Suspended Sediment and Organic Contaminants in the 

San Lorenzo River, California, Water Years 2009–2010

Open-File Report 2011-1120

U.S. Department of the Interior 

U.S. Geological Survey 

Suspended Sediment and Organic Contaminants in the 

San Lorenzo River, California, Water Years 2009–2010

By Amy E. Draut, Christopher H. Conaway, Kathy R. Echols, Curt D. Storlazzi, and Andrew Ritchie

Open-File Report 2011-1120

U.S. Department of the Interior 

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director 

U.S. Geological Survey, Reston, Virginia: 2011 

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Suggested citation:

Draut, A.E., Conaway, C.H., Echols, K.R., Storlazzi, C.D., and Ritchie, A., 2011, Suspended sediment and organic 

contaminants in the San Lorenzo River, California, water years 2009–2010: U.S. Geological Survey Open-File 

Report 2011-1120, 24 p. and data tables [http://pubs.usgs.gov/of/2011/1120/].

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted material contained within this report.
iii

Contents

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

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

Project Objectives...................................................................................................................3 

Study Site................................................................................................................................3 

Methods................................................................................................................................6 

 Suspended-Sediment Concentration and Grain Size.....................................................................6 

 Sediment-Load Calculation............................................................................................................7 

 Polycyclic Aromatic Hydrocarbon Analysis....................................................................................8 

Results ...............................................................................................................................9

Discussion........................................................................................................................................17 

 Sediment Load and Hysteresis.....................................................................................................17 

 PAH Contaminants........................................................................................................................18 

Conclusions.........................................................................................................................20 

Acknowledgments.................................................................................................................20 

References Cited..................................................................................................................21 

Figures

1. Location map of the San Lorenzo watershed, coastal California. 

2. Aerial photograph of the lower San Lorenzo River and mouth. 

3. Photographs showing sampling station in Santa Cruz. 

4. Hydrographs of the San Lorenzo River. 

5. Suspended sediment concentration and discharge for water years 2009 and 2010. 
iv

6. Suspended sediment concentration during January 2010 flood. 

7. Low molecular weight polycyclic aromatic hydrocarbons and discharge for water year 2010. 

8. High molecular weight polycyclic aromatic hydrocarbons and discharge for water year 2010. 

Tables http://pubs.usgs.gov/of/2011/1120/of2011-1120_tables/ 

1. Suspended-sediment data for water years 2009 and 2010. 

2. Sediment load calculated for water years 2009 and 2010. 

3. List of PAH compounds analyzed. 

4. Concentration of PAHs in water samples from the San Lorenzo River. 

5. Summary statistics of PAH analyses. 
Suspended Sediment and Organic Contaminants in the 

San Lorenzo River, California, Water Years 2009–2010

By Amy E. Draut1 , Christopher H. Conaway1, 2, Kathy R. Echols3, Curt D. Storlazzi1, and Andrew Ritchie1,4

Abstract 

This report presents analyses of suspended sediment and organic contaminants measured during 

a two-year study of the San Lorenzo River, central California, which discharges into the Pacific Ocean 

within the Monterey Bay National Marine Sanctuary. Most suspended-sediment transport occurred 

during flooding caused by winter storms; 55 percent of the sediment load was transported by the river 

during a three-day flood in January 2010. Concentrations of polyaromatic hydrocarbons can exceed 

regulatory criteria during high-flow events in the San Lorenzo River. These results highlight the 

importance of episodic sediment and contaminant transport in steep, mountainous, coastal watersheds 

and emphasize the importance of understanding physical processes and quantifying chemical 

constituents in discharge from coastal watersheds on event-scale terms. 

Introduction 

The San Lorenzo River drains a 350 km2 watershed in the mountains of coastal California, 

debouching into Monterey Bay at the city of Santa Cruz (fig. 1). The San Lorenzo Basin primarily 

consists of uplifted and faulted marine sedimentary rocks that reach elevations of more than 760 m, and 

 

1

U.S. Geological Survey Pacific Coastal and Marine Science Center, Santa Cruz, CA 95060 

2

Institute of Marine Sciences, University of California, Santa Cruz, CA 95060 

3

U.S. Geological Survey Columbia Environmental Research Center, Columbia, MO 65201 

4University of Washington, Seattle, WA 98195 
2

have steep, unstable slopes and erodible soils (Griggs and Paris, 1982). It is, therefore, an example of 

the class of streams with typically high sediment yield (small, steep, mountainous watersheds in 

tectonically active areas) known to contribute substantial quantities of sediment to the world’s oceans 

(Milliman and Syvitski, 1992). The San Lorenzo ecosystem includes terrestrial aquatic habitat used by 

endangered and threatened species, such as steelhead (Oncorhynchus mykiss), tidewater goby 

(Eucyclogobius newberryi), western pond turtle (Actinemys marmorata), and red-legged frog (Rana 

draytonii), and the river discharges into sensitive marine habitats of the Monterey Bay National Marine 

Sanctuary. Many parts of the river corridor and the coastal region around its mouth are used extensively 

for human habitation and recreation, and the beaches near its mouth are economically important to the 

Santa Cruz area. 

The U.S. Geological Survey (USGS) completed an event-based sampling program of the San 

Lorenzo River in Santa Cruz during water years 2009 and 2010 (October 1, 2008, through September 

30, 2010) to analyze the amount and composition of suspended sediment and to assess quantities of 

some persistent organic pollutants transported by the river and entering Monterey Bay during high-flow 

events. Because high sediment loads and organic contaminants can negatively affect coastal ecosystems, 

data from this study support ongoing and future assessments of ecosystem health in the San Lorenzo 

watershed and Monterey Bay. 
3

Figure 1. Location map showing the San Lorenzo watershed, central California. 

Project Objectives

The objectives of this study were twofold: (1) to quantify suspended-sediment concentration and 

grain size delivered by the San Lorenzo River into Monterey Bay, and (2) to measure concentrations of 

polycyclic aromatic hydrocarbons (PAHs) in the San Lorenzo River during high-flow events. Measuring 

suspended-sediment concentration and grain size supported complementary USGS investigations of 

sediment transport in shallow coastal waters offshore of Santa Cruz (Buscombe and others, 2010; Lacy 
4

and others, 2010; Rubin and others, 2010). Measurements of persistent organic pollutants in the river 

discharge are relevant to all recent and ongoing environmental and ecological studies of the San 

Lorenzo and nearshore Monterey Bay ecosystems (Copeland, 1986; Best and Griggs, 1991; Cahill, 

2006; Central Coast Long-term Environmental Assessment Network, 2007; National Marine Fisheries 

Service, 2010). 

Study Site

Rainfall events, which are concentrated in late fall, winter, and early spring in the area’s 

Mediterranean-type climate, commonly increase the San Lorenzo River flow by three to four orders of 

magnitude. Because of this hydrologic response to rain, and the opportunity for high flows to access 

potential sediment- and pollutant-source areas more readily than would low flows, water-sampling 

efforts were focused on high-flow events. Streamflow on the San Lorenzo River is monitored 

continuously by the USGS Water Resources Discipline at USGS gaging station 11161000 (San Lorenzo 

River near Santa Cruz; http://waterdata.usgs.gov/ca/nwis/uv/?site_no=11161000, last accessed 

February 20, 2011). The gaging station is approximately 4 km upstream of the river mouth (fig. 2). The 

3.5-km reach of the San Lorenzo River between U.S. Highway 1 and the river mouth is confined 

entirely by man-made levees (Griggs and Paris, 1982). 

Water samples were collected from a bridge-box sampling platform installed on a pedestrian 

bridge that spans the San Lorenzo River 2 km upstream of the river mouth (figs. 2 and 3). This sampling 

location was chosen as the downstream-most site where the sediment and contaminant content of the 

river could be assessed while involving minimal tidal influence and allowing safe personnel access. The 

sampling bridge is 200 m upstream of the confluence of ungaged Branciforte Creek and the San 

Lorenzo River (fig. 2), therefore, samples discussed in this report do not include any sediment or 
5

contaminants introduced by Branciforte Creek. For safety reasons, samples were collected only during 

daylight hours. 

Figure 2. Aerial photograph showing the lowermost 6 km of the San Lorenzo River, Santa Cruz, California, 

the locations of U.S. Geological Survey gaging station 11161000, and the sampling bridge from which water 

samples were collected for this study. The river discharges into the Pacific Ocean within the Monterey Bay 

National Marine Sanctuary. Image from Google Earth, 2009. 
6

Methods

Suspended-Sediment Concentration and Grain Size

Following standard methods, water samples for suspended-sediment analysis were collected 

using a US D-95 sampler (Edwards and Glysson, 1999; Federal Interagency Sedimentation Project, 

2000) mounted on a bridge-box platform at a fixed location above the thalweg of the San Lorenzo River 

(fig. 3). The sampler was deployed using a hand-operated reel winch to maintain a constant transit rate 

during sampling. Depth-integrated samples were collected in 1-liter bottles using a 0.25-inch-diameter 

nozzle. 

Suspended-sediment samples were analyzed at the USGS Water Resources Discipline laboratory 

in Marina, California. To analyze the concentration and the proportion of sand-sized material the 

samples were decanted after 7–10 days of settling time; excess water and organic matter were removed 

using wet suction from a vacuum pump, and the sand fraction was separated from silt and clay by 

filtering through a 0.063-mm sieve. Both fractions (sand and silt-plus-clay) of each sample were dried in 

crucibles in an oven at 103oC and then weighed. On selected samples, grain size of the sand fraction was 

analyzed in further detail. The sand fraction of each sample was separated by wet-sieving with 

deionized water into 0.125-, 0.25-, 0.5-, and 1.0-mm fractions, oven-dried, and weighed. For several 

samples collected during a large flood in January 2010, the grain size distribution of the silt-and-clay 

fraction (divisions at 0.002, 0.004, 0.008, 0.016, and 0.031 mm between clay, very fine silt, fine silt, 

medium silt, and coarse silt, respectively) was analyzed by pipette separation. 
7

Figure 3. Field photographs of A, the bridge-box sampling platform; and B, the US D-95 sampler lowered 

from the pedestrian bridge into the San Lorenzo River, Santa Cruz, California. 

Sediment-Load Calculation

A suspended-sediment rating curve was formulated by using all measurements made during 

water years 2009 and 2010. On the basis of the same measurements, the total San Lorenzo River 

sediment load was calculated using a sum of values derived from rating curves and interpolation 

between discrete sample values and 15-minute-resolution “instantaneous” discharge data from USGS 

gaging station 11161000. For the purposes of estimating sediment load, a separate suspended-sediment 

rating curve (not shown) was developed using only discharge values below 100 m3/s. Sediment load for 

discharge less than 100 m3/s was then calculated using 15-minute discharge data and the linearregression equation for that specialized rating curve, or, when samples were available, by interpolating 

between suspended-sediment concentration (SSC) in samples collected during these low-flow periods. 

Sediment loads for events with discharge greater than 100 m3

/s were calculated using instantaneous 

discharge data (assuming a 5-minute lag time between discharge at the gaging station and at the 

sampling station) and values interpolated between SSC samples collected during these high-flow 

periods. For those load calculations and for SSC values in table 1, we assumed a 15-minute lag time to 
8

account for the transit time for flows less than 28 m3/s to travel from the gaging station downstream to 

the sampling station, and we assumed a 5-minute lag time for flows above 28 m3/s. Interpolation of 

discharge to assume a 5-minute lag time between the stream gage and the sampling station during high 

flow was calculated by assuming that discharge changed in a linear fashion between 15-minute 

measurements at the gaging station. Interpolation of sediment load was done by calculating the ratio of 

SSC to discharge for samples and assuming that any change in that ratio between discrete samples 

occurred in a linear fashion with time. The product of this estimated ratio paired with the instantaneous 

discharge data was then used to estimate sediment load for discrete 15-minute intervals and summed 

into annual values. 

Polycyclic Aromatic Hydrocarbon Analysis from Whole Water Samples 

Fifteen samples were collected at the same location as noted above, with the same US D-95 

sampler for analysis of 27 polycyclic aromatic hydrocarbons (PAHs); however, specialized sampling 

and handling protocols were observed. The depth-integrated samples were collected using acid-cleaned, 

methanol-rinsed Teflon bottles and nozzles. Samples from the Teflon US D-95 bottles were transferred 

carefully to 2.3 or 4.0 L amber glass bottles that were pre-cleaned for trace organic and trace element 

sampling (I-CHEM). Samples were transported immediately to a laboratory where 30 mL of pesticidegrade dichloromethane (DCM) was added to the amber glass containers and then stored at 4o

C until the 

samples were shipped to the USGS Columbia Environmental Research Center (CERC) in Columbia, 

Missouri, for PAH extraction and instrumental analysis. 

Samples received at CERC were spiked with 100 ng of deuterated PAH surrogates (see table 3 

for list of compounds in the mix), predominately priority-pollutant PAHs. Water samples were then 

liquid-liquid extracted with three 50-mL portions of dichloromethane for approximately 1 L of sample; 
9

samples larger than 1 L had 2–3 extractions. The DCM extracts were combined for samples of more 

than 1 L and evaporated to 1 mL using a Caliper Turbovap. Following evaporation the samples went 

through a two stage cleanup: (1) flash gel permeation chromatography, followed by (2) a flash basic 

alumina column fractionation. PAHs are eluted in the second fraction off of alumina. The resulting 

fraction is evaporated under nitrogen gas and transferred to vials for full-scan gas chromatography mass 

spectrometry (GC/MS) analysis of PAHs. The final volume was 200 μL with 100 ng d14 p-terphenyl 

added as an internal standard. Three μL was injected as a cool-on column technique. The GC/MS was 

a bench-top Thermo Voyager GC Trace system. The column was a 30 mm by 0.15 mm by 0.1 μm film 

BPX-5 (SGE, Austin, Texas). The temperature program started at 100°C held for 1 minute and was 

increased by 3 degrees per minute to 338°C. Full-scan MS was done spanning 50–820 amu. Method 

blanks (n=3) showed little or no PAH background, and the process matrix blank had only a slight 

background of naphthalene and 2-methyl naphthalene. 

Results 

Water year 2009 (WY09) was a dry year for the San Lorenzo River, with discharge substantially 

below average throughout most of the year (fig. 4) and an annual runoff of 0.05 km3

 compared to an 

average annual value of 0.11 km3

 (table 2). Water year 2010 (WY10) was substantially wetter than 

WY09. A tropical storm (named Melor) caused the first flood on October 13, 2009, followed by larger 

flood peaks in January 2010; thereafter, daily streamflow followed the average 1953–2008 daily 

discharge trend line for the remainder of WY10 (fig. 4B). The peak flow during the study interval was 

359 m3

/s on January 20, 2010, during a winter storm that lasted from January 19 to 21. A flood peak of 

that magnitude has an estimated return interval of 4–5 years in the San Lorenzo watershed. The annual 

runoff for WY10 was 0.12 km3

, slightly greater than the average annual value (table 2). Suspended-
10

sediment concentration and discharge for water years 2009 and 2010 (fig. 5) show a significant, positive 

linear relationship (p<0.05, R2=0.884 from linear regression). A reverse hysteresis relationship in the 

data is apparent at the high-flow portion of the rating curve (fig. 5), representing the rising and falling 

limb of the WY 2010 peak flood event (fig. 6). 

The results for annual and major storm-event suspended-sediment loads are shown in table 2. 

The WY09 annual load was 7,236 metric tons of suspended sediment, and the annual load in WY10 was 

41,237 metric tons. Approximately 63 percent of the WY09 annual load was transported during a storm 

from February 15 to 17, 2009, that had a peak discharge of 126 m3

/s. In WY10, approximately 65 

percent of the annual load was transported during the three-day storm in January 2010 mentioned above, 

which also accounted for 55 percent of the sediment load for years WY09 and WY10 combined. 

The concentration of PAH compounds in San Lorenzo River water is shown in table 4 and 

summarized in table 5. The sum concentrations of low- and high-molecular-weight PAH compounds 

show a linear increase with stream discharge between zero and about 50 m3

/s (figs. 7 and 8). The sum 

concentration of PAHs is relatively constant between 50 and 100 m3/s (figs. 7 and 8), although this is 

somewhat obscured by large variations in naphthalene in the low molecular weight PAH data (table 4 

and fig. 7). Above a discharge of 100 m3

/s, there is high variation in the sum of PAH concentrations 

(figs. 7 and 8), including during the January 2010 major storm event. 
11

Figure 4. Hydrographs of the San Lorenzo River, Santa Cruz, California, showing discharge measured at 

USGS gaging station 11161000 (location on fig. 2). A, Daily average discharge for water years 1987 through 2010, 

the interval from which a continuous record is available for this station. Water years 2009 and 2010 are shaded 

gray. B, Daily average discharge (black line) measured during WY09 and WY10. Red dots indicate instantaneous 
12

peak discharge of the largest floods during those years. The blue dashed line shows the mean value of daily 

average discharge for each day for water years 1988 through 2009. Note the logarithmic scale on the vertical axis 

in (B). 

To generate a first-order approximation of PAH loading to Monterey Bay from the San Lorenzo 

River, we assumed a linear relationship between stream discharge and PAH concentration. The total 

concentration of low molecular weight PAHs in whole water samples shows a weak but significant 

positive correlation with discharge (p=0.04, R2

=0.15 from linear regression). Variation in naphthalene 

concentrations are the most confounding factor in the degree of correlation (R2). Using this simple 

linear-regression analysis, the WY10 annual load of low molecular weight PAHs can be estimated at 7 

kg per year. On average, 75 percent of the sum concentration of low molecular weight PAHs was made 

up of naphthalene and phenanthrene (table 4). The sum concentration of high molecular weight PAHs in 

whole water samples shows a stronger, significant positive correlation with discharge (p=0.001, R2=0.55 

from linear regression). Using this simple linear-regression analysis, the WY10 annual load of high 

molecular weight PAHs is estimated at 20 kg. The contribution of individual compounds to the high 

molecular weight PAH sum is more evenly weighted than in the low molecular weight PAH sum, with 

individual compounds contributing about 6±4 percent of the total (average value with standard 

deviation). 
13

Figure 5. Suspended-sediment concentration during high flow in the San Lorenzo River, Santa Cruz, 

California, during water years 2009 and 2010. The rating curve best-fit line, y=10.117x, and correlation coefficient, 

R2=0.8845, were calculated using all data from water years 2009 and 2010. 

The concentration of some PAH compounds in the San Lorenzo River water samples presented 

here exceeded U.S. Government water-quality criteria (table 5), specifically the California Toxics Rule 

(CTR), which applies to total recoverable PAH in water samples. The CTR is a U.S. Environmental 

Protection Agency published water-quality objective to limit concentrations of toxic substances in 

California inland waters (including enclosed bays and estuaries), including some PAHs that are known 
14

to be human health risks (U.S. Environmental Protection Agency, 2000). Median values for the 

concentration of benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, and 

benzo[a]pyrene exceeded the CTR criteria by factors of 2 to 5, and maximum concentrations of these 

compounds exceeded the CTR by factors of 10 to 50. The California Ocean Plan is water-quality control 

plan developed by the State of California for marine waters (outside of enclosed bays, estuaries, and 

coastal lagoons) that provides 30-day average concentration criteria for some PAHs known to affect 

human health adversely. The PAHs listed in the California Ocean Plan are: acenaphthylene, fluorene, 

phenanthrene, anthracene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, 

benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3-c,d]pyrene, dibenz[a,h]anthracene, and 

benzo[g,h,i]perylene (California Environmental Protection Agency, 2005). The California Ocean Plan 

requires that the sum of the 30-day average concentrations of these PAHs shall not exceed 8.8 ng/L. 

Although PAH measurements in this study represent instantaneous values in river water, and they were 

compared in this report to a 30-day average criteria for ocean water, it is notable that the sum of 

California Ocean Plan-listed PAHs (table 5) can exceed the numeric value of the water-quality criteria 

by an order of magnitude or more in individual samples. The sum concentration of California Ocean 

Plan-listed PAHs shows a significant positive correlation with discharge (p=0.0000002, R2

=0.86 from 

linear regression). Using this simple linear regression analysis, the WY10 annual load of California 

Ocean Plan listed PAHs can be estimated to be 10 kg. 

 

 
15

 

Figure 6. Suspended-sediment concentration measured during a three-day flood from January 19 to 21, 

2010, San Lorenzo River, Santa Cruz, California, showing reverse hysteresis—sediment concentrations were 

substantially higher on the falling limb of the hydrograph than on the rising limb. Sampling times are marked 

between January 20, 2010, at 1045 and 1730 PST. The flood peak of 359 m3/s occurred at approximately 1330 

PST on that day. 
16

Figure 7. Sum concentration of low molecular weight polycyclic aromatic hydrocarbons measured in the San 

Lorenzo River, Santa Cruz, California, during water year 2010. 
17

Figure 8. Sum concentration of high molecular weight polycyclic aromatic hydrocarbons measured in the 

San Lorenzo River, Santa Cruz, California, during water year 2010. 

Discussion

Sediment Load and Hysteresis

The transport of most (more than 60 percent) of the annual sediment loads during a few days in 

WY09 and WY10, and the fact that 55 percent of the total sediment load for the two-year study was 
18

accounted for in just three days of WY10, emphasizes the episodic nature of small mountain streams in 

general, and of the San Lorenzo River system in particular. A comparison of WY09 and WY10 

sediment loads to average annual values (140,000 metric tons per year; Best and Griggs, 1991) is 

somewhat misleading because such values are biased by the largest, rare floods (50–100-year return 

interval) and rarely are attained. Taken together, these estimated sediment loads emphasize the 

importance of single events and “flashy” hydrology on a multiyear time scale. 

Although it is common for flood events to have higher suspended-sediment concentrations on 

the rising limb of the flood compared to those on the falling limb, a result of decreasing sediment supply 

during the flood (for example, Topping and others, 2000), the opposite pattern (that is, reverse 

hysteresis) occurred during the largest San Lorenzo River flood of this study interval. As shown in 

figure 6, the SSC data presented here for the January 19–21, 2010, flood indicated reverse hysteresis, 

such that sediment concentrations on the flood’s falling limb were higher than those on the rising limb 

by a factor of 1.7. This may have been caused by some change in flow regime in the watershed, with 

potential causes being saturation-initiated mass wasting or increased overland flow; news reports at the 

time mentioned mudslides and flooding in the upper San Lorenzo watershed (Santa Cruz Sentinel, 

2010). Another possible explanation for the reverse hysteresis is the suspended sediment peak lagging 

the discharge peak, as described by Dinehart (1998) and specifically illustrated in his figures 19 and 20. 

Although debris flows can produce episodic, large-volume sediment discharge from intense rain events 

in the Santa Cruz Mountains (Wiezorek, 1987), none is known to have happened in the San Lorenzo 

River during the January 19–21, 2010, storm. 

PAH Contaminants

The results of this study show that the concentrations of PAH compounds in San Lorenzo River 

water are elevated during rain events, consistent with these compounds entering the river in storm water 
19

runoff. The results also are consistent with the suggestion that higher concentrations of high molecular 

weight PAHs are caused by combustion products washing off roadways (CCLEAN, 2009). In addition, 

the PAH concentrations can exceed U.S. Federal and State water-quality criteria (table 5). These 

findings confirm that the San Lorenzo River can be a substantial source of PAH compounds to the 

Monterey Bay National Marine Sanctuary, as previously indicated by monitoring studies in the region 

(CCLEAN, 2007). 

The relationship between PAH concentration and discharge (figs. 7 and 8) suggests a relatively 

predictable flux of PAH during low to moderate flows (less than 100 m3/s), punctuated by highly 

variable concentrations and fluxes during larger discharges and storm events (greater than 100 m3

/s). 

The highest PAH concentrations are associated with high flow “washing” out contaminants that likely 

had accumulated in the watershed during several years; before the 359 m3

/s flood peak in January 2010, 

a flow that high had not occurred since December 2005. Because PAHs are strongly sorbed to sediment 

particles, contaminant transport in the San Lorenzo River system, as in many Pacific coastal streams, is 

highly episodic and driven by intense rainstorms. Mixtures of PAH compounds have been shown to 

cause abnormalities in fish embryo development (Incardona and others, 2004). An exposure assessment 

for sensitive species (which is beyond the scope of this report) would require a more detailed 

consideration of the relationship among hydrologic conditions, contaminant sources, and contaminant 

concentrations; however, it is clear that timing and intensity of rainfall events are important variables in 

quantifying or modeling changes in PAH exposure to aquatic organisms. 

Concentration of PAH in nearshore waters of northern Monterey Bay have been shown to 

exceed the California Ocean Plan water-quality objective for the protection of human health, such as in 

March 2006, which was one of the rainiest periods of 2000–2010 (CCLEAN, 2009); however, PAH 

concentrations usually are below this objective value. Nevertheless, the episodic nature of PAH delivery 
20

from the San Lorenzo River to Monterey Bay shown in this report, as well as the previous 

demonstration that PAH loads from the San Lorenzo River can exceed the contribution of other rivers to 

Monterey Bay (CCLEAN, 2007), suggests that the San Lorenzo River is a substantial source of PAHs to 

Monterey Bay. Delivery of contaminant pulses to the coastal ocean from river discharge could pose an 

even greater environmental concern under future scenarios of altered hydrologic connectivity among 

landscapes, groundwater, and streams owing to climate change and increased urbanization of this and 

other watersheds (for example, Intergovernmental Panel on Climate Change, 2007; Kaushal and others, 

2008, 2010). 

Conclusions 

The data presented in this report show the episodic transport of sediment transport in a steep, 

mountainous Pacific coastal watershed and demonstrate the importance of changing mass-transport 

conditions with increasing river discharge. Further, the contaminant data suggest that PAH 

concentrations can exceed regulatory criteria during high-flow events in the San Lorenzo River. 

Implications of these findings include the importance of maintaining USGS streamflow gaging on 

Pacific coastal streams to provide long-term records of potential sediment and contaminant transport. In 

addition, the results emphasize the importance of understanding physical processes and quantifying 

chemical constituents in loads from coastal streams on event-scale terms. 

Acknowledgments 

We thank Kevin O’Toole for designing the bridge-box sampling station. We thank Larry 

Freeman, Jonathan Warrick, Jessica Lacy, Peter Swarzenski, Carl Orazio, and Dane Hardin for project 

design and discussion. Sampling assistance was provided by Jackson Currie, Jessica Lacy, Albert 
21

Stricker, and Jonathan Warrick, and field equipment repairs were performed or assisted by Peter Dal 

Ferro, Peter Harkins, and Cordell Johnson. Allan Mlodnosky from the USGS California Water Science 

Center Sediment Laboratory supervised and performed suspended-sediment concentration and grainsize analyses. Kevin Feltz and Shannon Earhart from the USGS Columbia Environmental Research 

Center performed the PAH extraction and analysis. Reviewers Peter Swarzenski and Jonathan Warrick 

contributed numerous excellent suggestions and a timely review of our work. We thank the City of 

Santa Cruz for site access to the San Lorenzo River pedestrian bridge. Funding was provided by the 

USGS Mendenhall Posdoctoral Research Fellowship to C.H. Conaway. 

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