing downstream turbidity and 

lowering pH.

Input

Input

Figure 32. Water-displacement hypothesis, showing the proposed mechanism by which the drawdown of Timothy 

Lake increases transport of bottom water from North Fork Reservoir, Clackamas River basin, Oregon.
68 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

The September 2011 event also provided an opportunity 

to examine how water-column chlorophyll-a levels and 

turbidity respond to the increased flow in the river during the 

drawdown (fig. 31B). Even though the drawdown was one of 

the main differences between these dates, there was a jump 

in flow of 190 ft3/s in the lower river that did not cause much 

of a response from the chlorophyll-a sensor. This was not 

totally unexpected because it is probably the more senescent 

algae that would detach during the drawdown (if it were 

benthic algae). If blue-green algae cells were released from 

the reservoirs, those also would not cause a large response 

from the chlorophyll-a probe; phycocyanin probes are much 

better suited for detecting blue-green algae. Although the full 

time-series data, which has considerable within-day variation, 

does not show any apparent increase in chlorophyll-a during 

drawdown, the daily median values (48 daily measurements) 

do show a very small increase (fig. 31C) that may have been 

caused by algal sloughing. These increases were not large, 

however, but the small jumps in streamflow that occurred at 

the Oregon City streamflow gage do seem to line up, more 

or less, with these small increases in chlorophyll-a, possibly 

indicative of scouring. This process has been noted previously 

in May 2003, for example (U.S. Geological Survey, 2012). 

The increases in chlorophyll-a in 2011 were, however, quite 

small and probably within instrument error. While this does 

add to the weight of the evidence that algae are a source of 

DBP precursors and provides a possible mechanism that could 

enrich source waters, these data are not sufficient to prove or 

disprove the hypothesis regarding algal sloughing.

During this possible sloughing event, the THMs in 

finished-water samples from the CRW DWTP actually 

decreased, while those from the LO DWTP increased. The 

lower THM4 values at CRW were not, however, attributable 

to the use of PAC, because it was not used during the active 

data-collection phase of the study but was employed later in 

October 2011 as taste and odor problems were developing.

Wastewater

Although the Clackamas River indirectly or directly 

receives wastewater effluents from three wastewater treatment 

plants (fig. 1) and effluents from possibly thousands of septic 

tanks in the basin, the effluents did not produce a carbon 

signature that could be construed as being definitively related 

to wastewater. Although sampled less frequently (n=3), there 

was also no definitive wastewater signal from Deep Creek, 

which receives wastewater-treatment-plant discharges from 

the city of Sandy and town of Boring through Tickle Creek 

and North Fork Deep Creek, respectively. 

Strategies for Managing and Reducing 

Disinfection By-Products

The strong terrestrial signature of the carbon in the 

Clackamas River suggests watershed-management strategies 

aimed at controlling soil erosion from forestland might reduce 

the input of DBP precursors to the river. Timber harvesting 

and associated road construction can cause erosion that 

commonly leads to sedimentation of streams and causes 

elevated turbidity. Soil erosion in the Clackamas River basin 

is widespread, and continuous monitoring since 2002 has 

shown high turbidity after storms every year (U.S. Geological 

Survey, 2012).

In many parts of the upper basin, unstable geology 

and steep slopes contribute to mass wasting and erosion, 

particularly in the Fish Creek and Collawash River Basins, 

and along portions of the upper Clackamas River where the 

potential for surface erosion is relatively high (Metro Regional 

Services, 1997). In the lower basin, tributaries become highly 

turbid after rain events, delivering sediment (and pesticides) 

to the main-stem Clackamas River (Carpenter and others, 

2008); tributaries also contained high concentrations of DBP 

precursors. Clackamas County Water Environment Services 

and others are working to reduce storm-water discharge 

pollution, rates, and volumes in the Sieben and Rock Creek 

watersheds, which may reduce peak concentrations of DBP 

precursors in the Clackamas River (Andrew Swanson, 

Clackamas County Water Environment Services, written 

commun., 2012), although these controls were not evaluated 

for their efficacy in removing DBP precursors.

Certain forest-management activities such as fertilizing 

with urea-nitrogen, burning of post-harvest slash, and 

disturbing and exposing soils to direct precipitation could all 

contribute to leaching of DOM into the shallow groundwater 

system and eventual transport to the river. Although forest 

fertilization on Federal forestland largely ended with the 

Northwest Forest Plan, this practice was common until 1996 

in the Clackamas River basin, where applications peaked 

at over 1 million pounds of nitrogen annually (Carpenter, 

2003). Adams and others (2005) found that, although variable, 

fertilization with urea-nitrogen increased soil leaching of DOC 

in another Pacific Northwest Douglas-fir forest in Washington 

State, but it is unclear whether this has been important in the 

Clackamas River basin or not.

Suppression of wildfire also may increase abundance of 

microbes in forest soils as was found in another Douglas‐fir 

forest in Colorado (Switzer and others, 2012). Although there 

have been a few small fires in the Clackamas River basin 
Conclusions, Implications for Drinking-Water Treatment and River Management, and Possible Future Studies 69

over the past 20–30 years, there have been no large fires 

comparable to some of the larger burns in 1930 and 1940, for 

example (Taylor, 1999). From 1900 to about 1940, seven large 

fires burned 2,000 to over 11,000 acres in the Clackamas basin 

(Carpenter, 2003). It is not known whether the lack of large 

fires has increased microbial activity or the export of carbon 

and DBP precursors from forested areas, but this could be an 

area for future study.

It is important to understand the primary sources of DBP 

precursors and processes that control their transport to the 

river. It is equally important to determine the capacity of the 

forest to store rather than leach organic carbon, because this 

may ultimately control concentrations of DBP precursors in 

source water and DBPs in finished drinking water, in concert 

with precipitation, snowpack, and other factors that affect flow 

and dilution rates and other influences that affect the annual 

growth of benthic algae and phytoplankton in this system.

Fluorescence Technology for 

Drinking-Water Management

Optical properties measured in the lab were highly 

correlated to DOC concentrations and THMFP and HAAFP in 

watershed samples. Similarly, in-situ FDOM sensors provided 

an excellent proxy for continuous DOC concentration in the 

Clackamas River, demonstrating great promise as a costeffective tool to better understand DBP precursor sources, 

seasonality and trends, and possible management of precursor 

compounds. Because DOM concentration is a driving factor 

in the production of DPBs in finished water and a good 

predictor for DBPFP for all watershed samples, continuous 

in-situ FDOM measurements are potentially valuable as an 

early-warning system, and for understanding and possibly 

forecasting finished-water DBP concentrations. In this study, 

for example, FDOM was highly correlated with finished-water 

HAA5 concentrations, thus this relation was used to estimate 

continuous HAA5 concentrations over time (fig. 24).

Although both laboratory measurements of FDOM 

and UVA254 were effective proxies for DOC concentration 

(fig. 23 and table 11), measurement of fluorescence has 

several advantages over absorbance (Henderson and others, 

2009; Kraus and others, 2010; Bridgeman and others, 

2011). In particular, absorbance measures are more prone 

to metal quenching and particle interference compared with 

fluorescence methods, and absorbance requires filtration prior 

to analysis. The fact that these flat-faced and flow through 

fluorometers can measure fluorescence directly without 

filtration is a tremendous advantage. In this study, FDOM 

produced as high or higher correlations with finished-water 

DBP concentrations and laboratory DBPFPs compared 

with more traditional absorbance measures such as UVA254

(table 11). This corroborates findings from a similar study of 

the nearby McKenzie River (Kraus and others, 2010).

Conclusions, Implications for 

Drinking-Water Treatment and 

River Management, and Possible 

Future Studies

While some of the initial results of this study were 

used to inform recent upgrades to the water-treatment plant 

at the LO DWTP (Kari Duncan, City of Lake Oswego, oral 

commun., 2010), it is hoped that future studies build on this 

research and identify specific areas or activities that produce 

DPB precursors so that watershed-management efforts can be 

targeted and wisely prioritized.

The DOM precursor pool in the Clackamas River 

basin was strongly influenced by season, streamflow, and 

perhaps most notably, the effects of storms. However, not 

all storms were alike in terms of the amount or character of 

carbon exported, which reveals that factors such as time of 

year, antecedent flow, snowpack conditions, and patterns in 

rainfall and runoff affect carbon export. This study identified 

the primary sources of organic matter that contributed to 

DBP precursors in raw source-water supplies in the lower 

Clackamas River, which turn out to be primarily dissolved 

organic compounds that are terrestrial in nature. Multiple 

lines of evidence also supported the hypothesis that algae 

may, at times, be contributors to the DBP precursor pool, 

especially THMs. 

Higher DBPFPs in some unfiltered samples compared 

to filtered samples suggested that, at times, a considerable 

amount of the total DBP precursor pool was composed of 

filterable particles such as detritus, soil particles, and algae. 

This suggests that filtration prior to chlorination could reduce 

finished-water DBP concentrations. Differences between 

watershed THM and HAA precursor sources, as well as their 

treatability, were evident, suggesting different actions may be 

necessary to manage for these types of DBP precursors. 

Although terrestrial sources of carbon dominated the 

Clackamas River in 2010–11, some of the results obtained 

here suggest that algae also contributed some carbon to the 

river. While benthic algae reached nuisance levels in the 

mainstem and caused large and synchronized swings in 

dissolved oxygen concentrations and pH—key photosynthesis 

indicators—conditions did not lead to clogged intakes or 

substantial sloughing as has occurred years past. Such material 

also contributes carbon that, to some degree, contains DBP 

precursors. Future in-situ, high-frequency monitoring of 

FDOM may present opportunities not offered during this 

study to evaluate the degree to which algae may affect 

source-water quality and DBP concentrations in treated 

water by providing an early warning that allows for timely 

sampling of such conditions. Although algal blooms appear 

to be a regular phenomenon, weather conditions and growth–

senescence processes are dynamic in the Clackamas River as 

algae accumulates on the riverbed or as blooms develop in 
70 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

the reservoirs. Sampling finished-water DBPs during periods 

when water-column chlorophyll-a is high at the Oregon 

City (or Estacada) monitors, or during periods of obvious 

sloughing of benthic algae or after particularly large blooms 

of blue-green algae in the reservoirs, could provide data to 

further evaluate this hypothesis. The occasional taste and 

odor event also provides opportunities to identify where in 

the system these compounds are coming from, and may be 

ideal times to screen for blue-green algae toxins (Graham 

and others, 2010). The high specific THMFP value from 

North Fork Reservoir suggests this source is worthy of future 

monitoring if THM concentrations continue to be a concern. 

Studies that examine the temporal changes in organic matter 

reactivity and potential production of DBP precursors, taste 

and odor compounds, and algal toxins over the course of 

a bloom could be helpful for understanding the effect that 

algae may have on downstream water quality. In addition 

to providing a better understanding of watershed processes, 

this knowledge might help guide future DBP-treatability 

approaches that remove DOC, TPC, and DBP precursors. This 

could yield great benefits, especially if algal-derived organic 

matter is less amenable to removal by coagulation methods 

employed at these direct-filtration plants.

High frequency, in-situ measurements of FDOM 

proved to be an excellent proxy for DOC concentration in 

the Clackamas River, suggesting further development and 

refinement of these sensors have the potential to provide 

information that can inform DWTP operations and upgrades. 

The in-situ FDOM measurements revealed short-term, rapid 

changes in DOC concentrations in the river in response 

to storms. For this system, the close association between 

source‐water in-situ FDOM and finished-water HAA5 

concentrations demonstrates the utility of these instruments 

in providing a robust proxy for DBPs continuously, in 

real-time. This technology represents a new tool that can 

be used to optimize treatment-plant operations by adjusting 

water-intake rates or modifying coagulant doses during critical 

time periods, for example, to minimize the DOC and DBPprecursor content of treated water (Kraus and others, 2010). 

The link between DOM fluorescence and the presence of 

DBP precursors for other classes of DBPs other than THMs 

and HAAs, including for example nitrogenous DBPs, which 

are of emerging concern, clearly warrants future study (Hua 

and others, 2007; Henderson and others, 2009; Chen and 

Westerhoff, 2010). Research into the use of fluorescence to 

monitor and predict treatability, estimate biological oxygen 

demand, identify and quantify wastewater inputs, and act as 

an early-warning system for contaminants also shows promise 

(Bieroza and others, 2008; 2009a, b; Hudson and others, 

2008; Henderson and others, 2009; Baghoth and others, 2011; 

Bridgeman and others, 2011; Goldman and others, 2012). 

Other studies could sample forest soils and streams to 

refine the current understanding of what processes lead to 

DOM leaching, where DBP precursors are most prevalent, 

and how these processes are trending over time. Information 

on how carbon is stored and released would help modeling 

and prediction of DBP precursors and be especially helpful 

for understanding the effects of potential changes in land 

management, fire, climate, precipitation patterns, and 

other factors. Bacterial and fungal activity within soils and 

decomposing wood on the forest floor likely play important 

roles in mediating carbon sequestration and losses over time, 

and more information on their status would be useful. Future 

studies might also examine how forest management influences 

forest-soil carbon dynamics and the types of organic matter 

(or “components” as described here) that are most prone 

to leaching, and which ones contribute DBP precursors. 

Additional measures of bromide and chloride concentrations 

from Austin Hot Springs in the upper basin would be helpful 

for detecting possible trends over time, and might provide 

insight into the seasonality of DBP speciation in finished 

drinking water.

Monitoring of in-situ DOM using high-frequency 

continuous FDOM sensors can provide information regarding 

trends in the amount and composition, and thus origin 

and reactivity, of organic matter present in the river and 

in source water. These data have not only the potential to 

provide real-time information that can be used to manage 

DWTP operations, better understand DOM treatability, 

and predict finished-water DBP concentrations, but also 

provide information about watershed hydrology and 

processes that affect carbon dynamics in both terrestrial and 

aquatic ecosystems.

Acknowledgments

Many individuals from several organizations contributed 

to this study. Technical and financial support came from the 

Water Research Foundation, the WaterRF Project Advisory 

Committee members, and the two water utilities that 

participated in the study, Clackamas River Water and the City 

of Lake Oswego. The authors wish to personally thank staff at 

CRW, especially Suzanne DeLorenzo for providing historical 

data from the CRW DWTP, Rob Cummings for helpful 

discussions regarding the water-treatment processes, and Lee 

Moore for his support for the project. Also at CRW, special 

thanks to Mike Avery and Tracy Triplett for conducting jar-test 

experiments and helping to service the in-situ sensors. The 

authors also thank and acknowledge the valuable contributions 

from Kari Duncan (City of Lake Oswego) for her help with 

project design and implementation and for providing historical 

data from the LO DWTP. Reliable assistance with sourceand finished-water sample collection came from the DWTP 

operators at Clackamas River Water and City of Lake Oswego, 

and their efforts are greatly appreciated. 

This study benefited from the six-station network of 

water-quality and streamflow monitors in the basin, which is 

supported by the Clackamas River Water Providers (CRWP), 
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Clackamas County Water Environment Services (CC WES), 

and Portland General Electric (PGE). The authors would 

like to thank Kimberly Swan (CRWP), Andrew Swanson 

(CC WES), and John Esler (PGE) for their support and 

help with funding this network. The authors also appreciate 

the contributions from Randy Kuntz, Promontory Park 

concessionaire, who provided daily reports on algal conditions 

in North Fork Reservoir and for generously providing access 

to a pontoon boat for sampling the reservoir.

The contributions made by staff at the Oregon and 

California USGS Water Science Centers are gratefully 

acknowledged—Stewart Rounds was instrumental in 

providing the software and computer programming that 

allowed continuous in-situ data retrieval and display in 

near-real-time; David Piatt, Michael Sarantou, Steven 

Sobieszczyk, and Tara Chestnut helped with sample 

collection and processing; Kathryn Crepeau conducted 

the DBPFP experiments; Micelis Doyle maintained the 

network of continuous water-quality monitors, and Rick 

Kittleson, Doug Cushman, Roy Wellman, Jay Spillum, 

Greg Lind, and Greg Olsen provided streamgage-operation 

and streamflow‐measurement support; Matt Johnston, Amy 

Brooks, and Melanie North provided telecommunications and 

computer support; and assistance with multivariate statistical 

analyses using R was provided by Ian Waite. Thanks also go 

to Kenna Butler for her assistance with various laboratory 

analyses and Travis von Dessonneck for data processing. 

Steven Ingebritsen (USGS, Menlo Park, CA) provided 

geochemical information on Austin Hot Spring, and Brian 

Pellerin (USGS, Sacramento, CA), Djanette Khiari (Water 

Research Foundation), and the Water Research Foundation 

Project Advisory Committee provided helpful comments on an 

earlier draft of the report.

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Appendixes and Data Quality Assurance 77

Appendixes and Data Quality Assurance

Evaluation of Quality-Assurance Data

Quality-assurance (QA) water samples consisted of 

field and laboratory blanks, field and laboratory (churn split) 

replicates, matrix spike samples, and standard reference 

samples. Appendix A lists the various types of QA samples 

for each type of data; appendix B provides the QA results 

for dissolved organic carbon (DOC) and optics; appendix C

shows QA data for disinfection by-products (DBPs) in finished 

water; appendix D shows QA data for DBP formation potential 

(DBPFP) measurements; appendix E shows QA results 

for nutrients.

Organic carbon and optical properties—QA 

samples for carbon and optical properties included four 

laboratory replicates (churn split) and two field replicates 

(appendix B). While the average relative percent differences 

(RPDs) for replicate samples were within 5–6 percent for 

total fluorescence, DOC, ultraviolet absorbance (UVA), 

fluorescence index (FI), and four of five carbon component 

loadings, higher average RPDs were found for component 

C5 (11 percent), spectral slopes (8–11 percent), humic 

index (HIX) and peaks B, T, and N (20–36 percent). The 

one finished-water replicate had some relatively high RPDs 

(appendix B), but in many cases, these represented small 

actual differences in concentrations that were at or near the 

detection limit. 

More variability in C5 values produced higher standard 

deviations compared with other components, possibly because 

the protein-like organic matter is highly variable amongst 

sites and seasonal trends (Yamashita and Tanoue, 2003). 

Reproducibility and precision of measurements of protein-like 

peaks in natural waters are reduced at shorter wavelength, 

because of interference in background fluorescence 

(Yamashita and Tanoue, 2003). Instability of the protein 

peaks can be due to changes in the complex structures of 

the protein. The fluorescing component of proteins found in 

organic matter resides in the residuals of the protein folds. 

Tryptophan and tyrosine display high anisotropies that are 

usually sensitive to protein conformation and the extent of 

motion during the excited state. This leads to highly variable 

natural lifetimes of proteins and makes it difficult to have 

accuracy in the reproducibility of the fluorescence of these 

particular fluorophores. 

DBP—QA samples for DBPs included “in-house” 

samples prepared by the laboratory and one blank, three 

replicates, and one standard reference sample submitted 

blindly during the project. Each set of 10 samples analyzed 

by the laboratory included 1 blank sample, pre- and postCCV (continuing calibration verification) samples, and 1 

matrix-spiked sample (regular sample spiked with 0.01 mg/L). 

The 1 blank sample submitted blindly contained no 

detections, and 12 in-house laboratory blanks for chloroform 

and bromodichloromethane also resulted in no detections. The 

standard reference sample for DBPs indicated a high percent 

relative difference for a few compounds, notably bromoform 

and chloroform (appendix C). The percent recovery for 

chloroform was just 87 percent, and although this may 

represent a low bias, this represented only a small difference 

between expected and reported concentrations of 0.01 mg/L 

(appendix C). In-house matrix spike samples (0.01 mg/L) for 

these two compounds resulted in average percent recoveries 

of 98±13 percent and 104±9 percent, respectively, whereas 

recovery ranges were 74–125 percent and 78–117 percent. The 

RPDs among replicate spike samples were about 3 percent 

for both compounds (Adriana Gonzalez-Gray, Alexin 

Laboratories, written commun., 2012). Field replicate values 

showed more variation in trihalomethanes (THMs) (notably 

chloroform) compared with haloacetic acids (HAAs), although 

absolute differences between replicates was again small, 

0.001–0.004 mg/L (appendix C).

DBP Formation Potentials—One blank sample 

contained a low-level concentration of chloroform at the 

detection limit of 0.002 mg/L (appendix B). Given the much 

higher concentrations present in environmental samples, 

this low-level detection does not affect the results or 

interpretations. Field DBPFP replicates showed variations 

of 5–10 percent for THMs and 5–20 percent for HAAs; the 

highest relative differences were for trichloroacetic acid 

(TCAA [appendix D]). Samples not meeting the upper 

acceptable range of 30 percent generally had low DPB 

concentrations (less than 0.010 mg/L), thus the absolute 

differences were small. Differences between field duplicates 

were largest for chloroform differences between field 

duplicates were largest for chloroform, which is more volatile 

compared with the other DBPs, and thus most prone to loss 

due to the occasional air bubble. The occurrence of bubbles 

in some DBPFP samples may have resulted in a low bias for 

the highly volatile compounds such as chloroform, but as the 

standard reference results presented below show, this does not 

appear to be a widespread issue.

The generally accepted range for THM and HAA 

recoveries is between 70 and 130 percent. The RPD between 

expected and reported results for the seven DHBA samples 

tested was 0.2–16.2 percent (average 7.6 percent), well 

within this range (appendix D). Errors associated with these 

analyses include the preparation of 3,5-dihydroxy-benzoic 

acid (DHBA) standard solution from a solid powder, steps 

involved with chlorine dosing and quenching, potential loss 

of the volatile THMs due to bubble formation or air leaks, 

and variations in the determination of the THM compounds 
78 Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water

by the laboratory. In addition, because of high concentrations 

of chlorinated DBPs in the formation potential samples, 

analyses usually required dilution. Although this additional 

step could introduce errors, no QA or quality control (QC) 

issues were apparent in replicate or standard reference 

samples. Nevertheless, some loss of the more volatile 

compounds, particularly chloroform, could have occurred 

during sample transfer for the high-concentration samples 

requiring this dilution step and may cause a low bias in 

reported concentrations.

Nutrients—The nutrient data had considerable QA 

issues, including numerous reported total phosphorus (TP) 

concentrations that were less than soluble reactive phosphorus 

(SRP) concentrations and unacceptably high variation (high 

RPDs) in laboratory replicate “churn-split” samples for SRP, 

TP, ammonium (NH4), silica (Si), total particulate carbon 

(TPC), and total particulate nitrogen (TPN) (appendix E). 

Nitrate concentrations appeared more reliable; laboratory split 

replicates were within 5 percent of one another. Given the 

concerns about the quality of the nutrient data, these data were 

utilized sparingly during analysis, and nutrients were not good 

explanatory variables during the study. 

The average RPDs for TPC and TPN for three laboratory 

replicate split samples also were relatively high—25 and 

34 percent, respectively (appendix E). Given that TOC 

concentrations were calculated by summing the TPC and DOC 

concentrations, the potential errors in TPC values could affect 

calculations of the percentage of particulate carbon compared 

to dissolved, which is duly noted.

The appendix data files are included in an Excel© 

Workbook and are available for download at http://pubs.

usgs.gov/sir/2013/0779. This workbook consists of the 

following worksheets.

Appendix A. Number and Type of Quality-Assurance Samples.

Appendix B. Quality-Assurance Data for Dissolved Organic 

Carbon and Selected Optical Properties.

Appendix C. Quality-Assurance Data for Disinfection 

By-Products in Finished Water.

Appendix D. Quality-Assurance Data for Disinfection 

By-Product Formation Potentials.

Appendix E. Quality-Assurance Data For Nutrients and Total 

Particulate Carbon.

Appendix F. Spearman Rank Correlations for Select Groups of 

Samples and Sites.

Appendix G. Discrete Data Used in the Analysis, Including 

Watershed, Finished-Water, and Jar-Test Samples.
Publishing support provided by the U.S. Geological Survey 

Publishing Network, Tacoma Publishing Service Center

For additional information contact:

Director, Oregon Water Science Center

U.S. Geological Survey

2130 SW 5th Avenue

Portland, Oregon 97201

http://or.water.usgs.gov
Carpenter and others— Sources and Characteristics of Organic Matter, Clackamas River, Oregon, Related to Disinfection By-Products in Drinking Water—Scientific Investigations Report 2013–5001