Summary of Optical-Backscatter and 

Suspended-Sediment Data, 

Tomales Bay Watershed, California, 

Water Years 2004, 2005, and 2006

By Jennifer A. Curtis

Scientific Investigations Report 2007–5224

U.S. Department of the Interior

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

DIRK A. KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Curtis, J.A., 2008, Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, California, Water Years 2004, 

2005, and 2006: U.S. Geological Survey Scientific Investigations Report 2007–5224, 16 p. 
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Study Area..............................................................................................................................................3

Acknowledgments................................................................................................................................3

Methods of Data Collection and Analysis .................................................................................................3

Sampling and Analysis of Suspended-Sediment Samples............................................................4

Optical Sensor Description and Operation.......................................................................................4

Optical Sensor Calibration ..................................................................................................................5

Storm Event Analysis............................................................................................................................5

Results. .............................................................................................................................................................5

Calibration Equations ...........................................................................................................................5

Monitoring Site Data ............................................................................................................................5

Storm Events..........................................................................................................................................8

Summary........................................................................................................................................................15

References Cited..........................................................................................................................................15

Figures

Figure 1. Map showing location of Tomales Bay, Lagunitas Creek watershed, Walker Creek 

watershed, and sediment and streamflow gaging stations ........................... 2

Figure 2. Graphs showing optical-backscatter sensor calibration, (A) first sensor deployed 

November 21, 2003, through January 21, 2005, and (B) second sensor deployed 

January 21, 2005, through June 1, 2006, at Lagunitas Creek at Samuel P. Taylor 

State Park, California ..................................................................... 6

Figure 3. Graphs showing optical-backscatter sensor calibration, (A) first sensor deployed 

November 21, 2003, through January 21, 2005, and (B) second sensor deployed 

January 21, 2005, through June 1, 2006, at Walker Creek near Marshall, 

California .................................................................................... 7

Figure 4. Graphs showing instantaneous streamflow, measured suspended-sediment 

concentrations, and estimated suspended-sediment concentrations, Lagunitas 

Creek at Samuel P. Taylor State Park, California, (A) water year 2004, (B) water year 

2005, and (C) water year 2006 ............................................................ 9

Figure 5. Graphs showing instantaneous streamflow and uncalibrated/edited opticalbackscatter data, Lagunitas Creek near Point Reyes Station, California, 

(A) water year 2004, (B) water year 2005, and (C) water year 2006 .................. 10

Figure 6. Graphs showing instantaneous streamflow, measured suspended-sediment 

concentrations, and estimated suspended-sediment concentrations, Walker Creek 

near Marshall, California, (A) water year 2004, (B) water year 2005, and 

(C) water year 2006 ........................................................................ 11
iv

Figure 7. Graphs showing ratio of suspended-sediment concentration to opticalbackscatter sensor-output voltage as a function of suspended particles finer than 

0.063 millimeter, (A) first sensor deployed November 21, 2003, through January 

21, 2005, and (B) second sensor deployed January 21, 2005, through June 1, 2006, at 

Lagunitas Creek at Samuel P. Taylor State Park, California ................................. 12

Figure 8. Graphs showing ratio of suspended-sediment concentration to opticalbackscatter sensor-output voltage as a function of suspended particles finer 

than 0.063 millimeter, (A) first sensor deployed November 21, 2003, through 

January 21, 2005, and (B) second sensor deployed January 21, 2005, through 

June 1, 2006, at Walker Creek near Marshall, California ................................. 13

Tables

Table 1. Annual seasonal suspended-sediment yield at Tomales Bay watershed stream 

gaging stations during water years 2004, 2005, and 2006 .............................. 8

Table 2. Percentages of valid optical backscatter data collected at Tomales Bay 

watershed stream gaging stations during water years 2004, 2005, and 2006 ...... 8

Table 3. Storm event data for Tomales Bay watershed gaging stations during 

water years 2004, 2005, and 2006 ......................................................... 14

Appendixes: Data collected at Lagunitas Creek at Samuel P. Taylor State Park, Lagunitas 

Creek near Point Reyes Station, and Walker Creek near Marshall, California.

 Appendix A. In Excel format

 Appendix B. In Excel format
v

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Area

square mile (mi2

) 2.590 square kilometer (km2)

Volume

cubic foot (ft3

) 0.02832 cubic meter (m3)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3

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

SI to Inch/Pound

Multiply By To obtain

millimeter (mm) 0.03937 inch

Dual units are used in this report. English units are used throughout with the exception of 

suspended-sediment concentration, grain size, and optical backscatter data, which are 

presented in milligrams per liter (mg/L), millimeters (mm), and millivolts (mV).

Abbreviations

ADAPS automated data processing system

BU backscatter units

FBU formazin backscatter units

NTU nephelometric turbidity units

OBS optical backscatterance sensor

PRS Point Reyes Station

SFRWQCB San Francisco Bay Regional Water-Quality Control Board

SPT Samuel P. Taylor (State Park)

SSC suspended-sediment concentration

TMDL total maximum daily loads
This page intentionally left blank.
Abstract

The U.S. Geological Survey, in cooperation with Point 

Reyes National Seashore, is studying suspended-sediment 

transport dynamics in the two primary tributaries to Tomales 

Bay, Lagunitas Creek and Walker Creek. Suspended-sediment 

samples and continuous optical backscatter (turbidity) data 

were collected at three locations during water years 2004–06 

(October 1, 2003–September 30, 2006): at two sites in the 

Lagunitas Creek watershed and at one site in the Walker Creek 

watershed. Sediment samples were analyzed for suspendedsediment concentration, grain size, and turbidity. Data were 

used to estimate mean daily and annual seasonal suspendedsediment discharge, which were published in U.S. Geological 

Survey Annual Water-Data Reports. Data were utilized further 

in this report to develop field-based optical-backscatter calibration equations, which then were used to derive a continuous time series (15-minute interval) of suspended-sediment 

concentrations. Sensor fouling and aggradation of the channel bed occurred periodically throughout the project period, 

resulting in data loss. Although periods of data loss occurred, 

collection of optical sensor data improved our understanding 

of suspended-sediment dynamics in the Lagunitas Creek and 

Walker Creek watersheds by providing continuous time-series 

storm event data that were analyzed to determine durations of 

elevated sediment concentrations (periods of time when suspended-sediment concentration was greater than 100 mg/L).

Data derived from this project contributed baseline 

suspended-sediment transport information that will be used to 

develop and implement sediment total maximum daily loads 

for Tomales Bay and its tributary watersheds, and provides 

supporting information for additional total maximum daily 

loads (pathogens, nutrients, and mercury) and restoration 

efforts for four federally listed aquatic species that are affected 

directly by sediment loading in the Tomales Bay watershed. 

In addition, this project provided an opportunity to evaluate 

the suitability of using optical data as a surrogate for more 

traditional labor-intensive methods of measuring suspendedsediment transport in steep coastal watersheds.

Introduction

The Federal Clean Water Act requires states to identify 

impaired water bodies and to define total maximum daily 

loads (TMDLs) that a water body can receive and still meet 

water-quality criteria and attain beneficial uses. Tomales Bay 

and its two primary tributary watersheds, Lagunitas Creek 

and Walker Creek, are listed as “impaired by sediment” 

by the San Francisco Bay Regional Water Quality Control 

Board (SFRWQCB). Sediment TMDLs are scheduled for 

development by 2007 http://www.waterboards.ca.gov/tmdl/

docs/303dlists2006/final/r2_final303dlist.pdf. Data derived 

from this project provide baseline information that will be 

used to develop and implement sediment TMDLs for Tomales 

Bay and its watersheds, and to provide supporting information 

for additional TMDLs (pathogens, nutrient, and mercury) and 

restoration efforts for four federally listed aquatic species that 

are affected directly by sediment loading in the Tomales Bay 

watershed. The federally listed species include coho salmon, 

steelhead trout, California freshwater shrimp, and California 

red-legged frogs.

Data collection included daily and seasonal (October 

to May) suspended-sediment monitoring and more frequent 

(15-minute) optical-backscatter (turbidity) monitoring at two 

U.S. Geological Survey (USGS) streamflow-gaging stations: 

Lagunitas Creek at Samuel P. Taylor State Park, California 

(Lagunitas Creek SPT) and Walker Creek near Marshall, 

California, (Walker Creek). Available funding was limited 

for data collection at the Lagunitas Creek near Point Reyes 

gaging station (Lagunitas Creek PRS). Data collection at this 

site included periodic seasonal suspended-sediment sampling, 

and more frequent (15-minute) optical-backscatter monitoring. 

Lack of sufficient sediment sampling (n=14 over the 3-year 

project period) at Lagunitas Creek PRS resulted in insufficient 

data to develop a reasonable field-based calibration equation; 

thus, only the uncalibrated raw sensor-output voltages are 

available for this station. Locations of the stream-discharge 

gaging stations are shown on figure 1.

Summary of Optical-Backscatter and SuspendedSediment Data, Tomales Bay Watershed, California, Water 

Years 2004, 2005, and 2006

By Jennifer A. Curtis
Figure 1. Location of Tomales Bay, Lagunitas Creek watershed, Walker Creek watershed, and sediment and streamflow gaging 

stations.

EXPLANATION

0 5 10 Miles

0 5 10 Kilometers

Tomales

Bolinas

Marshall

Inverness

 Point Reyes 

Station

 Walker Creek 

 near Marshall 

(11460750)

123°00'00" 122°52'30" 122°45'00" 122°37'30"

37°52'30"

38°00'00"

38°07'30"

38°15'00"

Roads

Stream discharge gaging station 

 and map identifier

Towns

Lagunitas Creek watershed Stream channel

Walker Creek watershed

Tomales Bay

 Lagunitas Creek near

 Point Reyes Station

 (11460600)

 Lagunitas Creek at 

 Samuel P. Taylor State Park

 (11460400)

Pacific Ocean

Olema

11460400

 San 

Francisco

C

A

L I F

O

R

N

I A

Map

area

 Los

Angeles

2 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Although sediment discharge in Lagunitas Creek and 

Walker Creek can vary significantly due to changes in concentration and streamflow, collection and calibration of opticalbackscatter data provided a continuous time-series of concentrations under changing streamflow- and sediment-supply 

conditions. These continuous time-series sediment data are 

valuable to the TMDL process and habitat restoration efforts 

because they capture temporal variability and provide detailed 

information about the timing of sediment peaks and duration 

of elevated sediment concentrations.

Purpose and Scope

This report summarizes suspended-sediment and optical-backscatter data collected by the USGS at three gaging 

stations in Lagunitas Creek and Walker Creek during water 

years 2004–06. These data were used to determine mean 

daily, annual, and event-based suspended-sediment transport 

within the two primary tributaries to Tomales Bay. Traditional 

suspended-sediment sampling was supplemented with more 

frequent (15-minute) optical-backscatter monitoring at all 

three gages to capture the rapid temporal variations in sediment concentration and to test the use of optical backscatter as 

a sediment surrogate in this environment.

Daily mean and total seasonal suspended-sediment discharge for stations Lagunitas Creek SPT and Walker Creek are 

published in the USGS Annual Water-Data Reports (Webster 

and others, 2005; U.S. Geological Survey, 2006; 2007). This 

report describes data-collection methods and summarizes 

suspended-sediment concentration, grain size, turbidity, 

and optical-backscatter data. Optical-backscatter calibration 

curves, plots of edited and calibrated optical-backscatter data, 

and a table of storm-event data also are presented.

Study Area

Lagunitas Creek and Walker Creek are the two primary 

tributary watersheds to Tomales Bay. Land use in the 103-mi2

Lagunitas Creek watershed is mixed, with about half of the 

watershed used for public purposes such as water supply and 

for a State Park and the other half used for limited agriculture, 

grazing, and rural residential development. Land use in the 

76-mi2

 Walker Creek watershed consists primarily of privately 

owned beef and dairy operations.

Pervasive cutbank and hillslope erosion throughout the 

study area delivers sediment to mainstem channels, resulting in aggradation of lower tributary channels. Analyses 

of digitized bathymetric maps from 1861, 1931, 1957, and 

1994 indicate that the highest sedimentation rates in Tomales 

Bay occurred from 1931 to 1957 (Rooney and Smith, 1999). 

Haible (1980) documented chronic erosion (channel incision) 

of 3–15 ft in the upper reaches of Walker Creek and sedimentation (aggradation) of 3–6 ft in the lower reaches of Walker 

Creek over the period 1915–75. Comparison of topographic 

maps published between 1862 and 1976 indicates that accumulation of sediment at the Lagunitas delta was greatest from 

1861 to 1931 (Neimi and Hall, 1996). Although rates have 

slowed, erosion and sedimentation in both watersheds continues, Anima and others (1988) reports the accumulation of 10 

million ft3 of sediment [roughly more than 37,000 dump truck 

loads (at 10 yd3 each)] on the Lagunitas and Walker deltas 

after the 1981–82 flood.

Acknowledgments

This project was funded by the joint National Park 

Service—USGS Water Quality Assessment and Monitoring 

partnership program and was developed through coordination 

with Point Reyes National Seashore. Brannon Ketcham (Point 

Reyes National Seashore) and Scott Wright (USGS–Sacramento, California) were responsible for project development. 

Neil Ganju (USGS–Sacramento, California) provided insightful advice throughout the project. Reviews by Scott Wright 

and Mark Uhrich (USGS–Portland, Oregon) improved the 

original manuscript.

Methods of Data Collection and 

Analysis

Streamflow, optical-backscatter, and suspended-sediment 

data were collected and analyzed using USGS methods for 

streamflow (Rantz and others, 1982), in situ water-quality 

monitors (Wagner and others, 2006), and fluvial sediment 

(Edwards and Glysson, 1999). Daily mean streamflow and 

suspended-sediment discharge data, as well as annual minimum and maximum streamflow for the study period, are published separately in USGS Annual Water-Data Reports (Webster and others, 2005; U.S. Geological Survey, 2006; 2007). 

The published streamflow and suspended-sediment data also 

are available at http://waterdata.usgs.gov/nwis/. Streamflow, 

gage heights, edited optical-backscatter data, and estimated 

suspended-sediment concentrations (calculated from calibrated 

optical-backscatter data) are presented at 15-minute intervals 

in Appendix A (see at AppendA.xls). Suspended-sediment 

concentrations, grain-size, and turbidity data for individual 

samples are presented in Appendix B (see at AppendB.xls).

Methods of Data Collection and Analysis 3
Sampling and Analysis of Suspended-Sediment 

Samples

Depth-integrated, single-vertical, and multi-vertical 

suspended-sediment samples were collected during water 

years 2004–06 following standard USGS procedures (Edwards 

and Glysson, 1999). Samples at two gaging stations (Lagunitas 

Creek SPT and Walker Creek) were collected 2–7 days per 

week, depending on hydrologic conditions, with increased 

frequency of sampling during periods of higher streamflow. 

A total of 14 periodic samples were collected throughout the 

3-year project period at Lagunitas Creek PRS. Typically, two 

sequential suspended-sediment concentration (SSC) samples 

were collected during each visit, and additional samples were 

collected periodically throughout the water year for grain-size 

and turbidity analysis. Concentrations (SSC), grain size, and 

turbidity were measured at the USGS sediment laboratory in 

Marina, California, using methods described by Guy (1969) 

and Anderson (2005).

Optical Sensor Description and Operation

Optical backscatter has been used successfully as a surrogate for suspended-sediment concentration in low-gradient 

rivers such as the Lower Sacramento River (Schoellhamer 

and Wright, 2003) and steep mountain rivers such as the Yuba 

River (Curtis and others, 2006). Developed and tested by 

Downing and others (1981), optical backscatter sensors (OBS) 

emit infrared light into the water column, which is reflected on 

contact with suspended particles. A series of photodiodes positioned around the emitter detects the light reflectance (backscatter), and an empirical calibration is used to convert sensor 

output voltage into suspended-sediment concentration. One 

can think of the calibration as the relation between the mass 

of sediment per unit volume of water (suspended-sediment 

concentration) and the number of particles per unit volume of 

water (backscatter).

Backscatter is related inversely to particle size such that 

the sensitivity of the OBS changes with particle size by more 

than an order of magnitude; therefore, large changes in the 

particle size distribution of suspended material can be problematic when developing calibrations. For example, during a 

storm event, increasing streamflow results in the suspension 

of larger sized particles. As progressively more sand travels 

in suspension, the mass of sediment (per unit volume of water 

in suspension) increases at a different rate than the number of 

particles (per unit volume of water), resulting in a nonlinear 

calibration relation. Also, there is an upper limit to the concentrations that optical sensors can measure that may be exceeded 

in smaller, steeper creeks with flashier hydrographs. This 

complication can be addressed by decreasing the sensitivity of 

the OBS, which is accomplished by adjusting the sensor gain, 

whereby the highest output voltage (2,500 mV) corresponds to 

the maximum expected sediment concentration.

Continuously recording optical backscatter sensors 

(OBS-3) were installed at three gaging stations [Lagunitas 

Creek at Samuel P. Taylor (SPT) State Park, Lagunitas Creek 

at Point Reyes Station (PRS), and Walker Creek]. Data loggers stored output in millivolts from the optical sensors at 

15-minute intervals and recorded data were telemetered from 

the sites or downloaded during site visits. Raw sensor output 

data (in millivolts) were archived in the USGS automated 

data-processing system (ADAPS) and retrieved and edited to 

remove invalid data. The criteria for identifying and removing invalid data included: (1) small groups (one to three) of 

anomalously high data points (assumed to represent debris or 

fish passage); (2) anomalous periods of moderate-to-high sensor output with no associated changes in the streamflow record 

or concentrations of sediment samples measured in the laboratory (assumed to represent bed aggradation); (3) progressive 

increases in sensor output prior to sensor cleaning followed by 

drastic reductions after cleaning (assumed to represent fouling); and (4) data that exceeded the upper limit of concentrations that the optical sensors were capable of measuring.

Sediment samples and optical data were collected seasonally (October to May), and measurement errors were assessed 

periodically throughout the water year. Measurement errors 

can lead to data loss and may occur due to instrumentation 

or environmental effects including: electronic sensor drift, 

fouling, and changes to the morphology of the channel bed. 

Sensors were field checked for electronic drift at the beginning 

and end of each water year using a 4,000-nephelometric turbidity unit (NTU) formazin standard, which was diluted using 

deionized water into a series of calibration solutions ranging 

from 40 to 1,200 NTU. Sensors were cleaned and immersed in 

the calibration solution and the voltage output was recorded. 

Sensor drift ranged from 1 to 4 percent, which fell within 

the recommended 5 percent calibration criteria (Wagner and 

others, 2006). Sensor fouling and aggradation of the channel bed occurred periodically throughout the project period, 

resulting in data loss. Fouling, due to biological or chemical 

particles accumulating on the sensors, occurred at all three 

sites and was of concern primarily during low-flow periods. 

Aggradation of the channel bed was problematic at Lagunitas 

Creek SPT and Walker Creek, resulting in extended periods of 

invalid data. Data loss was minimized by cleaning the sensor 

one to two times per week, as flow conditions allowed, and 

by periodically adjusting the distance of the sensors above the 

channel bed.

4 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Optical Sensor Calibration

Calibration of sensor output voltage to suspended-sediment concentration can vary significantly with particle size 

and color (Conner and de Visser, 1992; Levesque and Schoelhammer, 1995; Sutherland and others, 2000). In lieu of using a 

laboratory-based calibration, the optical sensors were calibrated using field data on a site-specific basis. Therefore, the 

raw data, which represents sensor output voltages, is reported 

in millivolts (mV), as opposed to turbidity units such as backscatter units (BU) or formazin backscatter units (FBU).

Although the data collected at Lagunitas Creek PRS 

were not sufficient to develop a reasonable calibration, 

adequate data were available to develop empirical calibrations 

for Lagunitas Creek SPT and Walker Creek. OBS at these 

two sites were calibrated using SSC data from width/depthintegrated, suspended-sediment samples. The SSC and OBS 

data were nonlinear and displayed nonconstant variance of the 

residuals (heteroscedasticity). In cases such as this, Hersel and 

Hirsch (1992) recommend transforming the data (for example, 

log x, 1/x, x1/2) to achieve constant variance of the residuals (homoscedasticity) required for ordinary least-squares 

regression. Analysis of the residuals indicated that only a log 

transformation resulted in a satisfactory decrease in the variance at higher concentrations and sensor output voltages such 

that homoscedasticity could be achieved.

The SSC and associated OBS data (collected contemporaneously) were transformed to base-10 logarithmic values, 

and the log values then were used to calculate a linear leastsquares regression:

log SSC = log a + b log OBS (1)

The regression converted to power form is:

SSC = a OBS b (2)

Storm Event Analysis

The first significant runoff event at the beginning of each 

water year (first flush) and all subsequent storm events (greater 

than 200 percent of the mean daily flow) were analyzed if 

valid data were available. Mean daily streamflows at Lagunitas 

Creek SPT, Lagunitas Creek PRS, and Walker Creek were 44, 

93, and 35 ft/s, respectively. Peak streamflow and storm durations were estimated using 15-minute streamflow data (Appendix A). Suspended-sediment yield and periods of time during 

which suspended-sediment concentrations were greater than 

100 mg/L were estimated using calibrated optical backscatter 

data (Appendix A).

Results

Calibration Equations

Empirical calibrations for optical sensors, developed 

using regression analysis and used to convert sensor output 

voltage into SSC, are as follows and also are shown in 

figures 2 and 3:

Lagunitas SPT first sensor (n = 114): 

SSC = –1.229 (OBS)1.204 (3)

Lagunitas SPT second sensor (n = 249): 

SSC = –2.253 (OBS)1.507 (4)

Walker Creek first sensor (n = 94):SSC = 

–1.287 (OBS)1.287 (5)

Walker Creek second sensor (n=429):

SSC = –2.282 (OBS)1.60 (6)

Significant scatter in the calibration datasets resulted 

in moderate-to-poorly fit calibration equations such that the 

calibrations may not be reliable for actual suspended-sediment 

predictions.

The total variation in predicted suspended-sediment 

concentrations using the calibration equations is given by r2. 

The strength of the regression relation can be measured using 

the r2 values, which vary from 0.583 to 0.863 (figs. 2 and 3). 

There is a lower likelihood for predicting suspended-sediment 

concentrations closest to the true values using calibration 

equations with lower r2 values. Although the r2 values indicate 

a moderate fit for Walker Creek and the first probe at Lagunitas SPT, the assumed log-linear relation does not hold for the 

second probe at Lagunitas SPT.

Monitoring Site Data

Suspended-sediment data collected in the Tomales Bay 

watershed were used to estimate mean daily and annual 

seasonal suspended-sediment discharge for Lagunitas Creek 

SPT and Walker Creek, which were published in USGS 

Annual Water-Data Reports (Webster and others, 2005; U.S. 

Geological Survey, 2006; 2007). Annual seasonal suspendedsediment yield at Tomales Bay watershed stream-discharge 

gaging stations is shown in table 1. Although the drainage 

area at the Lagunitas Creek SPT and Walker Creek gaging 

stations are similar, 34.3 and 31.1 mi2

, respectively, the annual 

seasonal suspended-sediment yield at Walker Creek is greater. 

Throughout the 3-year project period, Walker Creek transported 61–68 percent of the combined measured suspendedsediment discharge at the two gaging stations.

Results 5
Figure 2. Optical-backscatter sensor calibration, (A) first sensor deployed November 21, 2003, through January 21, 2005, and 

(B) second sensor deployed January 21, 2005, through June 1, 2006, at Lagunitas Creek at Samuel P. Taylor State Park, California. 

SSC = -1.229 (OBS)1.204

 r 2 = 0.796

1

10

100

1,000

10,000

1

10

100

1,000

10,000

SSC = -2.253 (OBS)1.507

 r 2 = 0.583

1 10 100 1,000 10,000

OPTICAL BACKSCATTER SENSOR OUTPUT, IN MILLIVOLTS

Regression line Sample datapoints

95 percent prediction interval

A

B

EXPLANATION

SUSPENDED-SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

6 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Figure 3. Optical-backscatter sensor calibration, (A) first sensor deployed November 21, 2003, through January 21, 2005, and 

(B) second sensor deployed January 21, 2005, through June 1, 2006, at Walker Creek near Marshall, California. 

1

10

100

1,000

10,000

1

10

100

1,000

10,000

1 10 100 1,000 10,000

Regression line Sample datapoints

95 percent prediction interval

A

B

EXPLANATION

OPTICAL BACKSCATTER SENSOR OUTPUT, IN MILLIVOLTS

SUSPENDED-SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

SSC = -1.287 (OBS)1.287

 r 2 = 0.682

SSC = -2.282 (OBS)1.60 

r

2

 = 0.863

Results 7
The maximum number of OBS data points collected per 

year, when collected seasonally from October 1 to May 31, is 

23,424 data points (96 data points per day for 244 days). The 

actual number of usable data points in this study was considerably less due to invalid data. The usable percentage of valid 

optical backscatter data collected at Tomales Bay watershed 

stream gaging stations ranged from 20 to 87 percent during 

the 3-year project period (table 2). Interruptions in the OBS 

data record at all three gaging stations were caused by OBS 

measurements that exceeded the upper measurement limit, 

malfunctioning of the sensor and (or) recording equipment, 

aggradation and scour of the channel bed, and fouling of the 

sensor. The upper limit of measurable concentrations was 

violated periodically at all three gaging stations throughout the 

3-year project period primarily during peak runoff events. At 

Lagunitas SPT, the sensor and (or) the data recorder malfunctioned during water year 2005, resulting in data loss from 

October 1, 2004, through January 21, 2005. Replacement sensors capable of measuring higher concentrations (the sensor 

gain was turned up) at Lagunitas Creek SPT and Walker Creek 

were deployed on January 21, 2005. Aggradation of the channel bed periodically buried the sensors at Lagunitas Creek SPT 

during water years 2004 and 2005 and at Walker Creek during 

water year 2005, resulting in anomalously high sensor output 

for associated low measured SSC values. This especially is 

evident in the calibration data for the second sensor at Lagunitas Creek SPT (fig. 2).

Invalid data were identified and deleted resulting in 

edited datasets for each gaging station. Streamflow, estimated 

SSC time-series data, and measured SSC data from samples 

collected at Lagunitas SPT during water years 2004–06 are 

presented in figure 4A–C. Streamflow and edited (invalid 

data removed) but uncalibrated sensor output data collected at 

Lagunitas PRS during WY 2004–06 are presented in figure 5A–C. Streamflow, estimated SSC time-series data, and 

measured SSC data from samples collected at Walker Creek 

during water years 2004–06 are presented in figure 6A–C.

Grain size analysis of suspended-sediment samples collected at Lagunitas Creek SPT and Walker Creek (Appendix 

B) shows that fine-grained sediment (percent less than 0.063 

mm) dominates the suspended load. Fine-grained sediment 

represented approximately 70 percent of the suspended material collected at Lagunitas Creek SPT and Walker Creek; 

whereas, only 22 percent of the samples collected at Lagunitas 

Creek SPT and 28 percent of the samples collected at Walker 

Creek contained sand-sized particles.

Calibration of the optical-backscatter data requires a relatively stable particle-size distribution. This condition generally 

was met at Lagunitas Creek SPT, but not at Walker Creek, 

and may have contributed to scatter in the calibration datasets, 

thereby resulting in moderate-to-poorly fit calibration equations. Plots of the ratio of suspended-sediment concentration 

to sensor output voltage, as a function of suspended particles 

finer than 0.063 mm (figs. 7 and 8) show that at higher concentrations Walker Creek has greater percentages of sand in 

suspension.

Storm Events

The OBS continuous time-series data captured the rapid 

temporal variations in sediment concentration during storms 

throughout water years 2004–06. Peak streamflow, storm duration, estimated suspended-sediment yield, and duration of elevated suspended-sediment concentrations for individual storm 

events are presented in table 3. Although regression analyses 

indicate that OBS calibrations may not be reliable for actual 

suspended-sediment predictions, the OBS storm data provide 

a means with which to characterize the relative magnitude and 

duration of storms that may result in deleterious effects on atrisk aquatic species in the Tomales Bay watershed.

Table 1. Annual seasonal suspended-sediment yield at Tomales 

Bay watershed stream gaging stations during water years 2004, 

2005, and 2006.

[See figure 1 for station locations. mi2, square mile; tons/mi2, ton per square 

mile]

Gaging Station

Water Year

2004 

(tons/mi2

)

2005 

(tons/mi2)

2006 

(tons/mi2)

Lagunitas Creek SPT 

(34.3 mi2

)

140 64 1,080

Walker Creek (31.1 mi2

) 223 125 2,300

Table 2. Percentages of valid optical backscatter data collected 

at Tomales Bay watershed stream gaging stations during water 

years 2004, 2005, and 2006.

[See figure 1 for station locations]

Gaging Station Water Year

2004 2005 2006

Lagunitas Creek SPT 38% 38% 87%

Lagunitas Creek PRS 60% 81% 81%

Walker Creek 20% 79% 65%

1

Table footnote

8 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Figure 4. Instantaneous streamflow, measured suspended-sediment concentrations, and estimated suspended-sediment 

concentrations, Lagunitas Creek at Samuel P. Taylor State Park, California, (A) water year 2004, (B) water year 2005, and 

(C) water year 2006. 

1

10

100

1,000

10,000

1

10

100

1,000

10,000

1

10

100

1,000

10,000

1

10

100

1,000

10,000

10/1/2004 11/30/2004 1/29/2005 3/30/2005 5/29/2005

10/1/2005 11/30/2005 1/29/2006 3/30/2006 5/29/2006

10/1/2003 11/30/2003 1/29/2004 3/29/2004 5/28/2004

INSTANTANEOUS STREAMFLOW, IN CUBIC FEET PER SECOND

SUSPENDED-SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

B

C

A

Streamflow

Estimated SSC

Measured SSC

1

10

100

1,000

10,000

1

10

100

1,000

10,000

DATE

Results 9
Figure 5. Instantaneous streamflow and uncalibrated/edited optical-backscatter data, Lagunitas Creek near Point Reyes Station, 

California, (A) water year 2004, (B) water year 2005, and (C) water year 2006. 

10/1/2003 11/30/2003 1/29/2004 3/29/2004 5/28/2004

Streamflow

Edited OBS

10

100

1,000

10,000

1

10

100

1,000

10,000

A

INSTANTANEOUS STREAMFLOW, IN CUBIC FEET PER SECOND 

EDITED OPTICAL BACKSCATTER DATA, IN MILLIVOLTS

B

C

10/1/2004 11/30/2004 1/29/2005 3/30/2005 5/29/2005

10

100

1,000

10,000

1

10

100

1,000

10,000

10/1/2005 11/30/2005 1/29/2006 3/30/2006 5/29/2006

10

100

1,000

10,000

1

10

100

1,000

10,000

DATE

10 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Figure 6. Instantaneous streamflow, measured suspended-sediment concentrations, and estimated suspended-sediment 

concentrations, Walker Creek near Marshall, California, (A) water year 2004, (B) water year 2005, and (C) water year 2006. 

10/1/2003 11/30/2003 1/29/2004 3/29/2004 5/28/2004

10

100

1,000

10,000

1

10

100

1,000

10,000

A

INSTANTANEOUS STREAMFLOW, IN CUBIC FEET PER SECOND 

B

C

10/1/2004 11/30/2004 1/29/2005 3/30/2005 5/29/2005

1

100

1,000

10,000

1

10

100

1,000

10,000

10/1/2005 11/30/2005 1/29/2006 3/30/2006 5/29/2006

1

100

1,000

10,000

1

10

100

1,000

10,000

Streamflow

Estimated SSC

Measured SSC

1

10

10

SUSPENDED-SEDIMENT CONCENTRATION, IN MILLIGRAMS PER LITER

DATE

Results 11
Figure 7. Ratio of suspended-sediment concentration to optical-backscatter sensor-output voltage as a function of suspended 

particles finer than 0.063 millimeter, (A) first sensor deployed November 21, 2003, through January 21, 2005, and (B) second sensor 

deployed January 21, 2005, through June 1, 2006, at Lagunitas Creek at Samuel P. Taylor State Park, California. 

0

0.2

0.4

0.6

0.8

1.0

40

0

0.2

0.4

0.6

0.8

1.0

40

50 60 70 80 90 100

50 60 70 80 90 100

PERCENTAGE OF SUSPENDED-SEDIMENT PARTICLES FINER THAN 0.063 MILLIMETER

SUSPENDED-SEDIMENT CONCENTRATION/OPTICAL BACKSCATTER SENSOR OUTPUT, IN MILLIGRAMS PER LITER PER MILLIVOLTS 

B

A

12 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Figure 8. Ratio of suspended-sediment concentration to optical-backscatter sensor-output voltage as a function of suspended 

particles finer than 0.063 millimeter, (A) first sensor deployed November 21, 2003, through January 21, 2005, and (B) second sensor 

deployed January 21, 2005, through June 1, 2006, at Walker Creek near Marshall, California. 

0

0

0.4

0.8

1.2

1.6

2.0

40 50 60 70 80 90 100

0.4

0.8

1.2

1.6

2.0

40 50 60 70 80 90 100

PERCENTAGE OF SUSPENDED-SEDIMENT PARTICLES FINER THAN 0.063 MILLIMETER

B

A

SUSPENDED-SEDIMENT CONCENTRATION/OPTICAL BACKSCATTER SENSOR OUTPUT, IN MILLIGRAMS PER LITER PER MILLIVOLTS 

Results 13
Date

Lagunitas Creek SPT Lagunitas Creek PRS Walker Creek 

11460400 11460600 11460750

Peak 

Streamflow 

(ft3

/s)

Storm 

Duration 

(hours)

SuspendedSediment 

Yield 

(tons/mi2

)

Duration of 

SSC 

>100 mg/L 

(hours)

Peak 

Streamflow 

(ft3

/s)

Storm 

Duration 

(hours)

Peak 

Streamflow 

(ft3

/s)

Storm 

Duration 

(hours)

SuspendedSediment 

Yield 

(tons/mi2

)

Duration of 

SSC 

>100 mg/L 

(hours)

Water Year 2004

12/06/03 105 21 0.61 0 104 37 33 17 0.54 0

12/09/03 135 48 1.40 0 154 48 35 48 1.53 0

12/14/03 — — — — 192 34 85 34 — —

12/23/03 1,230 47 1.37 8 1,750 47 619 47 1.50 11

12/29/03 3,230 35 1.00 14 4,520 59 1,220 35 1.10 16

01/01/04 1,510 36 1.05 27 4,860 36 1,690 36 1.20 7

01/09/04 530 56 — — 935 56 224 56 1.79 0

02/02/04 747 89 — — 1,010 89 197 89 — —

02/16/04 1,320 124 3.60 40 3,120 124 1,360 91 2.90 28

02/25/04 1,170 93 — — 2,540 93 1,330 93 2.97 9

Water Year 2005

12/06/04 1,120 98 — — 1,570 98 337 98 3.13 22

12/26/04 1,770 74 — — 3,010 74 588 74 — —

12/29/04 766 149 — — 872 186 555 186 — —

01/07/05 351 81 — — 985 81 1,070 81 2.60 36

01/10/05 262 61 — — 796 61 344 61 — —

01/27/05 354 93 2.71 4 679 93 105 93 — —

02/15/05 151 44 1.28 0 263 44 53 44 — —

02/18/05 287 165 4.81 2 879 213 416 213 — —

02/26/05 558 243 7.08 4 1,190 195 471 195 — —

03/19/05 1,410 426 12.40 6 2,980 426 959 426 — —

04/08/05 308 137 3.99 0 514 137 172 137 — —

05/08/05 404 87 — — 793 87 148 87 — —

05/18/05 570 122 — — 863 122 240 122 — —

Water Year 2006

12/01/05 316 37 1.08 6 349 48 57 37 1.18 11

12/18/05 1,200 24 0.70 7 1,040 42 271 42 1.34 21

12/20/05 1,030 107 3.12 3 1,220 107 1,090 107 — —

12/25/05 1,110 36 1.00 15 3,460 54 2,440 54 1.70 53

12/30/05 7,080 103 — — 17,250 103 7,760 103 3.00 94

01/28/06 641 180 5.25 0 898 180 511 180 — —

02/27/06 864 70 2.04 10 3,020 70 1,380 70 2.20 57

03/04/06 757 106 3.09 0 2,510 106 1,060 106 — —

03/13/06 554 52 1.52 2 1,160 52 427 52 1.66 15

03/24/06 1,340 82 2.39 4 1,930 82 478 82 2.62 11

04/11/06 1,870 110 3.20 15 6,040 110 1,460 110 3.50 46

Table 3. Storm event data for Tomales Bay watershed gaging stations during water years 2004, 2005, and 2006.

[See figure 1 for station locations. ft3/s, cubic foot per second; mi2, square mile; tons/mi2; ton per square mile; —, no data; >, greater than]

14 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006
Summary

Suspended-sediment transport in the Tomales Bay 

watershed during water years 2004–06 is characterized in this 

report. The U.S. Geological Survey collected suspended-sediment concentration, grain size, turbidity, and optical backscatter data at three gaging stations: Lagunitas Creek Samuel P. 

Taylor (SPT) (station 11460400), Lagunitas Creek Point Reyes 

Station (PRS) (station 11460600), and Walker Creek (station 

11460750). Sediment samples analyzed for concentrations 

and grain size were used to calibrate optical sensor output to 

suspended-sediment concentrations at Lagunitas Creek SPT 

and Walker Creek. Insufficient sediment data were collected at 

Lagunitas Creek PRS to develop a calibration equation. These 

data will be valuable during the development and implementation of total maximum daily loads (TMDLs) for the Tomales 

Bay watershed, which is scheduled for completion in 2007, 

and for restoration efforts for four federally listed aquatic 

species that are affected directly by sediment loading in the 

Tomales Bay watershed. This project also provided an opportunity to evaluate the suitability of optical sensors in steep 

coastal watersheds, and a chance to compare optical sensors 

with daily sediment stations.

Significant optical-data loss occurred due to measurements that exceeded the sensor’s upper measurement limit, 

malfunctioning of the sensor and (or) recording equipment, 

aggradation of the channel bed, and fouling of the sensors. 

Percent valid data ranged from 20 to 87 percent during the 

3-year project period (water years 2004–06). Although periods of data loss occurred, collection of optical-sensor data 

improved our understanding of suspended-sediment dynamics in the Lagunitas Creek and Walker Creek watersheds by 

providing continuous time-series storm event data that were 

analyzed to determine durations of elevated sediment concentrations (periods of time where SSC was greater than 100 

mg/L).

Data collection is challenging in dynamic systems with 

high sediment yields, unstable channel bed conditions, and 

wide variations in the concentration and particle size of the 

suspended load. Poorly fit calibration equations decreased the 

reliability of using optical data to determine actual sediment 

discharge in this study. Improving the reliability of optical sensors as surrogates for more traditional labor-intensive methods 

of measuring suspended-sediment transport would require 

relocation of the optical sensors to more stable stream reaches 

to avoid changing channel bed elevations. Use of multiple 

optical sensors at a single site, with the gain of two sensors 

adjusted to measure average conditions (lower sensor gain) 

and peak conditions (higher sensor gain), also may significantly improve data collection. Limited funding was available 

for sediment data collection at Lagunitas Creek PRS, which 

has a relatively stable channel bed and deep gaging pool. The 

Lagunitas Creek PRS site shows the greatest potential for 

using an optical backscatter sensor (OBS) as a surrogate for 

more traditional sampling. Interruptions in the data record at 

Lagunitas Creek PRS primarily were due to biofouling; 

therefore, data loss could be improved easily at this site by 

regular cleaning and maintenance of the OBS.

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16 Summary of Optical-Backscatter and Suspended-Sediment Data, Tomales Bay Watershed, Ca., WY 2004, 2005, and 2006