Turbidity Analysis for 

Oregon Public Water Systems

Water Quality in Coast Range Drinking Water Source Areas

June 2010

Last Updated: 06/29/10

By: Joshua Seeds

DEQ 09-WQ-024

Report
This report prepared by:

Oregon Department of Environmental Quality

811 SW 6th Avenue

Portland, OR 97204

1-800-452-4011

Contact:

Joshua Seeds

(503) 229-5081

seeds.joshua@deq.state.or.us
i

Turbidity Analysis for Oregon Public Water Systems

Table of Contents

Table of Contents.......................................................................................... i

List of Figures ..............................................................................................iii

List of Tables...............................................................................................iiv

List of Maps.................................................................................................iiv

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

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

Applicable Turbidity Standards for Water Quality ............................................... 2

Public Water System Evaluations....................................................................... 3

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

Definitions of terms.........................................................................................................6

Summary of Results.......................................................................................................6

Case Studies................................................................................................ 8

Analysis of Public Water System Turbidity Data................................................. 9

Arch Cape Water District (PWS #4100802)...................................................................9

City of Astoria (PWS #4100055) ..................................................................................10

City of Carlton (PWS# 4100171)..................................................................................15

City of Falls City (PWS# 4100297)...............................................................................20

City of Forest Grove (PWS #4100305) ........................................................................23

Hillsboro-Cherry Grove (PWS# 4100985)....................................................................27

City of Siletz (PWS# 4100821).....................................................................................30

City of Yamhill (PWS# 4100968)..................................................................................33

Ecological and Economic Implications ....................................................... 37

Natural Disturbances and Land Use Have Potential Effects on Sediment/Turbidity ...37

Land Use May Alter Flow Regimes..............................................................................38

Climate Change Risks for Public Water Supplies ........................................................38

Potential Costs Related to Higher Turbidity .................................................................38

Land Management Effectiveness for Source Water Protection ...................................39

Potential Followup...................................................................................... 42

References................................................................................................. 44

Appendices ................................................................................................ 49
ii

Turbidity Analysis for Oregon Public Water Systems

Appendix A: Public Water Systems with All or Part of DWSA in the Coast 

 Range ......................................................................................................... 49

Appendix B: Relationship of Stream Turbidity and Suspended Solids in the 

 Siletz River Basin: Analysis of Data Collected by Confederated Tribes of

 Siletz Indians............................................................................................... 52
iii

Turbidity Analysis for Oregon Public Water Systems

List of Figures

Figure 1: Number of Days with High Raw Water Turbidity .................................... 6

Figure 2: Total Duration of Treatment Shutdowns by Month (Arch Cape)............. 9

Figure 3a: Influent Turbidity for City of Astoria .................................................... 10

Figure 3b: Turbidity in City of Astoria’s Source Water: Streams.......................... 11

Figure 3c: Turbidity in City of Astoria’s Source Water: Lakes ............................. 11

Figure 3d: Precipitation for City of Astoria........................................................... 12

Figure 3e: Raw Water Turbidity Trend for City of Astoria .................................... 12

Figure 3f: Number of Days with High Turbidity in Astoria’s Raw Water............... 13

Figure 3g: Trend for High Turbidity Days in Astoria’s Raw Water ....................... 14

Figure 4a: Influent Turbidity for City of Carlton.................................................... 15

Figure 4b: Precipitation for City of Carlton .......................................................... 16

Figure 4c: Raw Water Turbidity Trend for City of Carlton 

 (includes effects of 1999 landslide)..................................................... 17

Figure 4d: Raw Water Turbidity Trend for City of Carlton 

 (excludes effects of 1999 landslide)................................................... 17

Figure 4e: Number of Days with High Turbidity in Carlton’s Raw Water.............. 18

Figure 4f: Trend for High Turbidity Days in Carlton’s Raw Water........................ 18

Figure 4g: Storage Capacity in City of Carlton’s Reservoir ................................. 19

Figure 5a: Influent Turbidity for City of Falls City................................................. 20

Figure 5b: Precipitation for City of Falls City ....................................................... 21

Figure 5c: Raw Water Turbidity Trend for City of Falls City ................................ 21

Figure 5d: Number of Days with High Turbidity in Falls City’s Raw Water .......... 22

Figure 5e: Trend for High Turbidity Days in Falls City’s Raw Water.................... 22

Figure 5f: Turbidity Range & Mean for November 2006 Sedimentation Incident. 23

Figure 6a: Influent Turbidity for City of Forest Grove .......................................... 24

Figure 6b: Precipitation for City of Forest Grove................................................. 25

Figure 6c: Raw Water Turbidity Trend for City of Forest Grove .......................... 25

Figure 6d: Number of Days with High Turbidity in Forest Grove’s Raw Water .... 26

Figure 6e: Trend for High Turbidity Days in Forest Grove’s Raw Water.............. 26

Figure 7a: Influent Turbidity for Hillsboro-Cherry Grove...................................... 27

Figure 7b: Precipitation for Hillsboro-Cherry Grove ............................................ 28

Figure 7c: Raw Water Turbidity Trend for Hillsboro-Cherry Grove...................... 28

Figure 7d: Number of Days with High Turbidity in Hillsboro-Cherry Grove’s

 Raw Water........................................................................................ 29

Figure 7e: Trend for High Turbidity Days in Hillsboro-Cherry Grove’s

 Raw Water........................................................................................ 29

Figure 8a: Influent Turbidity for City of Siletz ...................................................... 30

Figure 8b: Precipitation for City of Siletz............................................................. 31
iv

Turbidity Analysis for Oregon Public Water Systems

Figure 8c: Raw Water Turbidity Trend for City of Siletz ...................................... 31

Figure 8d: Number of Days with High Turbidity in City of Siletz’ Raw Water....... 32

Figure 8e: Trend for High Turbidity Days in City of Siletz’ Raw Water................. 32

Figure 9a: Influent Turbidity for City of Yamhill ................................................... 33

Figure 9b: Precipitation for City of Yamhill.......................................................... 34

Figure 9c: Raw Water Turbidity Trend for City of Yamhill

 (includes effects of 1996 storms) ....................................................... 35

Figure 9d: Raw Water Turbidity Trend for City of Yamhill

 (excludes effects of 1996 storms) ...................................................... 35

Figure 9e: Number of Days with High Turbidity in Yamhill’s Raw Water ............. 36

Figure 9f: Trend for High Turbidity Days in Yamhill’s Raw Water........................ 36

Figure 10: Reductions in fine sediment generated from roads and

 delivered to streams in Seattle’s Cedar River watershed ................... 40

List of Tables

Table 1: Recommendations for future work based on turbidity status and trend.... 7

Table 2: Seasonally-adjusted turbidity & high turbidity day trends

 for Astoria’s water sources.................................................................... 13

List of Maps

Map 1: Drinking water source areas originating in Oregon’s Coast Range ............ 5

Map 2: Turbidity case studies’ drinking water source areas................................... 8
1

Turbidity Analysis for Oregon Public Water Systems

Executive Summary

After catastrophic storms and landslides hit the Oregon Coast in 2006 and 2007, the Oregon Department of 

Environmental Quality and Oregon Department of Human Services had concerns about the resiliency of public 

water systems that rely on surface waters in watersheds prone to heavy rains and landslides. Landslides in such 

areas can deposit large amounts of sediment that can increase turbidity and significantly lower the quality of 

drinking water obtained and treated by these water systems. Increasing levels of turbidity may interfere with 

public water system operations, can increase operational costs and can also cause shutdowns. In addition, DEQ 

and DHS were interested in the potential effects from climate change in these watersheds.

DEQ sent e-mail inquiries and interviewed staff at numerous public water systems in the Coast Range and 

Southwest Oregon. It also obtained and analyzed available daily turbidity data from seven water systems 

(Hillsboro-Cherry Grove and the cities of Astoria, Carlton, Falls City, Forest Grove, Siletz and Yamhill) and 

collected data on water treatment facility shutdowns from Arch Cape Water District.

DEQ evaluated as case studies a total of eight public water systems in the North and Middle Oregon Coast Range 

area and used available turbidity data to analyze long-term trends. DEQ‟s analysis concludes that there are longterm concerns about system viability or increased treatment costs at five systems (Arch Cape Water District and 

the cities of Astoria, Carlton, Siletz, and Yamhill), based on large numbers of high-turbidity days and/or 

increasing numbers of high-turbidity days recorded in daily water samples for these systems. Also, DEQ found 

that turbidity is increasing in the long term in Drinking Water Source Areas for five systems, remains stable at 

one system, and is decreasing at two others.

Drinking Water Source Areas for these Coast Range systems are predominantly forested. Only the City of Siletz 

has a significant amount of rural residential and/or agricultural land in the lower part of its source area.

Precipitation has the greatest impact on turbidity patterns for these systems, with the largest spikes in turbidity 

occurring during the autumn months. In addition to this seasonal variation, for the five systems with increasing 

turbidity trends, there are an increasing number of days reported with turbidity levels above 5 nephelometric 

turbidity units (a system of measuring water‟s clarity in terms of visible light), turbidity spikes of increasing 

magnitude and/or increasing baseline turbidity. In addition to the problems high turbidity can cause on drinking 

water treatment and system operation, changes in turbidity may also adversely affect aquatic life or other 

beneficial uses of the water, such as fishing and swimming. 

High turbidity levels at some of these eight public water systems may be the result of inherent watershed 

characteristics, natural events (such as storms, slides and bank erosion), and land-use activities (forestry,

agriculture, rural residential, road building and maintenance). Regardless of the source of turbidity, watershed 

protection and restoration activities can reduce unacceptable levels of sediment deposited into public water system 

sources and can reduce the cost of drinking water treatment.

This analysis was not designed or intended to answer cause-and-effect questions related to turbidity. Further 

statistical and Geographic Information System analysis is necessary to evaluate the effects of human-caused 

changes, natural disturbances and inherent watershed features on public water systems. But collection of new 

data, further analysis of existing data, enhanced source water monitoring and evaluation and analysis of sediment 

sources and movement will help DEQ and DHS further understand and manage risks to public water systems. In 

addition, changes in weather patterns and precipitation due to climate change should be considered as this topic is 

studied further.
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Turbidity Analysis for Oregon Public Water Systems

Introduction

The storms of December 2007 resulted in catastrophic flooding in the northern Coast Range of Oregon, 

destroying or harming homes, water supplies, fisheries, roads, farms, and businesses. Many of Oregon‟s Public 

Water Systems (PWSs) were directly impacted by the flooding, wind damage, and landslides in their municipal 

watersheds. The Department of Human Services (DHS) and the Department of Environmental Quality (DEQ) 

had previously mapped all of these watersheds and prepared Source Water Assessment (SWA) reports for the 

community officials (http://www.deq.state.or.us/wq/dwp/dwp.htm). The SWAs included maps of the source area 

boundaries, natural characteristics of the watersheds, and identified potential risks for the systems. 

The damage from the storms raised questions about the sensitivity of these PWSs as the number of severe storms 

could increase due to climate change. The landslide near Clatskanie in northwest Oregon completely destroyed 

Westport‟s water system. DHS was concerned about the potential for complete loss of, or severe damage to, the 

systems due to storms. As part of the existing drinking water protection planning and implementation, DEQ 

embarked on this analysis of potential turbidity problems for PWSs. The purpose of this first phase of analysis is 

to use existing data to identify the PWSs that have ongoing turbidity problems and to conduct a trend analysis if

data is available. 

Work was initiated in January 2008 with an email/phone questionnaire to 148 surface water systems in Oregon.

DEQ asked them if there were any recent landslides in their watershed that impacted the intake(s). We indicated 

our purpose was to evaluate whether we need to put more resources into this problem at the state level. Out of 

this initial query, we received information from 17 different systems that had turbidity impacts, but not 

necessarily from landslides. Several system operators asked for assistance to determine what was causing the 

turbidity and to assist them in reducing the loading to the source stream. At that point, both DHS and DEQ 

determined that we should proceed with an analysis of the turbidity issues and follow this with recommendations 

for reducing the risks for those systems.

Applicable Turbidity Standards for Water Quality

The turbidity water quality standard (OAR 340-41-0036) is: 

Turbidity (Nephelometric Turbidity Units, NTU): No more than a ten percent cumulative increase in natural stream 

turbidities may be allowed, as measured relative to a control point immediately upstream of the turbidity causing activity. 

However, limited duration activities necessary to address an emergency or to accommodate essential dredging, construction 

or other legitimate activities and which cause the standard to be exceeded may be authorized provided all practicable 

turbidity control techniques have been applied and one of the following has been granted:

(1) Emergency activities: Approval coordinated by the Department with the Oregon Department of Fish and Wildlife under 

conditions they may prescribe to accommodate response to emergencies or to protect public health and welfare;

(2) Dredging, Construction or other Legitimate Activities: Permit or certification authorized under terms of section 401 or 

404 (Permits and Licenses, Federal Water Pollution Control Act) or OAR 14l-085-0100 et seq. (Removal and Fill Permits, 

Division of State Lands), with limitations and conditions governing the activity set forth in the permit or certificate.

In Oregon, there are many waters with low NTU levels and a 10 % increase is very difficult to measure in a clear 

stream. In addition, the original 10% Turbidity water quality standard was based on Jackson turbidity units 

(JTUs) which are measureable to 25 JTU with an accuracy of ± of 1 JTU. The standard switched to NTUs which 

are measureable to 1 NTU with an accuracy of ± 0.1 NTU. This has led to claims that DEQ‟s turbidity standards 

cannot by implemented. In reality, the 10% exceedence criteria was designed to take into account the fact that 
3

Turbidity Analysis for Oregon Public Water Systems

natural conditions in streams do not produce consistent low turbidity at all times. DEQ recognizes that there is 

some natural variation and this creates difficulty in determining whether turbidity levels exceed the criteria of a 

10% increase. There are several characteristics of turbidity by which a 10% increase could be documented: the 

baseline turbidity of a stream, the number of spikes in a stream‟s turbidity, the median or 75th percentile of 

turbidity, and/or the statistical trend that the stream‟s turbidity is following. However, the standard relates to a 

control point immediately upstream and is not applicable at the watershed scale but at the reach scale. The 

turbidity WQS may be revisited in 2009 or 2010.

Turbidity is one of the primary pollutants regulated in finished drinking water under the Safe Drinking Water Act 

(SDWA). To protect human health, the EPA has set the level of allowable turbidity in finished drinking water at:

0.3 Nephelometric Turbidity Units (NTU) in 95% of samples (1 NTU absolute maximum) for conventional

(sedimentation and rapid sand filtration) and direct (rapid sand only) filtration; and 1 NTU in 95% of samples (5 

NTU absolute maximum) for membrane, cartridge, slow sand, and diatomaceous earth filtration (US EPA 1998, 

US EPA 2001, US EPA 2002b). Turbidity limits prevent drinking water from having excessive levels of 

suspended fine sediment (US EPA 1999). Suspended sediment is of concern for drinking water safety as it can 

reduce the effectiveness of disinfection treatments (LeChevallier et al 1981), harbor pathogens (e.g. Chang et al

1960, Tracy et al 1966, Sen & Jacobs 1969, Meschke & Sobsey 1998), contribute to formation of disinfection byproducts (Nikolaou et al 1999, US EPA 2002a), and carry nutrients, heavy metals, pesticides, and other toxic 

chemicals (Lick 2008). Unpleasant tastes and odors frequently co-occur with excessive turbidity (US EPA 1998). 

Prevention or removal of fine sediment pollution from water reduces these risks to acceptable levels (US EPA 

2001).

Most drinking water treatment facilities run by Public Water Systems (PWSs) have the capacity to remove 

turbidity-causing sediments during treatment of raw water; however, the amount of turbidity that can be 

effectively removed depends on the treatment technology in use (US EPA 1999, personal communication with 

PWS managers). For example, water can be treated using a flocculent/coagulant agent to collect fine sediments 

into larger particles which are then removed by rapid pressurized filtration through a bed of sand. Another 

common drinking water treatment system, slow sand filtration, allows water to slowly percolate through a large 

bed of sand to be collected through an underdrain. When source water turbidity exceeds 5 NTU, a treatment plant

using these treatment systems will usually need to shut down (National Drinking Water Clearinghouse (NDWC) 

1996, pers comm with PWS managers). PWSs with additional pre-filtration or pre-sedimentation basins can treat 

source water with turbidity higher than 5 or 10 NTU (US EPA 2004, pers comm with PWS managers). Some 

systems in Oregon with frequent high turbidity install advanced filtration systems that can treat water with

turbidity higher than 50 or 100 NTU (e.g. City of Coquille). These advanced filtration systems are expensive to 

install and maintain and may not be affordable for all small PWSs. Some studies have shown that prevention of 

turbidity and fine sediment pollution is a cost effective way to ensure that current PWS treatment can meet the 

SDWA Maximum Contaminant Level (MCL) for turbidity (Brown 2000).

Public Water System Evaluations

Some Public Water Systems have historic and/or current problems with turbidity in their source water. There are 

161 PWSs with all or part of their Drinking Water Source Areas (DWSAs) in the Coast Range and mountains of 

Southwestern Oregon (Appendix A). This report focuses on 8 PWS Case Studies in the North Coast and Mid 

Coast regions that included interviews with PWS directors/supervisors and analysis of available raw water 

turbidity data. We also conducted field visits in the DWSAs for the Arch Cape, Siletz, and Yamhill PWSs. The 

PWS examined in this report are:

Arch Cape Water District,

City of Astoria,

City of Carlton,

City of Falls City,
4

Turbidity Analysis for Oregon Public Water Systems

City of Forest Grove,

Hillsboro-Cherry Grove,

City of Siletz,

City of Yamhill.

In addition, the following PWS were also interviewed and status reports were written based on the interview. The 

status reports give some history of each PWS, the operators‟ opinions of the current situation, and their concerns 

about potential threats to the PWS. Those status reports are not included in this report because we could not get 

turbidity data or conduct a site visit:

City of Clatskanie,

City of Coquille,

Langlois Water District,

City of Myrtle Point,

City of Port Orford,

City of Powers,

Rainier Water Department,

Seaside Water Department,

City of Waldport,

Westport Water Association.

Some water systems did not have daily turbidity data available in an accessible format, and not all PWSs collect 

precipitation data. Several were affected by a large landslide in December 2007 above Highway 30 near the 

Columbia River. One (Westport Water Association) had their intake and associated infrastructure completely 

destroyed by the slide. Out of the 18 water systems that were interviewed, 14 had problems or expressed 

concerns about high and/or increasing turbidity in their source water. 
5

Turbidity Analysis for Oregon Public Water Systems

Methods

We received beginning-of-the-day turbidity data from 7 PWSs, giving a snapshot of turbidity conditions at about 

8am each day. One PWS (Arch Cape) gave us data with the dates of treatment shutdowns due to turbidity. Data 

were organized and counts of high turbidity days per water year were done using Microsoft Excel 2007. Minitab 

v15.1 was used for graphing and data analysis, including time series trend analysis for high turbidity days using

the method of Box and Jenkins (1994) and seasonally-adjusted trend analysis of turbidity data over time 

(Makridakis et al 1998). The trend analysis of high turbidity days showed whether the number of days with 

turbidity ≥5 NTU (“high turbidity days”) is changing directionally over time. We fitted linear, quadratic, and 

exponential models and chose the model with the best accuracy, as shown by lower Mean Absolute Percent Error 

Map 1
6

Turbidity Analysis for Oregon Public Water Systems

(MAPE), Mean Absolute Deviation (MAD), and Mean Squared Deviation (MSD) scores. In no cases did the 

exponential model have the best fit. The seasonally-adjusted trend analysis of turbidity separated the data into 

seasonal and linear trend components. The decomposition of seasonal and linear trends allowed examination of 

long-term change while accounting for the tendency of stream systems to have elevated turbidity at the beginning 

of the autumn rainy season. Trend equations take the form:

Linear: Yt=α+βt

Quadratic: Yt=α+βt+γt2

where Ytis the turbidity at time t, α is the y-axis intercept, and β and γ are the coefficients of change for t and t

2

, 

respectively.

We used ArcGIS v9.3, operator interviews, and field visits to evaluate patterns in land ownership and conditions 

on the ground.

Definitions of terms

Low Turbidity Status―Turbidity baseline lower than 5 NTU greater than or equal to 50% of the time

High Turbidity Status―Turbidity baseline lower than 5 NTU less than 50% of the time

Occasional Spikes―Sudden increases in turbidity that exceed 5 NTU occurring less than 30 times/year

Frequent Spikes―Sudden increases in turbidity that exceed 5 NTU occurring more than 30 times/year

Increasing Trend―β > 0.000150 for seasonally-adjusted turbidity trend

Decreasing Trend―β < -0.000150 for seasonally-adjusted turbidity trend

Stable Trend―0.000150 ≥ β ≥ -0.000150 for seasonally-adjusted turbidity trend

Summary of Results

Conditions ranged from low turbidity with occasional spikes to frequent large spikes that require intervention by 

operators to consistently high turbidity throughout the late autumn, winter, and early spring. Turbidity trends 

range from historic problems that are improving to steady turbidity patterns (whether high or low) to increasing 

turbidity leading to more 

frequent problems. There 

is significant variation 

among Public Water 

Systems, with some 

having noticeably more 

high turbidity days than 

others (Figure 1.) 

Currently, the Cities of 

Astoria and Falls City 

have the most consistently 

low turbidity while the 

Cities of Carlton, Siletz, 

and Yamhill have frequent 

high turbidity days. Arch 

Cape was analyzed 

differently because daily 

turbidity data were 

unavailable.

Below are suggested 

approaches for PWSs or 

agencies to address a 

situation, generally 

Figure 1: Count of days when influent water turbidity equal or exceeds 5 NTU by 

water year for 1994 through 2008. The water year is October 1st through September 30th.

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

300

250

200

150

100

50

0

Water Year

Days w/ Turbidity >5 NTU

Astoria

Carlton

Falls City

Forest Grove

Hillsboro

Siletz

Yamhill

PWS

Number of Days with High Raw Water Turbidity

Selected Public Water Systems: WY1994-WY2008
7

Turbidity Analysis for Oregon Public Water Systems

speaking. Specifics will vary from system to system based on their particular watershed(s) and the level of 

resource protection within it. It is also important to note that many PWSs do not have resources beyond those 

needed for day-to-day operations and maintenance; additional protection measures require additional funding 

(Drinking Water Needs Survey 2009).

Table 1: Recommendations for Future Work Based on Turbidity Status and Trend

Low Turbidity

(Occasional Spikes)

Low Turbidity

(Frequent Spikes)

Consistent 

High Turbidity

Trend of Decreasing 

Turbidity

N/A Evaluate situation/ 

Consider DWSA 

protection measures

Find causes/Consider 

additional DWSA 

protection measures

Trend of Stable 

Turbidity

Evaluate situation/

Consider DWSA 

protection measures

Find causes/Consider 

additional DWSA 

protection measures

Find causes/Change 

DWSA mgmt. & 

implement restoration

Trend of Increasing 

Turbidity

Find causes/ 

Consider additional 

DWSA protection 

measures

Find causes/Change 

DWSA mgmt. & 

implement restoration

Find causes/Change 

DWSA mgmt. & 

implement restoration
8

Turbidity Analysis for Oregon Public Water Systems

Case Studies

Map 2
9

Turbidity Analysis for Oregon Public Water Systems

Analysis of Public Water System Turbidity Data

Arch Cape Water District (PWS# 4100802)

Thomas Merrell, Director

503-436-2790

Increasing Trend [Reported by PWS]: Low Turbidity (Occasional Spikes) transitioned to Low Turbidity 

(Frequent Spikes)

Arch Cape is small coastal community in southern Clatsop County with a mixture of year-round and part-time 

residents. The treatment plant uses chemical coagulation and pressurized (rapid) sand filtration to remove 

particulates from the source water. The district has two drinking water intakes in its 1,250 acre Drinking Water 

Source Area (DWSA): on Asbury Creek, a medium-sized coastal stream, and on Shark Creek, a medium-sized 

tributary of Asbury Creek. The primary intake on Shark Creek is behind a small dam that forms an intake pool, 

and the secondary intake is on Asbury Creek below its confluence with Shark Creek. The land of the forested 

DWSA has been owned by a succession of private industrial timber companies. Arch Cape Water District does 

not have a backup source of water.

Arch Cape Water District 

has difficulty with 

operations due to high 

turbidity and spends a 

significant proportion of 

each month shutdown due 

to turbidity with the 

exception of the summer 

months (Figure 2). Arch 

Cape Water District does 

not have a backup source 

of water. If the PWS 

cannot use the Shark 

Creek intake, then they 

must use the Asbury 

Creek intake (downstream 

of the confluence of Shark 

and Asbury Creeks). If 

neither intake is usable, 

then the PWS must shut 

down water purification 

and use stored water (3 to 

7 day supply).

Further increases in suspended sediment in Arch Cape‟s DWSA could be very problematic. As little as 0.5 inches 

of rain raises stream turbidity to the point that Arch Cape‟s rapid sand filtration can not remove enough fine 

sediment to meet finished water standards (T. Merrell, pers comm). For the last 3 years, the intake pool has

needed to be dredged yearly by necessity where during the previous decade the PWS only dredged it once every 3 

or 4 years as routine maintenance. To cope with the increased turbidity, Arch Cape recently applied for and 

received over $1 million in American Recovery and Reinvestment Act (“stimulus”) funds to replace their rapid 

sand filtration and chlorination treatment with membrane filtration and UV disinfection. Leaking distribution 

pipes will also be replaced with these funds.

Figure 2: Number of days per month that Arch Cape Water District‟s treatment plant 

spent shutdown due to high turbidity from January 2007 through March 2010. Bar colors 

highlight the calendar year.

Mar 2010

Jan 2010

Nov 20 9

Sep 20 9

Jul 20 9

May 20 9

Mar 20 9

Jan 20 9

Nov 20 8

Sep 20 8

Jul 20 8

May 20 8

Mar 20 8

Jan 20 8

Nov 20 7

Sep 20 7

Jul 20 7

May 20 7

Mar 20 7

Jan 20 7

30

25

20

15

10

5

0

Shutdown Duration (Days)

Total Duration of Treatment Shutdowns by Month

Arch Cape Water District: 2007-March 2010
10

Turbidity Analysis for Oregon Public Water Systems

City of Astoria (PWS #4100055)

Jim Hatcher, Public Works Operations

503-325-3524

Increasing Trend: Low Turbidity (Occasional Spikes)

The City of Astoria is located on the Columbia River estuary. Astoria has water supply intakes on two lakes 

(Middle and Main Lakes) and two creeks (Cedar and Bear Creeks). They can switch among intakes as needed, 

depending on weather and source conditions. Astoria typically draws more water from the creeks during the 

summer and more from the lakes in winter due to turbidity fluctuations. This ability to change sources is an 

important asset. Astoria uses a slow-sand filter to remove particulates, and these filters can be easily clogged by 

turbidity higher than 5 NTU. Since 1956, the City of Astoria has owned nearly all of the 2,736 acre forested 

Drinking Water Source Area (DWSA), in addition to a small part of neighboring Wikiup‟s DWSA. The City 

does harvest timber from the DWSA. The Forest Stewardship Council (FSC) certified Astoria‟s management and 

harvest operations as “Well Managed” forestland (the highest classification) in 2003. A private industrial timber 

(PI) company owns less than 5% of Astoria‟s DWSA, of the southern and northern edges. Private industrial 

companies own the land surrounding Astoria‟s DWSA.

Turbidity for water 

entering Astoria‟s water 

treatment plant is shown 

below (Figure 3a). When 

the source supplying the 

treatment plant begins to 

be too turbid, then 

operators survey all 

sources to determine 

which has the lowest 

turbidity and is most 

suitable for use (Figures 

3b&c). Therefore, the

source in use is typically 

the one with the lowest 

turbidity. Rises in 

turbidity are frequently 

triggered by precipitation 

(Figure 3d). The turbidity 

patterns in Astoria‟s water 

sources are fairly stable 

and easily managed with 

the multitude of potential 

intakes. However, 

influent turbidity is 

slightly increasing overall (Figure 3e), albeit at low rate and well below levels that would drive serious concerns. 

All of the individual sources except Cedar Creek show at least some recent increase in turbidity (Table 2). Spur 

14 Creek has very low turbidity, so its fluctuations in turbidity may appear meaningful when in fact they are very 

small.

As the influent turbidity approaches 5 NTU, Astoria‟s drinking water system switches to another source with 

lower turbidity. High turbidity days in the influent water are therefore rare, even when several of the sources have 

elevated turbidity (Figure 3f). The flexibility among sources created a flat, and extremely low, trend in high 

turbidity days (for influent) over the last 6 years (Figure 3g). Bear Creek‟s trend in high turbidity days remained

Figure 3a: Turbidity for City of Astoria‟s drinking water system. Since the City can 

switch between sources as needed, this represents a composite of whichever source has 

the lowest turbidity at any given time. Other sources within the DWSA probably had 

higher turbidity at the time.

09/30/0801/01/0801/01/0701/01/0601/01/0501/01/0401/01/03

20

15

10

5

0

Turbidity (NTU)

5

Influent Turbidity for City of Astoria

Drinking Water Intake: 2002-2008
11

Turbidity Analysis for Oregon Public Water Systems

level after a jump in 2005, and Middle and Main Lakes had noticeable increasing trends in the last two years 

(Table 2). The turbidity increases in Bear Creek and the two lakes may be cause for concern and are likely 

contributed to by ground 

disturbed as trees toppled 

in large numbers during

the December 2007 

windstorm that struck 

Oregon‟s North Coast. 

Much of this blowdown 

was adjacent to clearcuts 

on neighboring private 

land.

09/30/0801/01/0801/01/0701/01/0601/01/0501/01/0401/01/03

30

25

20

15

10

5

0

Turbidity (NTU)

5

Middle Lake

Main Lake

Turbidity in Astoria's Source Water: Lakes

November 2002-September 2008

Figure 3b: Turbidity for the stream sources of Astoria‟s drinking water (Bear Creek, 

Cedar Creek, and Spur 14 Creek).

Figure 3c: Turbidity for the lake sources of Astoria‟s drinking water (Middle Lake and 

Main Lake).

09/30/0801/01/0801/01/0701/01/0601/01/0501/01/0401/01/03

80

70

60

50

40

30

20

10

5

0

Turbidity (NTU)

5

Bear Creek

Cedar Creek

Spur 14 Creek

Turbidity in Astoria's Source Water: Streams

November 2002-September 2008
12

Turbidity Analysis for Oregon Public Water Systems

09/30/0801/01/0801/01/0701/01/0601/01/0501/01/0401/01/03

5

4

3

2

1

0

Precipitation (inches)

Precipitation for City of Astoria

Astoria WSO Airport Co-op Station: 2002-2008

09/30/0801/01/0801/01/0701/01/0601/01/0501/01/0401/01/03

20

15

10

5

0

Turbidity (NTU)

5

MA PE 29.0221

MA D 0.3026

MSD 0.3719

R A ccuracy Measures aw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for Astoria

Multiplicative Model: November 2002-September 2008

Figure 3d: Precipitation at the Astoria WSO Airport co-op weather station, the nearest 

weather data to Astoria‟s DWSAs. The seasonal pattern of precipitation drives the 

seasonal pattern in turbidity.

Figure 3e: The turbidity trend for the influent (raw) water is gradually increasing 

(Yt=0.85+0.000183*t), but the trend is not strong. Generally, the turbidity remains low.
13

Turbidity Analysis for Oregon Public Water Systems

Seasonally-Adjusted Turbidity & High Turbidity Day Trends for Astoria’s Water Sources

Source Water Turbidity Trend

Equation

Direction of Change Days >5 NTU Trend 

Equation

Bear Creek Yt= 1.27+0.000455*t Increasing Yt= -4.27+8.44*t-0.82*t2

Cedar Creek Yt= 1.93+0.000065*t Stable Yt= 6.27+1.69*t

Spur 14 Creek Yt= 0.455+0.000169*t Increasing Yt= 1.20-0.057*t

Main Lake Yt= 0.975+0.000728*t Increasing Yt= 31.9-31.2*t+6.05*t2

Middle Lake Yt= 0.890+0.000572*t Increasing Yt= 11.3-11.4*t+2.66*t2

2003 2004 2005 2006 2007 2008

80

70

60

50

40

30

20

10

0

Days w/ Turbidity >5 NTU

Influent

Bear Cr

Cedar Cr

Spur 14 Cr

Middle Lk

Main Lk

Number of Days with High Turbidity in City of Astoria's Raw Water

WY2003-WY2008

Water Year

Figure 3f: Number of high turbidity days (>5NTU) per water year in Astoria‟s DWSA 

as measured at the drinking water treatment plant from the intake(s) in use (Influent) and 

as measured at each source. The water year runs from October 1st through September 

30th.

Table 2: Trend for seasonally-adjusted turbidity and trend in the number of days with high turbidity for each of 

Astoria‟s water source. Trends are calculated for time period between November 2002 and September 2008.
14

Turbidity Analysis for Oregon Public Water Systems

2003 2004 2005 2006 2007 2008

2.0

1.5

1.0

0.5

0.0

Water Year

Days w/ Turbidity >5 NTU

MA PE 60.7143

MA D 0.6571

MSD 0.5810

A ccuracy Measures

Count Data

Fitted Trend

Trend for Number of High Turbidity Days for Astoria's Raw Water

Linear Trend Model for WY2003-WY2008

Yt = 0.400 + 0.0286*t

Figure 3g: Trend in number of high turbidity days in Astoria‟s DWSA for 2003 through 

2008.
15

Turbidity Analysis for Oregon Public Water Systems

City of Carlton (PWS# 4100171)

Bryan Burnham, Public Works Supervisor

503-852-7575

bburnham@ci.carlton.or.us

Increasing Trend: Low Turbidity (Occasional Spikes) transitioned to Low Turbidity (Frequent Spikes) 

The City of Carlton is in Yamhill County near the eastern foothills of the Coast Range. Carlton has a reservoir on 

Panther Creek with their intake on the downstream end. Panther Creek is a tributary of the North Yamhill River 

on the east side of the Coast Range. The City has a dam and intake structure; a reservoir behind the dam stores

water for use during low flows. Carlton installed a floating intake structure in 2001 to draw water from the top of 

the reservoir rather than the bottom, as a means of reducing sediment entering the intake. The treatment plant 

filters out particulates by coagulation and flocculation with chemicals, followed by pressurized (rapid) sand

filtration. Carlton owns 17 acres of their nearly 2100 acre DWSA; the City‟s triangular parcel includes the 

reservoir and intake and a small section of forestland directly south of the reservoir. The remainder is a 

checkerboard of Bureau of Land Management (BLM) land and private industrial forestland (approximately 50% 

each). The BLM land is currently managed under NWFP. The private industrial landowners have clearcut most 

of their forestland in the watershed since 2000; some small harvests occurred in the1970‟s, 80‟s, and early 90‟s. 

The Panther Creek intake is the sole year-round water source; Carlton does not have an emergency or backup 

source.

Turbidity data collected 

by the Carlton PWS at 

their intake from February 

1998 through September 

2008 was used (Figure 

4a). In February 1999, a 

debris flow occurred on 

BLM land several 

hundred meters upstream 

of Carlton‟s reservoir. 

Turbidity levels in 1999 

and 2000 were higher than 

in 1998 and returned to 

1998 levels by 2001, 

although this decrease 

may be due to the change 

to a floating intake. In the 

last five years, baseline 

turbidity is rising and the 

frequency and magnitude 

of spikes is increasing. 

The seasonal weather 

patterns appear to have 

been consistent (Figure 

4b) and indicate that other 

factors may be contributing to the turbidity trends. The debris flow created a short-term impact to the water 

system that was resolved within two years. The event was of a great enough magnitude that the PWS‟s turbidity 

trend appears to be declining when the debris flow is included in the trend analysis (Figure 4c). However, trend

analysis of the post-recovery data (2001-2008) demonstrates that either turbidity in the DWSA is becoming 

steadily worse since 2001 or the sediment deposited by the debris flow is moving deeper into the reservoir and 

being pulled into the intake (Figure 4d). 

Figure 4a: Carlton DWSA intake turbidity levels from February 1998 through 

September 2008. In 1999, a debris flow occurred in a tributary 600ft upstream of the 

reservoir. Turbidity returned to 1998 levels by 2001. More recently, spikes in turbidity 

increased.

1/1/20 91/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 8

100

80

60

40

20

5

0

Turbidity (NTU)

5

Influent Turbidity for City of Carlton

Drinking Water Intake: 1998-2008
16

Turbidity Analysis for Oregon Public Water Systems

As stated above, days

with turbidity ≥5 NTU

were characterized as 

“high turbidity days” 

because at this level the 

PWS risks clogging its 

sand filter unless it has a 

means to pre-treat the raw 

water. Carlton‟s DWSA 

reached its maximum of 

271 high turbidity days in 

1999 before returning to a 

low of 13 days in 2001. 

Since then, the maximum 

number of high turbidity 

days has increased

gradually to 98 days in 

Water Year 2008, the 

maximum since 

stabilization of the 

tributary landslide (Figure 

4e). Analysis of the 

trend in high turbidity 

days shows a linear 

increase in high turbidity 

episodes over the last eight years (Figure 4f). Carlton has advanced filtration systems and is usually able to purify 

water adequately. The PWS supervisor reported that during the winter of 2007-08 a silt bar formed where Panther 

Creek flows into the reservoir. In addition, reservoir storage is being lost due to sediment settling out in the 

reservoir (Figure 4g). Carlton plans to dredge the reservoir to restore lost capacity and prevent future problems.

Figure 4b: Precipitation at Carlton‟s water treatment facility. The seasonal pattern of 

precipitation drives the seasonal pattern in turbidity. Many of the changes in turbidity 

appear seasonal and weather-driven, but other factors are also suspected.

1/1/20 91/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 8

6

5

4

3

2

1

0

Precipitation (inches)

Precipitation for City of Carlton

Drinking Water Intake: 1998-2008
17

Turbidity Analysis for Oregon Public Water Systems

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/192/1/19 8

100

80

60

40

20

5

0

Turbidity (NTU)

5

MA PE 116.612

MA D 3.896

MSD 96.728

Raw Da A ccuracy Measures ta

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for City of Carlton

Multiplicative Model: February 1998-September 2008

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 1

70

60

50

40

30

20

10

5

0

Turbidity (NTU)

5

MA PE 50.5578

MA D 1.7110

MSD 21.6825

Raw Data A ccuracy Measures

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for City of Carlton

Multiplicative Model: January 2001-September 2008 (excludes 1999 slide effects)

Figure 4c: The trend decreases over time (Yt=7.88-0.00139*t). The linear shape of the 

trend fails to capture the true trend of the data, which has a curved (quadratic) shape. The

accuracy measures are relatively high (MAD of 3.896 NTU), showing that the fit of the 

trend is somewhat poor.

Figure 4d: Turbidity levels in 2001 are similar to those in 1998 and subsequently show 

an increasing trend of turbidity levels (Yt=2.62+0.000796*t). The lower values of the 

accuracy measures (MAD of 1.711 NTU) reflect a better fitting trend than in Figure 2c.
18

Turbidity Analysis for Oregon Public Water Systems

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

300

250

200

150

100

50

0

Water Year

Days w/ Turbidity >5 NTU

*

*

Number of Days with High Turbidity in Carlton's Raw Water

WY1998-WY2008

* 1999 & 2000 show effects of landslide near intake

2001 2002 2003 2004 2005 2006 2007 2008

100

90

80

70

60

50

40

30

20

10

Water Year

Days w/ Turbidity >5 NTU

MA PE 22.213

MA D 9.735

MSD 130.591

A ccuracy Measures

Count Data

Fitted Trend

Trend for High Turbidity Days in Carlton's Raw Water

Linear Trend Model for WY2001-2008

Yt = 11.9 + 9.56*t

Figure 4e: Number of high turbidity days (≥5NTU) per water year at Carlton‟s drinking 

water intake. The water year runs from October 1st through September 30th. 

Figure 4f: Trend in number of high turbidity days in Carlton‟s DWSA for 2001 through 

2008. 
19

Turbidity Analysis for Oregon Public Water Systems

1998 2004 2008

30

25

20

15

10

5

0

Year of Measurement

Reservoir Capacity (millions of gallons)

Storage Capacity in City of Carlton's Reservoir

Figure 4g: Storage capacity of Carlton‟s reservoir from soundings done in 1998, 2004, 

and 2008. The decrease in capacity is thought to be due to a landslide that occurred in 

1999. 
20

Turbidity Analysis for Oregon Public Water Systems

City of Falls City (PWS# 4100297)

PWS # 4100297 

Don Poe

Public Works Supervisor

503-787-1305

Stable Trend: Low Turbidity (Occasional Spikes)

The City of Falls City is in Polk County near the Coast Range; Falls City‟s treatment facility uses slow sand 

filtration for removal of particulates. They cease water purification when turbidity goes above 5 NTU, as higher 

turbidity clogs the filters. Their 3,219 acre combined Drinking Water Source Areas are located on the eastern 

slopes of the Coast Range. Falls City has two drinking water intakes: one on Glaze Creek and the other on Teal 

Creek. The intake on Glaze Creek is only 1⁄2 mile from the springs, resulting in very low turbidity the majority of 

the time. The Glaze Creek intake is usually the sole intake in use during late autumn and winter, water from 

Glaze and Teal Creeks are mixed during spring and early summer, and only Teal Creek is used in the late summer 

and early autumn when flows are too low on Glaze Creek. The use pattern of the intakes probably keeps influent 

turbidity low by relying on Glaze Creek during the early rainy season when high turbidity events are most 

common (personal communication w/ PWS staff). Ownership of the Glaze Creek DWSA (648 acres) is split 

between a private industrial (PI) company and the Bureau of Land Management with each owning about half of 

the drainage. The Teal Creek DWSA (2571 acres) is ~90% owned by the PI company. The BLM owns ~ 10% of 

the drainage with an unidentified private non-industrial landowner owning ~50 acres near the intake. Falls City 

does not own any of the land in their DWSAs. Private landowners logged half of the Teal Creek DWSA and 

about 15% of the Glaze Creek DWSA in the last ten years. Water quality in the watershed is historically very 

good.

Turbidity data collected 

daily at approximately 

8am by the Falls City

PWS from November 

1999 through September 

2008 was used (Figure 

5a). [Falls City must 

shutdown their intake for 

1 to 3 days during stormtriggered turbidity spikes

greater than 5 NTU. Data 

were censored (>5 NTU 

but of indeterminate 

value) on days of 

treatment plant 

shutdowns due to high 

turbidity. These data are 

counted as 5 NTU in the 

analyses, but the values 

are likely to be higher.] 

The collected data 

represent the source water 

being used at the time, so 

they cannot characterize 

Teal Creek during the winter or Glaze Creek during the late summer, for example. Teal Creek could have high or 

low turbidity during the winter, but it cannot be evaluated with this data. The composite influent for Falls City 

has very low baseline turbidity with few spikes, relative to other PWSs in this report. The seasonal weather 

pattern had few large storms earlier in the decade (2001-2004; Figure 5b). This appears to have been a calm 

Figure 5a: Falls City‟s influent turbidity levels from November 1999 through 

September 2008. Water from Falls City‟s DWSAs generally has low baseline turbidity 

with infrequent spikes.

9/30/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/20

10

8

6

5

4

2

0

Turbidity (NTU)

5

Influent Turbidity for City of Falls City

Drinking Water Intake: 1999-2008
21

Turbidity Analysis for Oregon Public Water Systems

period relative to 2000 and 2005 onward. Seasonally-adjusted trend analysis of the data (1999-2008) 

demonstrates that influent

turbidity is remaining 

steady in the time period 

analyzed (Figure 5c). 

As stated above, days 

with turbidity ≥5 NTU 

were characterized as 

“high turbidity days” 

because at this level the 

PWS risks clogging its 

sand filter without the

means to pre-treat the raw 

water. Falls City‟s 

influent water reached its 

maximum of 8 high 

turbidity days in 2007 

(Figure 5d). Three years 

did not have any high 

turbidity days at all. 

Analysis of the trend in 

high turbidity days shows 

a slight quadratic increase 

in high turbidity episodes 

over the last eight years 

(Figure 5e). Falls City 

reports that they currently 

get spikes in turbidity 

during moderate-to-heavy 

rain that requires 

shutdowns. Nevertheless, 

Falls City experiences 

high turbidity days very 

rarely compared to other 

Coast Range PWSs 

examined in this report, 

so the physical 

significance of this trend 

may be minimal. If this 

trend continues, it would 

be cause for concern.

Figure 5b: Precipitation at the Rockhouse1 RAWS weather station, the nearest weather 

data to Falls City‟s DWSAs. The seasonal pattern of precipitation drives the seasonal 

pattern in turbidity.

Figure 5c: The trend remains relatively steady over time (Yt=0.62-0.000076*t). 

Censored data from treatment plant shutdowns due to high turbidity (in 2003, 2006, and 

2007) are entered as 5 NTU when the values were possibly higher. However, the trend 

does not change much and remains “stable” even if censored data are assigned higher 

values (15 NTU, Yt=0.61-0.000037*t; 50 NTU, Yt=0.62+0.000093*t).

9/30/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/20

10

8

6

4

2

0

Precipitation (inches)

Precipitation for City of Falls City

Rockhouse 1 RAWS Station: 1999-2008

9/30/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/20

20

15

10

5

0

Turbidity (NTU)

5

MA PE 65.6028

MA D 0.2532

MSD 0.2964

R A ccuracy Measures aw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for Falls City

Multiplicative Model: November 1999-September 2008
22

Turbidity Analysis for Oregon Public Water Systems

In November of 2006, 

Falls City had several 

days of high turbidity 

triggered by logging and 

hauling operations near 

the springs of Glaze 

Creek in the midst of 

heavy rains. Oregon 

Department of Forestry

later determined that there 

were violations of forest 

practice rules including 

wet weather hauling

limitations. Shortly after 

the incident, DEQ 

acquired minimum, 

maximum, and mean 

daily turbidity data for 

11/01/2006 through 

11/10/2006. We graphed 

this data with the 8am 

data used in this analysis 

of Falls City (Figure 5f). 

The 8am data is fairly 

close to the mean, but 

does not accurately show 

the extremes, especially 

the maximum. More 

systematic data collection 

that shows the range of 

daily turbidity is probably 

needed to characterize 

DWSAs accurately.

Figure 5e: Trend in number of high turbidity days in Falls City‟s influent water for 

2000 through 2008. 

Figure 5d: Number of high turbidity days (≥5NTU) per water year in Falls City‟s 

influent drinking water. The water year runs from October 1st through September 30th. 

2000 2001 2002 2003 2004 2005 2006 2007 2008

8

6

4

2

0

Water Year

Days w/ Turbidity >5 NTU Number of Days with High Turbidity in City of Falls City's Raw Water

WY2000-WY2008

2000 2001 2002 2003 2004 2005 2006 2007 2008

9

8

7

6

5

4

3

2

1

0

Water Year

Days w/ Turbidity >5 NTU

MA PE 58.2846

MA D 1.8620

MSD 4.8134

A ccuracy Measures

Count Data

Fitted Trend

Trend for Number of High Turbidity Days in Falls City's Raw Water

Quadratic Trend Model for WY2000-WY2008

Yt = 1.29 - 0.31*t + 0.090*t**2
23

Turbidity Analysis for Oregon Public Water Systems

Figure 5f: Comparison of daily means, minima, and maxima with 8am „snapshot‟ data 

during sedimentation incident in November, 2006. 8am data are close to the mean, but 

inexact. 8am data do not reflect maximum turbidity well.

1 /10/20 6

11/9/20 61 /8/20 61 /7/20 611/6/20 611/5/20 611/4/20 611/3/20 61 /2/20 61 /1/20 6

50

40

30

20

10

0

Turbidity (NTU)

Daily Maximum

Daily Minimum

Daily Mean

8am Measurement

Turbidity Range & Mean for November 2006 Sedimentation Incident
24

Turbidity Analysis for Oregon Public Water Systems

City of Forest Grove (PWS #4100305)

Randy Smith, Water System Supervisor

503-992-3259

Decreasing Trend: Low Turbidity (Frequent Spikes) trending towards Low Turbidity (Occasional Spikes)

The City of Forest Grove is located to the west of Portland in Washington County. The PWS removes 

particulates with a sedimentation basin, followed by chemical coagulation and flocculation with rapid sand 

filtration. They own 4300 acres, most of which is in their small DWSA on the east side of Oregon‟s north Coast 

Range. Forest Grove‟s ownership is ~75% of the DWSA; the balance is ~7% state forest, ~7% private industrial 

(various owners), and ~10% private non-industrial. The streams in the DWSA are Clear Creek, Deep Creek, 

Roaring Creek, Smith Creek, and Thomas Creek, all upper tributaries of the Tualatin River. These watersheds are 

used most of the year; however, Forest Grove gets water from the Joint Water Commission (JWC, Hillsboro, 

Beaverton, and other municipalities) in summer if stream levels get too low to keep the intake at an adequate 

depth or in November and December if flushing of sediment from the stream system forces a shutdown of 

drinking water treatment. In 2000, the City Council approved a Forest Stewardship Council (FSC)-certified 

management plan. Trout Mountain Forestry manages the watershed sustainably on the City‟s behalf. 

Turbidity data collected 

by the Forest Grove PWS 

at their intake from 

November 1994 through 

September 2008 was 

evaluated. There is a 

decreasing trend in 

turbidity (Figure 6a). The 

turbidity patterns are 

unlikely to be explained 

by weather, as the 

seasonal weather patterns 

appear to be consistent 

(Figure 6b). Trend 

analysis of the turbidity 

data shows the declining 

turbidity in the system 

since the mid-1990‟s 

(Figure 6c). In particular, 

1996 and 1997 showed a 

great many large spikes in 

turbidity before and after 

the February 1996 storms. 

With time, the Creeks 

flushed out the additional fine sediment. 

Days with turbidity ≥5 NTU can adversely affect the treatment of water for the Forest Grove PWSs. The number 

of high turbidity days for Forest Grove‟s DWSA reached a maximum of 103 days in 1997. The number of high 

turbidity days has stayed low during the last eight years (Figure 6d). Trend analysis shows the initial rapid 

decrease in high turbidity days is leveling off (Figure 6e). Their FSC-certified plan has allowed Forest Grove to 

generate revenue and solve the legacy problems from previous activity, including removal of risky/unnecessary 

roads and culverts, revegetation, and geoengineering. Highly erosive or steep areas are not harvested. Riparian 

protection zones are 75 to 200 feet wide with the following requirements:

Figure 6a: Turbidity for the Forest Grove DWSA intake from November 1994 through 

September 2008. Note that while the seasonal patterns remained, the baseline turbidity 

declined, and the frequency and intensity of spikes decreased.

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 5

120

100

80

60

40

20

5

0

Turbidity (NTU)

5

Influent Turbidity for City of Forest Grove

Drinking Water Intakes: Water Years 1995-2008
25

Turbidity Analysis for Oregon Public Water Systems

Timber removal 

is prohibited within 

riparian protection zones, 

except for restoration 

activities (no commercial 

harvest is allowed). 

Practices promote older 

forest structure and 

diversity. 

Skid trails or 

roads in riparian areas are 

retired or relocated 

wherever possible. 

Equipment 

operation in riparian areas 

is prohibited, except for 

restoration activities.

Restoration 

activities within riparian 

areas use methods with 

the lightest possible 

impacts.

(Forest Grove Watershed 

Plan, Chapter 4).

The current turbidity and 

fine sediment situation is 

easily managed by PWS 

staff because of the 

generally low turbidity, 

the identification of 

problem sources by their 

forester, and their ability 

to switch to JWC water 

when necessary. In 2001, 

Forest Grove received a 

League of Oregon Cities 

award for their efforts.

Figure 6b: Precipitation in Forest Grove‟s DWSA. The seasonal pattern of 

precipitation drives the seasonal pattern in turbidity.

Figure 6c: Turbidity trend for Forest Grove (Yt=3.97-0.000401*t). The decreases in 

both baseline turbidity and the frequency of spikes results in a downward linear trend. 

The trend without the effects of the 1996 flood is similar (Yt=3.90-0.000482*t; graph of 

1997-2008 not shown).

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 5

5

4

3

2

1

0

Precipitation (inches)

Precipitation for City of Forest Grove

Drinking Water Intakes: Water Years 1995-2008

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 5

100

80

60

40

20

5

0

Turbidity (NTU)

5

MA PE 88.7495

MA D 1.7979

MSD 27.9094

Raw Data A ccuracy Measures

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for Forest Grove

Multiplicative Model: November 1994-August 2008
26

Turbidity Analysis for Oregon Public Water Systems

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

100

80

60

40

20

0

Water Year

Days w/ Turbidity >5 NTU

Number of Days with High Turbidity in Forest Grove's Raw Water

WY1995-WY2008

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

100

80

60

40

20

0

Water Year

Days w/ Turbidity >5 NTU

MA PE 113.648

MA D 15.744

MSD 531.357

A ccuracy Measures

Count Data

Fitted Trend

Trend in High Turbidity Days for Forest Grove's Raw Water

Quadratic Trend Model for WY1995-WY2008

Yt = 63.7 - 6.46*t + 0.221*t^2

Figure 6e: Trend in number of high turbidity days in Forest Grove‟s DWSA. 

Figure 6d: Number of high turbidity days (>5NTU) per water year in Forest Grove‟s 

DWSA as measured at the drinking water intake. The water year runs from October 1st

through September 30th.
27

Turbidity Analysis for Oregon Public Water Systems

Hillsboro-Cherry Grove (PWS# 4100985)

Niki Iverson

Water Resources Manager

503-615-6770

nikii@ci.hillsboro.or.us

Stable Trend: Low Turbidity (Occasional Spikes)

[Formerly a Decreasing Trend: Transitioned from Low Turbidity (Frequent Spikes) to Low Turbidity (Occasional 

Spikes)]

The Hillsboro-Cherry Grove Public Water System (PWS) is the sole provider of drinking water to the community

of Cherry Grove and the primary provider for the City of Gaston, the community of Dilley, and the LA Water Coop. (The other provider is the Joint Water Commission, PWS# 4100379.) These communities are west of 

Portland near the Coast Range. The PWS has two water sources: the upper Tualatin River and Barney Reservoir, 

on the upper Trask River. Water from Barney Reservoir is piped into the upper reaches of the Tualatin River 

during drier months (June through October) to supplement the Tualatin‟s flow. The PWSs Slow Sand Filtration 

Plant (SSFP), which removes particulates by means of a settling pond (sedimentation) and slow sand filtration, is 

currently benefitting from low turbidity in the upper Tualatin River, but, as a precaution, the water district shuts 

the slow-sand plant when turbidity is 5 NTU or greater. The forested Drinking Water Source Area (DWSA) in 

the upper Tualatin River is over 15,500 acres of mixed public and private ownership. The terrain is landslide 

prone in the area near the slow-sand water filtration plant. The southeastern corner of the DWSA (near the intake)

is a checkerboard of land 

owned by the PWS (~3% 

of DWSA) and the Bureau 

of Land Management

(~5%). A small parcel 

(~1% of DWSA) of 

private non-industrial 

timberland straddles the 

southern edge. A private 

industrial (PI) company 

owns the Lee Creek 

drainage and the 

headwaters of Sunday 

Creek, amounting to the 

eastern edge and the 

northeastern third of the 

DWSA (~45% of the 

DWSA). The remaining 

(~45%) area of the 

DWSA is in the 

Tillamook State Forest, 

including most of Sunday 

Creek and nearly all of 

Maple Creek, in addition 

to the headwaters of the 

Tualatin River. Some partial cuts (thinning) occurred on state and federal lands. Since 2000, the major private 

industrial (PI) landowner has harvested approximately half of their land by clearcutting. The forested DWSA for 

the intake on Barney Reservoir is 5275 acres of mixed ownership. The PWS owns land surrounding the Reservoir 

(~8% of the DWSA). The lower (northern) part of the DWSA contains most of the Reservoir; it is in the 

Tillamook State Forest (~40%). The upper (southern) part is PI forestland (~45%). BLM owns scattered parcels

(~7%) around the edges of the Barney DWSA. The Barney Reservoir (North Fork Trask) DWSA experienced 

Figure 7a: Turbidity for the Hillsboro-Cherry Grove DWSA intake from May 1993 

through December 2008. The frequency and intensity of turbidity spikes decreased during 

the early part of this decade. Recently, the number and intensity of spikes has increased. 

Most of the 2003 data is missing.

12/15/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 51/1/19 4

5/19/19 3

140

120

100

80

60

40

20

5

0

Turbidity (NTU)

5

Influent Turbidity at Hillsboro-Cherry Grove Filtration Plant

Drinking Water Intake: 1993-2008
28

Turbidity Analysis for Oregon Public Water Systems

some scattered clearcuts 

in the last 10 years on PI 

land and some partial cuts 

on State Forest land. 

Conditions in the upper 

Tualatin River DWSA 

probably have the greatest 

impact on source water 

turbidity.

Figure 7a shows the 

improvement in turbidity 

in the upper Tualatin 

River watershed. While 

spikes in turbidity above 

5 NTU were common 12 

years ago, the watershed 

recovered from 

disturbances in the mid1990‟s and now has 

consistently low turbidity. 

Baseline turbidity has 

decreased also. Large 

spikes do occur from time 

to time, as do periods of 

high turbidity. These 

spikes do appear to be 

triggered by extreme 

precipitation events at 

times (Figure 7b), but 

other factors may 

contribute to surges in 

stream turbidity. 

Seasonally-adjusted trend 

analysis shows a 

downward trend in the 

watershed‟s turbidity 

(Figure 7c), although 

some recent winters had 

multiple large turbidity 

spikes that reduce the fit 

of the trend and may 

indicate potential 

problems.

The number of high 

turbidity days (≥ 5NTU)

decreased from a high of 

47 days during the 1996 water year to a low of 2 days in the 2001 water year (Figure 7d). This shows a healthy 

recovery from the effects of disturbances before and during the February 1996 floods. Analysis of the trend in 

high turbidity days shows that the number of days greater than 5 NTU decreased until 2001 when there were only 

Figure 7b: Precipitation at Hillsboro-Cherry Grove‟s slow-sand filtration plant. The 

seasonal pattern of precipitation drives the seasonal pattern in turbidity.

Figure 7c: Turbidity trend for the Hillsboro-Cherry Grove Slow Sand Filtration Plant 

(Yt=2.79-0.000297*t). After frequent large spikes in the 1990‟s, the frequency and 

intensity of high turbidity events declined. The trend without the effects of the 1996 flood 

is similar (Yt=2.33-0.000269*t; graph of 1997-2008 not shown).

12/15/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 51/1/19 4

5/19/19 3

7

6

5

4

3

2

1

0

Precipitation (inches)

Precipitation at Hillsboro-Cherry Grove Filtration Plant

Drinking Water Intake: 1993-2008

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 61/1/19 51/1/19 4

100

80

60

40

20

5

0

Turbidity (NTU)

5

MA PE 123.927

MA D 1.670

MSD 86.395

A ccuracy Measures Raw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition

Multiplicative Model: May 1993-September 2008

for Hillboro-Cherry Grove SSFP
29

Turbidity Analysis for Oregon Public Water Systems

2 high turbidity days 

(Figure 7e). Turbidity 

stayed low before 

climbing somewhat 

recently (14, 12, and 10 

days in 2006, 2007, and 

2008, respectively). It is 

unknown if the 2006-

2008 level represents the 

typical equilibrium for 

this watershed, or if the 

lower turbidity of 2001-

2004 is the norm. The 

result is a curved trend 

with sharp declines in 

high turbidity events 

followed by leveling off. 

If the trend continues, 

then the slight upward 

inflection at the end could 

become a steady upward 

climb in stream turbidity. 

Lee Creek drains into the 

Tualatin River 

approximately 1500 

meters upstream of the 

SSFP, so any individual 

or cumulative timber 

harvest effects could be 

felt quickly. The next 

several years will 

demonstrate whether land 

use activities are likely to 

increase turbidity and 

negatively impact water 

quality in the upper 

Tualatin River watershed.

Figure 7d: Number of high turbidity days (≥5NTU) per water year in Hillsboro-Cherry 

Grove‟s DWSA as measured at the drinking water intake. The water year runs from 

October 1st through September 30th.

Figure 7e: Trend in number of high turbidity days in Hillsboro-Cherry Grove‟s DWSA 

by water year.

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

50

40

30

20

10

0

Water Year

Days w/ Turbidity >5 NTU

MA PE 101.691

MA D 7.177

MSD 89.433

A ccuracy Measures

Count Data

Fitted Trend

Trend for High Turbidity Days in Hillsboro-Cherry Grove's Raw Water

Quadratic Trend Model for WY 1994-2008

Yt = 37.38 - 4.43*t + 0.167*t^2

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

50

40

30

20

10

0

Water Year

Days w/ Turbidity >5 NTU

Water Years 1994-2008

Number of Days w/ High Turbidity in Hillsboro-Cherry Grove's Raw Water
30

Turbidity Analysis for Oregon Public Water Systems

City of Siletz (PWS# 4100821)

Allen Middaugh, Superintendent of Public Works

541-444-2521

Increasing Trend: Low Turbidity (Occasional Spikes) transitioned to Low Turbidity (Frequent Spikes)

Baseline Turbidity Increasing

In the western foothills of the middle Coast Range, the City of Siletz is located on the banks of the Siletz River in 

Lincoln County. The City uses the Siletz River as their year round water source. The PWS accomplishes 

particulate removal with chemical coagulation and flocculation and rapid sand filtration. The City has a small 

storage tank for raw water. It does not hold more than a roughly two day supply, so Siletz is planning 

construction of a water storage pond to provide a backup supply of water when the River‟s turbidity is too high. 

Approximately 1/3 of the PWS‟s finished water is purchased by the Confederated Tribes of Siletz Indians. At 

131,540 acres, the Drinking Water Source Area is larger than any other PWS cited in this study. Most of the 

DWSA is made up of steep terrain with large amounts of rainfall. The majority (~75%) of the DWSA is privately 

owned by industrial timber companies. Landowners logged some of the Boulder Creek drainage in the 1980‟s, 

and, in 1988, Boise Cascade removed the Valsetz Lake dam. Mr. Middaugh (PWS supervisor) reports that the 

River is flashier now-more prone to sudden rises and falls in turbidity and flow. In the 1990‟s and 2000‟s, 

landowners logged extensively in the Gravel Creek and South Fork Siletz River drainages and along the mainstem 

Siletz River (“the Gorge”). The Siletz tribe owns scattered portions of the watershed (~2.5% of the DWSA). The 

low gradient properties along the river (just upstream of the City) are occupied by agricultural and rural 

residential lands, making up a very small (~7%) but important portion of the DWSA‟s area. Small landowners 

along the river in lower part of the DWSA occasionally conduct land disturbing activities, such as the illegal 

clearing of vegetation and fill operations by a riverfront landowner in September 2008, just a couple of miles 

upstream of Siletz‟ intake. Public ownership includes some state land in the south-east corner (~5% of the 

DWSA) and BLM land in the northern headwaters portion (~10% of the DWSA). BLM did some small scale 

clearcut logging on their land around the North Fork Siletz River and Boulder Creek in the 1980‟s. The City of 

Siletz does not own or manage any of the DWSA.

The City of Siletz PWS

started monitoring Siletz 

River turbidity year round 

in April of 2000, after 

they built a new water 

treatment plant and begin 

using the Siletz River as a 

water source all year 

long. From 1967 to 2000, 

they used Tangerman 

Creek except in the 

summer/early autumn. 

(Prior to that, Logan 

Creek was the water 

source. Source changes 

have been driven by 

water quality problems.) 

The years 2000 and 2001 

had low turbidity with 

few large spikes, but in 

subsequent years, large 

spikes in turbidity occur 

frequently before

Figure 8a: Influent turbidity for the City of Siletz‟ drinking water intake on the Siletz 

from April 2000 through December 2008. 

12/31/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 1

4/21/20

90

80

70

60

50

40

30

20

10

5

0

Turbidity (NTU)

5

Influent Turbidity for City of Siletz

Drinking Water Intake: 2000-2008
31

Turbidity Analysis for Oregon Public Water Systems

dropping back to the 

baseline (Figure 8a). The 

baseline turbidity itself is 

higher, especially during 

the winter rains.

Turbidity follows the 

seasonal precipitation 

pattern (Figure 8b), but 

the biggest storms 

frequently do not coincide 

with the high turbidity 

events. The seasonallyadjusted trend analysis 

demonstrates a gradually 

increasing trend (Figure 

8c); the relatively less 

turbid 2007/2008 rainy 

season slightly reduces 

the slope of the trend.

The Siletz River 

experienced few high 

turbidity (≥5NTU) days in 

the latter half of the 2000 

water year (4 days, AprilSeptember) and in the 

whole 2001 water year 

(15 days; Figure 8d). In 

contrast, the following 

water year (WY2002) had 

85 high turbidity days, the 

highest number since 

year-round use of the 

Siletz River began. The 

number of high turbidity 

days has not returned to 

the levels seen in 2001, 

being 52 days in the 2008 

water year. After 

excluding the incomplete 

data for WY2000, 

analysis of the count data 

show a generally 

increasing trend in 

turbidity (Figure 8e). The 

turbidity varies around a

central tendency since 

2002. It is possible that 

WY2000 and WY2001 were anomalously low. Alternately, events since WY2001 may also have created 

sediment sources, accounting for the elevated turbidity since 2002. Analysis of turbidity and Total Suspended 

Solids (TSS) data collected by the Siletz Tribe indicates that turbidity is spatially and temporally complex and 

Figure 8b: Precipitation at Siletz‟ water treatment facility. The seasonal pattern of 

precipitation drives the seasonal pattern in turbidity.

Figure 8c: Turbidity trend for the City of Siletz (Yt=2.87+0.000197*t). The frequency 

and intensity of high turbidity events increased starting in the autumn of 2001.

12/31/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 1

4/21/20

7

6

5

4

3

2

1

0

Precipitation (inches)

Precipitation for City of Siletz

Drinking Water Intake: 2000-2008

9/30/20 8

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 1

4/21/20

90

80

70

60

50

40

30

20

10

5

0

Turbidity (NTU)

5

MA PE 78.7126

MA D 2.0649

MSD 28.0932

A ccuracy Measures Raw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for City of Siletz

Multiplicative Model: April 2000-September 2008
32

Turbidity Analysis for Oregon Public Water Systems

generally higher farther down in the watershed (Appendix B). Source identification information and turbidity 

data prior to April 2000 would be useful to better evaluate the trend in the DWSA.

2000 2001 2002 2003 2004 2005 2006 2007 2008

90

80

70

60

50

40

30

20

10

0

Water Year

Days w/ Turbidity >5 NTU

Number of Days with High Turbidity in City of Siletz' Raw Water

WY2000-WY2008

* Data for 2000 begin midway through water year

2001 2002 2003 2004 2005 2006 2007 2008

90

80

70

60

50

40

30

20

10

0

Water Year

Days w/ Turbidity >5 NTU

MA PE 42.757

MA D 14.375

MSD 365.359

A ccuracy Measures

Count Data

Fitted Trend

Trend for High Turbidity Days in City of Siletz' Raw Water

Linear Trend Model for WY2001-WY2008

Yt = 41.7 + 2.15*t

Figure 8d: Number of high turbidity days (≥5NTU) per water year in the Siletz River as 

measured at the drinking water intake. The water year runs from October 1st through 

September 30th.

Figure 8e: Trend in number of high turbidity days at Siletz‟ intake. The linear trend 

model shows the increase in high turbidity days since 2001.
33

Turbidity Analysis for Oregon Public Water Systems

City of Yamhill (PWS# 4100968)

Richard Howard

Public Water System Supervisor

503-662-4344

pwsuper@cityofyamhill.com

Stable/Increasing Trend: Low Turbidity (Frequent Spikes)

The City of Yamhill is in Yamhill County near the foothills of the Coast Range, north of Carlton. Yamhill‟s PWS 

removes particulates by settling in their upper reservoir, chemical coagulation and flocculation, and rapid sand 

filtration. Yamhill‟s Drinking Water Source Area is 2900 acres of the Turner Creek watershed, a tributary of the 

North Yamhill River on the eastern slopes of the Coast Range. A small dam forming a pool keeps the intake 

submerged during lower flow periods. Another structure upstream forms an impoundment to store water to 

supplement low flows in the late summer and early autumn. The DWSA is mostly rolling hills with steeper 

slopes in the valleys and occasional steep slopes in the uplands. The watershed ownership is a patchwork pattern 

of public and private ownership with nearly all the private land harvested within the last ten or fifteen years. 

Yamhill owns 60 acres of mature forest just upstream of the intake (given to the City by President Calvin 

Coolidge) and another 10 acres on the south side of their water storage impoundment in the upper reaches of the 

watershed. BLM owns approximately 27% of the watershed. The remainder is private industrial forestland (70% 

of the DWSA total). There is a 75 acre parcel near the intake owned by a small woodland owner; the portion of it 

in the DWSA is roughly 20 acres (<1% of the DWSA).

We graphed and analyzed 

the turbidity data 

collected by the Yamhill 

PWS at their intake from 

January 1996 through 

September 2008 (Figure 

9a). Following the 

impacts of the 1996 

storm, the turbidity 

decreased with fewer 

large spikes and falling 

baseline turbidity. 

Turbidity then increases

starting in 2003 with 

frequent large spikes and 

a baseline that varies

around 5 NTU. The large 

storm that hit the northern 

Coast Range in December 

of 2007 washed sediment 

into Turner Creek and 

initiated a landslide above 

the intake, filling the pool 

behind the dam with fine 

sediment and gravel. Poor road placement and a blocked culvert contributed to the slide. The intake turbidity 

data represents stream turbidity after fine sediment settles out in the impoundment and, to a lesser degree, the

intake pool. The seasonal weather patterns appear to have been consistent, so storms alone cannot explain the 

changes in turbidity (Figure 9b). Seasonally-adjusted trend analysis shows a flat, stable trend when 1996 data is 

included (Figure 9c). Analysis without 1996 data (to account for the extreme and historically rare nature of the 

1996 storm and high-flow events) shows an upward linear trend in turbidity (Figure 9d).

Figure 9a: Turbidity for the Yamhill DWSA intake from January 1996 through 

September 2008. Note the effect of the 1996 storm followed by relatively low turbidity 

until 2003. More recently, large turbidity spikes occur with greater frequently.

1/1/20 91/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 6

300

250

200

150

100

50

0

Turbidity (NTU)

5

Influent Turbidity for City of Yamhill

Drinking Water Intake: 1996-2008
34

Turbidity Analysis for Oregon Public Water Systems

High turbidity days (≥5 NTU) are most problematic for PWSs. Yamhill‟s DWSA reached a maximum of 149 

high turbidity days in 2004; the low of 42 days was in 2001. The count of high turbidity days has stayed near 100 

days since 2004 and was 113 days in the 2008 water year (Figure 9e). This is much greater than the 2008 count 

of high turbidity days for Forest Grove and comparable to the current situation in Carlton. Statistical analysis of 

the trend shows a decline in high turbidity episodes until six years ago, followed by increased episodes in recent 

years (Figure 9f). Data 

prior to 1996 would help 

to evaluate whether the 

high turbidity events in 

the late 1990‟s are due to 

1996 flood effects or some 

other processes in the 

DWSA. The intake will 

eventually need to be 

moved to a more stable 

location and designed in a 

way that allows adequate 

fish passage. Yamhill 

needs to dredge the intake 

pool and the 

impoundment in the upper 

watershed to restore lost 

storage capacity.

Figure 9b: Precipitation at Yamhill‟s water treatment facility. The seasonal pattern of 

precipitation drives the seasonal pattern in turbidity. Analysis shows that many of the 

changes in turbidity are seasonal and weather-driven, but other factors are also at work.

1/1/20 91/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 6

10

8

6

4

2

0

Precipitation (inches)

0

Precipitation for City of Yamhill

Drinking Water Intake: 1996-2008
35

Turbidity Analysis for Oregon Public Water Systems

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 71/1/19 6

100

80

60

40

20

5

0

Turbidity (NTU)

5

MA PE 132.179

MA D 5.563

MSD 530.908

A ccuracy Measures

Raw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for City of Yamhill

Multiplicative Model: January 1996-September 2008

1/1/20 81/1/20 71/1/20 61/1/20 51/1/20 41/1/20 31/1/20 21/1/20 11/1/201/1/191/1/19 81/1/19 7

100

80

60

40

20

5

0

Turbidity (NTU)

5

MA PE 119.533

MA D 4.866

MSD 432.549

A ccuracy Measures

Raw Data

Model Predictions

Temporal Trend

Raw Water Turbidity Trend & Seasonal Decomposition for City of Yamhill

Multiplicative Model: January 1997-September 2008 (excludes 1996 flood effects)

Figure 9c: Turbidity trend for the City of Yamhill for 1996-2008 (Yt=6.88-0.000122*t). 

The frequency and intensity of high turbidity events decreased through 2001 and 

increased thereafter.

Figure 9d: Turbidity trend for the City of Yamhill for 1997-2008 

(Yt=4.58+0.000688*t). Removal of 1996 alters the trend noticeably.
36

Turbidity Analysis for Oregon Public Water Systems

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

160

140

120

100

80

60

40

20

0

Water Year

Days w/ Turbidity >5 NTU

Number of Days with High Turbidity in Yamhill's Raw Water

WY1996-WY2008

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

160

140

120

100

80

60

40

20

0

Water Year

Days w/ Turbidity >5 NTU

MA PE 27.062

MA D 21.769

MSD 807.333

A ccuracy Measures

Count Data

Fitted Trend

Trend for High Turbidity Days in Yamhill's Raw Water

Quadratic Trend Model for WY1996-WY2008

Yt = 139.9 - 11.3*t + 0.729*t**2

Figure 9e: Count of high turbidity days (≥5NTU) per water year in Yamhill‟s DWSA as 

measured at the drinking water intake. The water year runs from October 1st through 

September 30th. 

Figure 9f: Trend in count of high turbidity days in Yamhill‟s DWSA. The quadratic 

trend model reflects the dip in high turbidity days in 2000 and 2001. The trend is largely 

flat with substantial variation around the mean.
37

Turbidity Analysis for Oregon Public Water Systems

Ecological and Economic 

Implications

In 2002, Oregon Departments of Forestry (ODF) and Environmental Quality (DEQ) released a review of the thencurrent rules under the Forest Practices Act (FPA): “Sufficiency Analysis: A Statewide Evaluation of FPA 

Effectiveness in Protecting Water Quality” (ODF/ODEQ 2002). The report examined the ability of forest practice 

rules to meet some of DEQ‟s water quality standards, including turbidity and sedimentation (accumulation of 

bedded sediment). The Sufficiency Analysis (SA) discussion of turbidity and sedimentation focused primarily on 

the effects of roads and landslides, the largest sources of sediment. The SA states that roads built to post-1984 

road construction standards are less likely to fail, cause landslides, or otherwise contribute sediment. However, 

roads can generate substantial amounts of fine sediment the first few years after road construction and during wetweather log hauling. The SA also discussed the effects of yarding (moving logs across the ground) on erosion, 

the increased incidence of landslides on clearcut forestland, and the effects of landslides on stream channels. 

Changes to wet weather hauling, management of ground-based yarding on steep or erosive slopes, and 

management of landslide-prone locations were recommended. The SA does not discuss the effects of windthrow 

on sediment regimes or the adequacy of riparian buffers to prevent production of fine sediment or intercept 

sediment before it reaches water bodies. The SA identifies that the effects of practices along small non-fishbearing streams on downstream sediment regimes is an issue that needs research and that data are lacking on 

turbidity and fine sediment effects of forest practices. The data and scientific literature in this report may be able 

to meet some of these identified data gaps and research needs.

Natural Disturbances and Land Use Have Potential Effects on Sediment/Turbidity

Turbidity fluctuations are a normal part of stream systems and an inherent risk in the use of surface water for 

drinking water supplies. Disturbances affect even unmanaged watersheds in the form of bank erosion, landslides, 

fires, floods, and windthrow. The 1999 debris flow in Carlton‟s Drinking Water Source Area is an example of a 

natural disturbance resulting in impacts to a Public Water System. Generally, natural disturbances are infrequent 

and stream systems can flush the additional fine sediment from the stream (Reeves et al 1995, Gomi et al 2004, 

Gomi et al 2005). Episodic landslides move a much greater volume of fine sediment into stream networks than 

small, frequent disturbances; however, landslides are discrete events to which biological systems are adapted 

while chronic sedimentation creates chronic stress that has detrimental effects (Kirchner et al 2001). The woody 

debris and coarse sediment in landslides can enhance fish habitat, even though there are short term impacts to 

aquatic life and drinking water uses (Reeves et al 1995, Reeves et al 2006). The data for the Hillsboro-Cherry 

Grove and the Cities of Forest Grove and Yamhill show impacts from the February 1996 storms, but within two 

or three years, turbidity is equal to or lower than pre-storm levels. This may confirm the short-term impacts of 

natural disturbances discussed by Reeves and Kirchner. The Institute for Natural Resources completed ODF‟s 

Dynamic Ecosystem Policy Project in 2009, and the synthesis documents and seminar white papers give more 

information about the dynamic nature of forest ecosystems and the importance of planning for natural events 

(http://egov.oregon.gov/ODF/STATE_FORESTS/FRP/RP_Home.shtml#Dynamic_Forest_Ecosystems).

Forests are the major land use in the Drinking Water Source Areas described in this report, but DWSAs 

throughout the state contain a variety of land uses. There are three major categories of forest ownership in 

Oregon: privately owned (industrial and non-industrial), state owned, and federally owned. There are also smaller 

amounts of forestlands owned by Native American nations, counties, cities, and public water systems. Impacts 

from land management (such as agriculture, silviculture, roads, and (sub)urban development) can create chronic 

sediment sources that take time and resources to resolve, such as production of fine sediment by forest roads (see 

Sufficiency Analysis and Reid & Dunne 1984). A temporary 2-4 hour increase in turbidity due to legitimate land 

use activities is allowable under DEQ‟s water quality standards, but chronic introduction of fine sediment is not 

allowed. It is also possible for management to increase the rate of other disturbances, as in the case of clearcut 
38

Turbidity Analysis for Oregon Public Water Systems

timber harvests increasing rates of landslides and windthrown trees (Sidle et al 1985, Grizzel & Wolff 1998, 

Robison et al 1999). Klein and others (2008) found a significant association of large scale clearcutting and 

increased turbidity in Northern California redwood forests. Agriculture and suburban development introduces 

sediment and toxics into source water, resulting in increased turbidity, toxic chemicals, and disinfection byproducts in finished drinking water (Ritter et al 2002, Carpenter et al 2008). PWSs may not have resources to 

mitigate disasters or to upgrade equipment. Future studies should examine how land management practices affect 

operations and costs of PWSs.

Land Use May Alter Flow Regimes

Fluctuations in stream flow and modifications due to land use are another potential risk for PWSs with human use 

in Drinking Water Source Areas. Urban or suburban development modifies stormwater flows, creating flashier 

systems with larger high flows, smaller low flows, and more concentrated pollutants, including fine sediment and 

toxics (Schueler 2000). Agricultural uses can consume large amounts of water during summer low flows, 

contribute to flashier stream systems, and pollute waterways (Hunter 1992, Perkins et al 2009). Silviculture,

particularly clearcutting and road building, increases soil moisture during the rainy season and the likelihood of 

damaging flood events and landslides (Keppeler & Brown 1998). Peak flows can increase during the first fall 

storms because of clearcutting (Grant et al 2008); flows can increase due to roads (Jones & Grant 1996), 

potentially moving more sediment through channels and carrying sediment from roads. Several years following 

harvest and replanting, rapidly growing young trees consume large amounts of water and may result in lower base 

flows (Jones & Post 2004). Summer flow deficits are especially great in 35-50 year-old plantations (Perry 2007). 

Fast-growing trees in previously harvested riparian areas consume large amounts of water and reduce low flows 

(Hicks et al 1991), while older riparian forests consume significantly less water (Moore et al 2004). It should be 

noted that any flow alterations due to timber harvest are small in comparison to effects of agriculture or 

urbanization. Regardless of the cause, changes in flow regimes can make planning for adequate water supplies 

difficult for PWSs and potentially contribute to water shortages during summer when water use is highest.

Climate Change Risks for Public Water Supplies

Global climate change compounds the risks of detrimental effects on streamflow and sediment regimes. 

Generally, wetter winters and higher storm intensity could create more risk of flooding and erosion, and drier, 

hotter summers could contribute to smaller low flows and greater risk of water shortages (Bates et al 2008). In 

the Coast Range and Western Cascades geologies, low flows are likely to stay similar but begin earlier in the year 

(Tague et al 2008, G. Grant personal communication). Scientists do not know whether the streamflow effects of 

changes in vegetation (such as re-growing trees) and climate will compound each other and create greater 

problems or whether those effects will negate each other to some extent. More storms of high intensity are likely 

to interact with land use effects to amplify sediment generation (Grizzel & Beschta 1993). For example, soil 

exposed by agriculture or log skidding is more vulnerable to erosion in intense storms, especially exposed soil 

near streams (Gomi et al 2005, Gomi et al 2006). The compounding effects of land use and climate change could 

severely impact public water supplies, especially for small PWSs. It is not yet known what the biggest effects of 

climate change are likely to be, what the risks are, or how these risks interact with vegetation changes such as 

harvest and succession. Scientists are studying this topic intensively.

Potential Costs Related to Higher Turbidity

Financial resources are a crucial concern for the operation of small communities‟ Public Water Systems. A 

typical small PWS has sufficient income to maintain facilities, pay staff, and fund other operating costs, but water 

rates are not usually high enough to absorb substantial cost increases or the need for upgraded equipment. Several 

examples of additional costs attributable to higher turbidity are:

Increased use of coagulation/flocculation chemicals to remove sediment from raw water;

Dredging of sediment-filled reservoirs, impoundments, and intake pools;
39

Turbidity Analysis for Oregon Public Water Systems

Increased staff time to monitor turbidity, flush filters, inspect waterways and facilities, and apply for 

funding and permits to do additional maintenance work such as dredging;

Equipment upgrades such as new filter systems and pre-filters to remove high levels of fine sediment if 

high turbidity conditions are or become chronic;

Installation of new intake structures and/or switching to another water source, such as groundwater or a 

neighboring PWS;

In the case of extended shutdowns during high turbidity events, there is a risk of catastrophic loss of

structures if a fire occurs when there is insufficient water in the PWS‟s reservoirs with which to fight the 

fire. (Falls City and Arch Cape officials expressed this concern to the Salem Statesman-Journal

newspaper and to DEQ staff, respectively);

Costs to procure water from a vendor or neighboring PWS if extended shutdowns are too prevalent; 

Short-term public health risks due to loss of a water supply, the inability to sufficiently purify water, 

and/or toxics; and

Long-term health risks and costs as a result of toxic chemicals (such as pesticides, solvents, and heavy 

metals) adsorbed onto the surface of fine sediment (Ambachtsheer et al 2007).

Monitoring effectiveness of land use BMPs is necessary to ensure that land management is not increasing public 

health risks and PWS operating costs.

Land Management Effectiveness for Source Water Protection

It is cheaper and more effective to protect source water quality than it is to clean up degraded water prior to use 

(Freeman et al 2008). The recent award of $1 million in ARRA funds to Arch Cape for membrane filtration is 

driven in large part by rising turbidity. In addition, modern treatment methods are not able to remove all 

contaminants (Stackelburg et al 2004). Of particular concern are wastewater contaminants like pharmaceuticals 

and personal care products (Benotti et al 2009) and the synergistic effects of multiple contaminants (Hayes et al

2006). These contaminants can sorb to or move with turbidity-causing particles. Postel and Thompson (2005) 

summarize the hydrological services provided by healthy watersheds and the economic costs of losing or reducing 

those services. Some municipalities embarked on source water protection and restoration approaches to both 

reduce present and future costs and to ensure that their delivered water is free of contaminants that treatment 

cannot effectively remove. Seattle, Washington, and New York City, New York, are two notable examples, but 

there are others such as Boston, Massachusetts. Their examples showcase potential management directions that 

may be of use to Public Water Systems in Oregon.

New York City needed to correct water quality problems. Faced with US Environmental Protection Agency 

(EPA) requirements to upgrade treatment facilities, New York applied for a waiver (Limited Alternative to 

Filtration) and took the more cost-effective approach of engaging land owners in their municipal watershed, 

changing land management practices, and monitoring watershed conditions and water quality. In 1995, 

negotiations took place around watershed protection and included representatives of EPA, New York City, New 

York State, environmental groups, local governments, and residents of the watershed. New York City protected 

their resource and created benefits for the watershed communities through a comprehensive program that includes

land acquisition, conservation easements, regulatory revisions, stream restoration, and incentives for farm- and 

forest-owners to keep land safe from development and encourage Best Management Practices that protect water 

quality (Watershed Agricultural Program and Watershed Forestry Program, respectively). The watershed 

communities benefit from economic development money, infrastructure improvements, and a healthier 

environment. New York City benefits by saving money and protecting its source water for the long-term. 

Through forward-thinking efforts and working closely with partners, New York City‟s approach saves millions of 

dollars, improves the quality of life for watershed residents, and protects and restores the environment in their 

municipal watershed ( http://www.nyc.gov/watershed, NRC 2000).

Seattle, in Washington State, gets 2/3 of its water supply from the Cedar River. After buying up the Cedar River 

watershed over the last 100+ years, Seattle owns nearly the entire 90,000 acre watershed. (The remaining 1/3 of 
40

Turbidity Analysis for Oregon Public Water Systems

the water supply comes from the Tolt River‟s 11,000 acre watershed of which Seattle owns 70%.) Active logging 

in the Cedar River watershed until the middle of the 1990‟s left numerous roads and legacy effects from 

clearcutting. Seattle built an $80 million disinfection system early in this decade, but they also needed to build a 

new $100 million filtration plant because of turbidity from roads and landslides. (The cost to build the plant 

would now be $150 million with an additional $3-5 million per year in operating costs.) Instead, Seattle Public 

Utilities (SPU) received permission to use a Limited Alternative to Filtration under SDWA, requiring watershed 

protection with an annual survey of conditions. Their comprehensive protection strategy includes roads, culverts, 

bridges, bank stabilization, security, dealing with problematic downed trees, road removal and maintenance, and 

cultural resources protection. SPU crafted the Cedar Transportation Plan to manage their 650 miles of roads and 

reduce sediment pollution. They have a road inventory for their watersheds and use WARSEM (Washington 

Road Surface Erosion Model) to find the worst road segments and estimate the cumulative effects on the 

watershed. They have decommissioned 105 miles of roads as of September, 2008, and will decommission almost 

300 miles of roads over a 10 year period. Reductions in fine sediment pollution are already apparent (Figure 10, 

graph provided by Seattle Public Utilities). Combined with the other measures, such as prohibiting public access 

to the watershed to prevent microbial contamination and vandalism, the restoration and protection measures taken 

by Seattle create significant savings in money, materials, and energy (Source: Todd Bohle, Seattle Public 

Utilities).

The City of Portland has special management and legislative protections on the US Forest Service land (the Bull 

Run watershed) that provides drinking water to the largest city in Oregon. These protections give Portland clean, 

easy-to-treat source water that is free of contamination. The Cities of Astoria and Forest Grove own their 

watershed forestlands; both are Forest Stewardship Council (FSC) certified and use FSC guidelines to manage the 

Figure 10: Reductions in the amount of fine sediment generated from roads and delivered to 

streams in Seattle‟s Cedar River watershed. These reductions are due to the decommissioning of 

the worst actors for sediment delivery.
41

Turbidity Analysis for Oregon Public Water Systems

forests in their watersheds (Forest Stewardship Council 2005). According an analysis by EPA (US EPA 2005), 

FSC guidelines are at least as effective in protecting source water quality and salmonid fisheries as EPA and 

NOAA-Fisheries guidelines. FSC certification also allows the harvested wood to be sold with a price premium 

that adds an economic benefit to FSC management (Bensel et al, in press). Other private land management 

methods (such as SFI, the Sustainable Forestry Initiative) can potentially protect source water quality, but they 

need to be monitored and evaluated to insure that they are in fact adequate. Currently, Oregon State Forest 

Management Plans, the federal Northwest Forest Plan, FSC-certified private operations, and other forest 

management schemes meeting or exceeding EPA and NOAA-Fisheries guidelines are the demonstrated means 

most likely to protect and restore source water quality and limit turbidity impacts while producing timber.

The examples of New York City and Seattle suggest that restoration and protection of source water is an effective 

means of reducing treatment costs and ensuring safe, high quality water. Source water protection is a critical part 

of the Multi-Barrier Approach to providing safe drinking water (NRC 2000, Barnes et al 2009). Roads are the

greatest source of fine sediment and landslide initiation sites (Montgomery 1994) and can be significant sources 

of toxic pollutants (Christensen et al 1997). Therefore, road removal can be an extremely effective means of 

protecting source water quality (Switalski et al 2004). When roads are needed, proper road placement and 

maintenance contribute to water quality protection (Jones et al 2000). A 30-year turbidity study of managed 

forestland in Western Washington by Reiter et al (2009) found that turbidity decreased over the life of the study 

with the implication that road improvement and sediment controls were responsible. Thinning and multi-age 

management in plantations avoids large changes in peak flows (Grant et al 2008) and increases resiliency (Bisson 

et al 2003, Brown et al 2004), leaving forests and streams less vulnerable to climate change and disturbances such 

as fires. For example, the management strategies for the Tillamook and Clatsop State Forests incorporate 

thinning and multi-age management in addition to a robust riparian protection strategy (ODF 2001). As a result, 

water quality in state forests are on a generally improving trend (Rockwell et al 2006). The Northwest Forest 

Plan Aquatic Conservation Strategy, coupled with restoration in key watersheds, is also effective in improving 

water quality (Gallo et al 2005). Boston, Massachusetts, uses active management of its Quabbin watershed to 

increase resiliency and protect water quality and reduce treatment costs (Barten et al 1998). Watershed protection 

can be a very important part of providing safe drinking water in a manner that is cost effective overall (Brown 

2000), although costs associated with protection can be a significant economic challenge for timber producers (Ice 

et al 2006). Incentives or ecosystem services payments could help producers meet economic challenges 

associated with source water protection and prevent conversion to non-forest use. The economic benefits of 

riparian protection are potentially massive (Palone & Todd 1997). New York City‟s example particularly 

suggests that voluntary measures by the owners and managers of working forests and farms can protect source 

water quality.
42

Turbidity Analysis for Oregon Public Water Systems

Potential Followup

There are several additional tasks that should be done to better understand turbidity dynamics and potential causeand-effect relationships between stream conditions and natural conditions, land use activities, and climate change. 

The first is to continue acquiring information about the issues that Public Water Systems face, especially in small 

communities. This should include turbidity, precipitation, roads, and land use data for DWSAs. Continuous 

turbidity monitoring will show patterns more fully than once-per-day measurements, as shown by the comparison 

of Falls City‟s daily data with the more detailed data from November 1st-10th, 2007. Light Ranging and Detection 

(LiDAR) data would be an extremely effective tool for evaluating landscape conditions and characteristics. The 

results of the Drinking Water State Revolving Fund Needs Survey can be used to help assess the financial 

challenges faced by PWSs. Understanding the risks posed by climate change is essential. Further collection, 

examination, and analysis of data and expanding the geographic area of analysis will enable an improved 

understanding of source water protection.

Improved turbidity monitoring and assessment of source waters is needed when consistent turbidity problems or 

identified risks are present. As needed, waterbodies with newly identified problems can be put on the 303(d) list, 

and TMDLs or other protection and restoration plans can be developed to meet the requirements of the Clean 

Water Act. DEQ plans to revisit the turbidity water quality standard in 2010. A revised turbidity standard would 

provide more effective implementation through its existing programs in NPDES permits, CWA 303(d) water 

quality assessment and impaired waters listings, 401 certifications, TMDL development, drinking water 

protection, and nonpoint source programs. Another critical part of addressing turbidity issues where risks have 

been identified is ensuring that nonpoint source programs at other agencies (e.g. Oregon Departments of 

Agriculture and Forestry) are adequately funded and staffed to ensure best management practices are being 

applied and water quality standards are being met.

In order to accurately assess sources of fine sediment that lead to elevated turbidity in Coast Range DWSAs, an 

analysis of land use patterns and disturbances and how they relate to source water turbidity is needed. A thorough 

analysis would also include landslides and significant windthrow events (if known), precipitation effects, and 

sediment contributions from roads, new and old. Washington‟s WARSEM model is one potential tool for 

modeling the effects of roads on DWSA sediment movement. Eugene Water and Electric Board have a source 

water monitoring and assessment program that provides methods which could be applied to small PWSs. Ice et al 

(2006) explore methods for evaluating the areal extent of riparian protection. Other evaluation methods and 

examples exist as well.

DEQ should provide technical assistance to PWSs whenever such assistance is wanted. In the short term, DEQ 

can assist PWSs in applying for SDWA grants for testing and data collection to enhance our knowledge of what is 

actually happening on the landscape. Since small communities typically do not have the expertise or resources to 

thoroughly examine the collected data, the Drinking Water Protection program (DWP) can assist with analysis 

and inform them of Best Management Practices that may be of use. In the long term, the DWP program can help 

communities to develop land use plans for their DWSAs that are consistent with protecting their drinking water 

for generations to come. Such plans may include land acquisition through fee title and/or conservation easements 

or incentives/ecosystem services payments to land owners if more protective management practices are found to 

be necessary. The DWP program can assist PWSs in identification of funding needs and sources, such as loans 

and grants to implement plans for source water protection or collection of fees from water system users for

ecosystem services payments. 

DEQ works closely with the Oregon Department of Human Services on drinking water issues. In addition, close

coordination with other state agencies such as Department of Forestry, Department of Land Conservation and 

Development, Department of Agriculture, and Department of Geology and Mineral Industries (DOGAMI) could 

enable the DWP to better target assistance to PWSs. DOGAMI, for example, has done excellent work recently to 
43

Turbidity Analysis for Oregon Public Water Systems

identify active landslides and landslide risk areas. EPA, NRCS, and NOAA-Fisheries are federal agencies with 

whom DEQ can increase cooperation to resolve concerns common to source water protection and implementation 

of the CWA and ESA.
44

Turbidity Analysis for Oregon Public Water Systems

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Turbidity Analysis for Oregon Public Water Systems

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49

Turbidity Analysis for Oregon Public Water Systems

Appendices

Appendix A: Public Water Systems with All or Part of 

DWSA in the Coast Range

ID # PWS Name Water Source County

Area 

(acres)

4100003 ADAIR VILLAGE WATER SYSTEM WILLAMETTE R. BENTON 234501.88

4100012 ALBANY, CITY OF S. SANTIAM CANAL LINN 7665.94

4100041 AMITY, CITY OF S. YAMHILL R. YAMHILL 17504.87

4100047 ASHLAND WATER DEPARTMENT ASHLAND CR. JACKSON 12736.58

4100055 ASTORIA, CITY OF BEAR & CEDAR CR., MIDDLE LAKE CLATSOP 2735.95

4100062

YOUNG'S RIVER-LEWIS & CLARK 

WD N. & S. FORKS BARNEY CR. CLATSOP 1899.51

4100063 WICKIUP WATER DISTRICT

LITTLE CR. & BIG/LITTLE FAT BUCK 

CR. CLATSOP 1359.47

4100074 BANDON, CITY OF GEIGER & FERRY CR. COOS 2555.48

4100124

FISHHAWK LAKE RECREATION 

CLUB FISHAWK CR. COLUMBIA 10004.88

4100164 CANNON BEACH, CITY OF W. FORK ELK CR. CLATSOP 5277.47

4100169 CANYONVILLE, CITY OF CANYON CR. DOUGLAS 22656.81

4100171 CARLTON, CITY OF PANTHER CR. YAMHILL 2075.02

4100194 CLATSKANIE, CITY OF ROARING & WEST CR. COLUMBIA 1799.90

4100199 BEAVER WATER DISTRICT BEAVER CR. TILLAMOOK 18647.16

4100205

COOS BAY-NORTH BEND WATER 

BD. PONY CR. COOS 2557.01

4100213 COQUILLE, CITY OF RINK CR. COOS 429.18

4100214

GARDEN VALLEY WTR 

ASSOCIATION CHINA CR. COOS 419.24

4100225 CORVALLIS, CITY OF

WILLAMETTE R, S.&N. FORKS ROCK 

CR, GRIFFITH CR. BENTON 47005.87

4100239 LONDON WATER CO-OP BEAVER CR. LANE 867.67

4100246 CRESWELL, CITY OF COAST FORK WILLAMETTE R. LANE 123064.39

4100248 DALLAS, CITY OF RICKREAL CR. POLK 17953.92

4100250 MILO ACADEMY S. UMPQUA R, LICKEY CR. DOUGLAS 21934.54

4100254 DEPOE BAY, CITY OF ROCKY &DEPOE BAY CR. LINCOLN 6695.81

4100260 DRAIN, CITY OF BEAR & ALAN CR. DOUGLAS 4017.17

4100276 ELKTON, CITY OF UMPQUA RIVER DOUGLAS 316072.91

4100297 FALLS CITY, CITY OF TEAL &GLAZE CR. POLK 3218.60

4100301 HECETA WATER DISTRICT CLEAR LAKE LANE 614.69

4100302 SILTCOOS HEIGHTS SILTCOOS LAKE LANE 39670.56

4100304

ALDERWOOD WATER 

DEVELOPMENT CO WOAHINK LAKE LANE 4334.51

4100305 FOREST GROVE, CITY OF

ROARING, CLEAR, THOMAS, DEEP, & 

SMITH CR. WASHINGTON 4856.58

4100323 GLENDALE, CITY OF COW, SECTION, & MILL CR. DOUGLAS 119381.66

4100324

KERNVILLE-GLENEDEN-LINCOLN 

BCH W D DRIFT CREEK LINCOLN 22229.85

4100326 GLIDE WATER ASSOCIATION NORTH UMPQUA RIVER DOUGLAS 428437.01

4100333 GOLD HILL, CITY OF ROGUE RIVER JACKSON 284024.61

4100337 ROCK CREEK HIDEOUT WATER CO UNNAMED CREEK POLK 26.74

4100342 GRANTS PASS, CITY OF ROGUE RIVER JOSEPHINE 170961.64

4100379

HILLSBORO-FOREST GROVEBEAVERTONTUALATIN R. & N. FORK TRASK R. 

(BARNEY RES.) WASHINGTON 120691.98

4100463 LAKESIDE WATER DISTRICT EEL LAKE COOS 3872.15

4100466 LANGLOIS WATER DISTRICT FLORAS CREEK CURRY 39049.91

4100483 LINCOLN CITY WATER DISTRICT SCHOONER CREEK LINCOLN 9591.86

4100497 MCMINNVILLE WATER AND LIGHT HASKINS & MCGUIRE RES. YAMHILL 6213.11

4100505

MANZANITA WATER 

DEPARTMENT N.&W. FORKS ANDERSON CR. TILLAMOOK 569.36
50

Turbidity Analysis for Oregon Public Water Systems

4100507 MAPLETON WATER DISTRICT BERKSHIRE CREEK LANE 501.69

4100513 MEDFORD WATER COMMISSION ROGUE RIVER JACKSON 290653.71

4100548

CLARKS BRANCH WTR. 

ASSOCIATION SOUTH UMPQUA RIVER DOUGLAS 84099.45

4100549 TRI-CITY WATER DISTRICT SOUTH UMPQUA RIVER DOUGLAS 106502.61

4100550 MYRTLE CREEK, CITY OF

S. UMPQUA R. & SPRINGBROOK 

SPRINGS DOUGLAS 4809.62

4100551 MYRTLE POINT, CITY OF NORTH FORK COQUILLE RIVER COOS 180885.59

4100554 NEHALEM, CITY OF BOB'S CREEK TILLAMOOK 425.21

4100556 NETARTS WATER DISTRICT EAST FALL CREEK TILLAMOOK 367.50

4100564 BAY HILLS WATER ASSOCIATION UNNAMED CREEK LINCOLN 26.21

4100566 NEWPORT, CITY OF BIG CR.&SILETZ R. LINCOLN 2150.12

4100568

BEVERLY BEACH WATER 

DISTRICT WADE CREEK LINCOLN 1460.17

4100581 OAKLAND, CITY OF CALAPOOYA CREEK DOUGLAS 64894.66

4100585 OCEANSIDE WATER DISTRICT SHORT CREEK TILLAMOOK 1304.36

4100603

PANTHER CREEK WATER 

DISTRICT PANTHER CREEK LINCOLN 1106.43

4100610

TIERRA DEL MAR WATER 

COMPANY BELTZ CREEK TILLAMOOK 160.06

4100624 PHILOMATH PUBLIC WORKS MARY'S RIVER BENTON 85972.99

4100670 PORT ORFORD, CITY OF HUBBARD CR. & GARRISON LAKE CURRY 2845.60

4100672 POWERS, CITY OF S. FORK COQUILLE R. & BINGHAM CR. COOS 94207.20

4100689 RAINIER WATER DEPARTMENT COLUMBIA R. & FOX CR. COLUMBIA 44917.84

4100699 REEDSPORT, CITY OF CLEAR LAKE DOUGLAS 1382.96

4100706 RIDDLE, CITY OF COW CREEK DOUGLAS 192495.04

4100707

LAWSON ACRES WATER 

ASSOCIATION COW CREEK DOUGLAS 7025.14

4100708 ROCKAWAY BEACH WATER DEPT JETTY CREEK TILLAMOOK 1310.52

4100712 ROGUE RIVER, CITY OF ROGUE RIVER JACKSON 69007.97

4100717 ROBERTS CREEK WATER DISTRICT S. UMPQUA RIVER DOUGLAS 3094.75

4100719

UMPQUA BASIN WATER 

ASSOCIATION N. UMPQUA RIVER DOUGLAS 33475.75

4100720

ROSEBURG, CITY OF -

WINCHESTER N. UMPQUA RIVER DOUGLAS 131134.54

4100731 SALEM PUBLIC WORKS N. SANTIAM RIVER & IG MARION 17149.09

4100792 SCAPPOOSE, CITY OF

GOURLEY, LAZY, & S.FK SCAPPOOSE 

CR COLUMBIA 6059.81

4100799 SEASIDE WATER DEPARTMENT NECANICUM R. CLATSOP 30069.02

4100802

ARCH CAPE WATER SERVICE 

DISTRICT ASBURY & SHARK CR. CLATSOP 1249.34

4100808 COUNTRY VIEW MH ESTATES ROGUE RIVER JACKSON 734031.45

4100811 SHERIDAN, CITY OF SOUTH YAMHILL RIVER YAMHILL 135375.06

4100821 SILETZ, CITY OF SILETZ R. & TANGERMAN CR. LINCOLN 131836.17

4100843 STAYTON WATER SUPPLY NORTH SANTIAM RIVER MARION 4535.87

4100847 SUTHERLIN, CITY OF CALAPOOYA & COOPER CR. DOUGLAS 57605.19

4100893 TILLAMOOK WATER COMMISSION KILLAM & FAWCETT CR. TILLAMOOK 6176.20

4100898 TIMBER WATER ASSOCIATION NEHALEM RIVER WASHINGTON 7787.26

4100899 TOLEDO WATER UTILITIES MILL CR. & SILETZ R. LINCOLN 2680.81

4100922 VERNONIA, CITY OF ROCK CREEK COLUMBIA 3561.18

4100925 S.W. LINCOLN COUNTY WTR. DIST. BIG, VINGIE, STARR, DICK‟S FORK CR. LINCOLN 3451.89

4100926 WALDPORT, CITY OF ECKMAN & WEIST CR. LINCOLN 3186.26

4100932 WARRENTON WATER SYSTEM LEWIS&CLARK R. CAMP C CR. CLATSOP 18375.81

4100950 WESTPORT WATER ASSOCIATION WEST CREEK CLATSOP 1326.04

4100952 WHEELER WATER DISTRICT VOSBURG & JARVIS CR. TILLAMOOK 365.23

4100953

CITY OF WILLAMINA WATER 

DEPARTMENT WILLAMINA CREEK YAMHILL 52478.62

4100954 WILSONVILLE, CITY OF WILLAMETTE RIVER CLACKAMAS 1050979.75

4100957

WINSTON-DILLARD WATER 

DISTRICT SOUTH UMPQUA RIVER DOUGLAS 111553.38

4100958 YONCALLA, CITY OF ADAMS & WILSON CR. DOUGLAS 1676.22

4100966 YACHATS, CITY OF SALMON & REEDY CR. LINCOLN 773.12

4100968 YAMHILL, CITY OF TURNER CREEK YAMHILL 2916.76

4100970 NESKOWIN REGIONAL WD HAWK CREEK TILLAMOOK 1542.70

4100971 CITY OF CAVE JUNCTION EAST FORK ILLINOIS RIVER JOSEPHINE 148775.61
51

Turbidity Analysis for Oregon Public Water Systems

4100985 HILLSBORO-CHERRY GROVE TUALATIN RIVER WASHINGTON 15565.00

4101012 PP&L-TOKETEE VILLAGE TOKETEE LAKE (N. UMPQUA) DOUGLAS 224207.72

4101062 RAINBOW ROCK VILLAGE MHP TAYLOR CREEK WELLS CURRY 1039.25

4101072

JOHNSON CREEK WATER SERVICE 

CO JOHNSON CREEK LINCOLN 660.07

4101092 USFS TILLER RANGER STATION SOUTH UMPQUA RIVER DOUGLAS 288525.43

4101095 USFS WOLF CREEK JOB CORPS LITTLE RIVER DOUGLAS 57801.95

4101139 MIDLAND WATER ASSOCIATION GRAHAM CREEK COLUMBIA 988.93

4101174 BUELL-RED PRAIRIE WATER ASSN GOOSENECK CREEK POLK 1057.58

4101361 RAINBOW ROCK CONDOMINIUMS UNNAMED CREEK CURRY 155.21

4101483 ANGLERS COVE/SCHWC ROGUE RIVER JACKSON 10705.24

4105246

PGE BEAVER GENERATING 

STATION COLUMBIA RIVER COLUMBIA 57081.64

4105581 WEISS ESTATES WATER SYSTEM FAHY'S LAKE COOS 1028.96

4105737 BERNDT CREEK WATER CORP ROCK CREEK COLUMBIA 35304.09

4190416

FORT JAMES OPERATING 

COMPANY COLUMBIA RIVER COLUMBIA 87190.52

4192139 TILLER ELEMENTARY, SD #15 SOUTH UMPQUA RIVER DOUGLAS 54986.13

4192152 POPE & TALBOT, INC., WILLAMETTE RIVER LANE 528459.96

4192674 USFS STAR RANGER STATION APPLEGATE RIVER JACKSON 178577.82

4194300

ROSEBURG FOREST PRODDILLARD SOUTH UMPQUA RIVER DOUGLAS 28863.00
52

Turbidity Analysis for Oregon Public Water Systems

Appendix B: Relationship of Stream Turbidity and 

Suspended Solids in the Siletz River Basin:

Analysis of Data Collected by the Confederated Tribes of Siletz 

Indians

Joshua Seeds, Nonpoint Source Analyst, Oregon DEQ

The Confederated Tribes of Siletz Indians collected winter turbidity data from numerous sites on the Siletz River 

and its tributaries in 2002 through 2006. For some of the sampling events, tribal staff also collected samples for 

measuring total suspended solids (TSS), a measure of sediment in the water column. The sites with TSS data 

include 11 tributaries and 8 sites along the mainstem of the Siletz River. As Public Water Systems collect 

turbidity data, and since DEQ wrote a report detailing turbidity in Coast Range PWSs in 2009, it is important to 

understand what is contributing to elevated turbidity levels when they are present. High turbidity is typically 

caused by fine sediment, but algae and other factors may also be responsible. We needed to establish to what 

degree fine sediment pollution is driving turbidity in Coast Range streams, so we used the available data for the 

Siletz River to test this relationship.

Analysis of Relationships between Turbidity and Total Suspended Solids

We graphed the relationships with turbidity as the independent variable (X axis) and TSS as the dependent 

variable (Y axis) for the tributaries (Figure 1) and the mainstem of the Siletz River (Figure 2).

0 50 100 150 200 250 300 350

700

600

500

400

300

200

100

0

Turbidity (NTU)

Total Suspended Solids (mg/L)

Steer C reek

Mill C reek (@ Logsden)

C edar C reek

Euchre C reek

Dewey C reek

Mill C reek (Town)

Bentilla C reek

Sam's C reek

Rock C reek

Big Rock C reek

Little Rock C reek

Linear Regressions of Turbidity vs. TSS for Siletz River Tributaries

Figure 1
53

Turbidity Analysis for Oregon Public Water Systems

0 50 100 150 200 250 300

500

400

300

200

100

0

Turbidity (NTU)

Total Suspended Solids (mg/L)

Below C edar C reek

@O jalla Bridge

@Green Bridge

@Town Bridge

@Twin Bridge

@Wildcat Bridge

@Logsden Bridge

@ Ly le Yeck's Property

Linear Regressions of Turbidity vs. TSS for Siletz River Sites

DEQ then graphed and analyzed the data using least-squares regressions to evaluate the relationship between 

turbidity and TSS in the Siletz River basin. We evaluated the combined data in three different groups: the pooled 

tributaries, the pooled mainstem sites, and both tributaries and mainstem sites (the whole system). 

To insure the validity of the least-squares regression analysis, we needed to make sure that the assumption that 

data are normally distributed was met. Least-squares regressions can be especially sensitive to outliers that skew 

the line and give a false impression of its accuracy. Histograms of turbidity and TSS for the three groups all show 

lognormal distributions for the data sets (Figures 3-8). Analysis using a Ryan-Joiner test for normality confirms 

that transforming the data with a 

base10 logarithm results in normal distributions for all the variables except TSS for the tributaries (Figure 4). 

Because the data appear lognormally distributed, the likelihood that failure of the normality test is due to a couple 

outliers, and other data transformations did not result in a normal distribution, we proceeded with the regression 

on the tributary data in addition to the other two groups. 

0 80 160 240 320 400 480 560

60

50

40

30

20

10

0

Turbidity (NTU)

Frequency

Loc 3.235

Scale 1.341

N 109

Lognormal

Histogram of Turbidity in Siletz River Tributaries

0 200 400 600 800 1000 1200

50

40

30

20

10

0

Total Suspended Solids (mg/L)

Frequency

Loc 4.348

Scale 1.171

N 109

Lognormal

Histogram of TSS in Siletz River Tributaries

Figure 2

Figure 3 Figure 4
54

Turbidity Analysis for Oregon Public Water Systems

0 200 400 600 800 1000 1200

70

60

50

40

30

20

10

0

Total Suspended Solids (mg/L)

Frequency

Loc 4.373

Scale 1.165

N 157

Lognormal

Histogram of TSS in Siletz River System

The least-squares regressions on the log transformed data for the tributaries, the mainstem, and the whole stream 

system show high correlations between turbidity and TSS (R2=81.4%, 89.9%, 82.5%, respectively; Figures 9-11). 

The accuracy of the regression is excellent for the lower and middle reaches of the river (Figure 10). Confidence 

and prediction intervals are narrow at the lower end of the turbidity and TSS ranges, but widen greatly near the 

upper end of the range, showing less certainty about values at the upper end of the ranges. Therefore, turbidity is 

a good predictor of the combined organic and mineral suspended solids in the Siletz River system, but caution 

must be used in translating turbidity values into TSS at turbidities greater than 100 or 150 NTU.

0 90 180 270 360 450 540 630

25

20

15

10

5

0

Turbidity (NTU)

Frequency

Loc 3.694

Scale 1.231

N 48

Lognormal

Histogram of Turbidity in Siletz River Sites

0 180 360 540 720 900 1080 1260

25

20

15

10

5

0

Total Suspended Solids (mg/L)

Frequency

Loc 4.431

Scale 1.163

N 48

Lognormal

Histogram of TSS in Siletz River Sites

0 100 200 300 400 500 600

80

70

60

50

40

30

20

10

0

Turbidity (NTU)

Frequency

Loc 3.376

Scale 1.322

N 157

Lognormal

Figure 7 Histogram of Turbidity in Siletz River System

Figure 5 Figure 6

Figure 8
55

Turbidity Analysis for Oregon Public Water Systems

0 50 100 150 200 250 300 350

1000

800

600

400

200

0

Turbidity (NTU)

Total Suspended Solids (mg/L)

S 0.219179

R-Sq 81.6%

R-Sq(adj) 81.4%

Regression

95% CI

95% PI

Regression of Turbidity vs. TSS for all Siletz River Tributaries

log10(TSS) = 0.7803 + 0.7885 log10(Turbidity)

0 50 100 150 200 250 300 350

1000

800

600

400

200

0

Turbidity (NTU)

Total Suspended Solids (mg/L)

S 0.160250

R-Sq 90.1%

R-Sq(adj) 89.9%

Regression

95% CI

95% PI

Regression of Turbidity vs. TSS for Siletz River Sites

log10(TSS) = 0.4855 + 0.8968 log10(Turbidity)

Figure 9

Figure 10
56

Turbidity Analysis for Oregon Public Water Systems

0 50 100 150 200 250 300 350

1000

800

600

400

200

0

Turbidity (NTU)

Total Suspended Solids (mg/L)

S 0.211880

R-Sq 82.6%

R-Sq(adj) 82.5%

Regression

95% CI

95% PI

Regression of Turbidity vs. TSS for Siletz River System

log10(TSS) = 0.7247 + 0.8012 log10(Turbidity)

Two tributaries of the Siletz River (Dewey and Bentilla 

Creeks) have much steeper slopes to their TSS vs. 

Turbidity lines, seen in Figure 1. Cedar Creek also has an 

outlier near the Dewey and Bentilla data. An analysis of 

variance on the logarithm-transformed ratio of TSS-toTurbidity showed a statistically significant difference 

among sites (F=3.13, p<0.001). Bentilla and Dewey 

Creeks do indeed have the highest mean ratios, but Little 

Rock Creek, Mill Creek (near the City of Siletz), and 

Sam‟s Creek also have higher than average TSS:Turbidity 

ratios.

ANOVA: TSS:Turbidity by Site

Source DF SS MS F P

Sites 18 13.80 0.767 3.13 0.000

Error 138 33.85 0.245

Total 156 47.66

S = 0.4953 R-Sq = 29.0% R-Sq(adj) = 19.7%

Figure 11

Yecks

Steer Cr

Sams Cr

Rock Cr

Mil Cr (Town)

Mil Cr (Logsden)

Lit le Rock Cr

Euchre Cr

Dewey Cr

Cedar Cr

Big Rock Cr

Bentil a Cr

Below Cedar

@Wildcat Br

@Twin Br

@Town Br@Ojal a Br

@Logsden Br

@Green Br

7

6

5

4

3

2

1

0

TSS:Turbidity Ratio

95% CI for the Mean

Interval Plot of TSS-to-Turbidity Ratio for Siletz River & Tributaries

Figure 12
57

Turbidity Analysis for Oregon Public Water Systems

Possible explanations for these 

differences in sediment characteristics 

among sites include soil type, 

differences in road surfacing or

density, land use differences, or a 

lower proportion of organic sediments 

in the total sediment in the stream. A 

USGS report (Madej et al 2002) 

indicates that organic sediments stay 

suspended in the water column and 

thereby contribute more to turbidity 

than mineral sediments of the same 

size. Some of the TSS samples 

collected by the tribe were analyzed 

for mineral and organic content. We 

plotted the percent of organic matter in 

TSS samples with that samples 

TSS:Turbidity ratio to see if there was 

a clear pattern, but no relationship exists (Figure 13). Soil type or other factors are therefore more likely to drive 

patterns in relationship between TSS and turbidity in the Siletz River watershed.

Conclusions

Turbidity is a useful predictor of sediment mass loads in the Siletz River and tributaries. I recommend using the 

regression equation for the whole system when estimating sediment loads(Figure 11), as the results are based on 

greater amounts of data and are thereby more robust. The equation from mainstem sites could be used on the 

middle and lower reaches of the mainstem where most of the mainstem sample sites were located (Figure 10). 

Caution should be used when applying the regression equations in the uppermost portions of the basin (above 

Gravel Creek or in the North and South Forks) because there are no TSS data for this part of the basin. Caution 

should also be used when turbidities exceed 100 NTU. Some tributaries (such as Bentilla and Dewey Creeks) 

have anomalous TSS-to-Turbidity relationships, so it may be necessary to derive individual regression equations 

for those streams.

References:

Madej, MA, M Wilzbach, K Cummins, C Ellis, and S Hadden. 2002. The contribution of suspended organic sediments to 

turbidity and sediment flux. Turbidity and Other Sediment Surrogates Workshop, Reno, NV.

Figure 13

20 30 40 50 60 70 80 90 100

10

8

6

4

2

0

%Organic Matter in TSS

TSS (mg/L):Turbidity (NTU)

Tributaries

Mainstem

Comparison of % Organic Matter in TSS and the TSS:Turbidity Ratio