U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2013–3023

April 2013

Sources of Fine-Grained Sediment to 

the Coast

The majority of the fine-grained sediment that enters and circulates through the 

coastal systems of California is part of the 

natural processes of erosion and transport 

associated with the geologic cycle that has 

been active for millions of years. In California, rivers are the dominant source of finegrained sediment that enters the ocean, with 

an average annual discharge of approximately 

34 million tons. The erosion of coastal cliffs 

and bluffs contributes roughly 3 million tons 

of fine-grained sediment per year. These 

natural supplies vastly exceed the combined 

sediment contributions from human activities, 

which include damming of rivers, emplacement of harbors, hydraulic mining, watershed 

land-use changes, and movement and management of coastal sediment. The construction of dams on California rivers has had 

perhaps the greatest impact on fine-grained 

sediment transport, resulting in a dramatic 

reduction in sediment supply to the coast—on 

the order of 25 to 50 percent since the 1960s.

Fine-grained sediment (silt and clay) enters coastal waters from rivers, eroding coastal 

bluffs, resuspension of seabed sediment, and human activities such as dredging and 

beach nourishment. The amount of sediment in coastal waters is an important factor in 

ocean ecosystem health, but little information exists on both the natural and human-driven 

magnitudes of fine-grained sediment delivery to the coastal zone, its residence time there, and 

its transport out of the system—information upon which to base environmental assessments. 

To help fill these information gaps, the U.S. Geological Survey has partnered with Federal, 

State, and local agencies to monitor fine-grained sediment dispersal patterns and fate in the 

coastal regions of California. Results of these studies suggest that the waves and currents 

of many of the nearshore coastal settings of California are adequately energetic to transport 

fine-grained sediment quickly through coastal systems. These findings will help with the 

management and regulation of fine-grained sediment along the U.S. west coast. 

Aerial-imagery mosaic 

showing sediment 

plumes near the mouth 

of the Tijuana River, 

south of Imperial 

Beach in southern 

California. Photos were 

taken during a U.S. 

Geological Surveyled demonstration 

project that tracked 

the dispersal of finegrained sediment from 

a beach-nourishment 

project to its eventual 

deposition on the 

seafloor. Imagery 

collected for the USGS 

by Ocean Imaging Corp.

Tijuana

River

Oneonta

Fine-Grained Sediment and 

Coastal Ecosystems 

Once discharged into the ocean, 

fine-grained sediment can have 

significant and important effects on 

coastal ecosystems, both negative and 

positive. Turbidity and sedimentation 

may reduce photosynthesis, decrease 

visibility for visual feeders, and bury 

sea-floor habitats. But fine-grained 

sediment is also an important source 

of nutrients to estuarine ecosystems, 

the continental shelf, and the 

California Current. Additionally, the 

deposition of fine-grained sediment 

along coasts may act as a mitigating 

factor to sea-level rise. These 

competing concepts of the potential 

effects of fine-grained sediment 

on California coastal ecosystems 

emphasizes the need for a better 

understanding of the pathways and 

physical processes of fine-grained 

sediment delivery, residence times, 

and dispersal in coastal ecosystems. 

Pacific Ocean

Los Angeles Los Angeles

Left, a satellite image showing river-sediment plumes off the southern California coast near 

Los Angeles. Image courtesy of NASA (National Aeronautics and Space Administration). 

Right, seafloor habitats near Santa Cruz, California. Photo by Curt Storlazzi. 

500 

METERS

1,000

FEET

Slough

Tijuana 

Estuary

Pacific 

Ocean

California

Fine-Grained Sediment Dispersal Along the California Coast
The Santa Cruz 

Harbor Demonstration 

Project was conducted 

in 2009 adjacent to 

the mouth of the Santa 

Cruz Harbor in central 

California. This project 

utilized over 7,600 

cubic meters (~10,000 

cubic yards) of sediment 

from Arana Gulch that 

was approximately 70 

percent fine-grained by 

mass. This sediment was 

placed about 60 meters 

offshore of the harbor 

mouth in 2-meter water 

depth over an interval 

of two and a half weeks. 

Sediment was observed 

to disperse quickly from 

the placement site on 

the inner continental 

shelf and ultimately was 

transported offshore 

towards the midshelf 

mud belt, and there 

did not appear to be 

significant net deposition 

on the seabed in or near 

the project site. 

Santa Cruz Harbor Demonstration Project

Left, USGS scientists in Santa 

Cruz Harbor retrieve a grain-sizemeasuring instrument, which uses 

a laser to determine the size and 

concentration of sediment particles 

in the water column. 

Right, map of suspened 

sediment concentration (in 

grams per liter), calculated by a 

model over an 18-hour period.

Above, sediment discharged by the San Lorenzo River (less than 1 km 

upcoast from the Santa Cruz Harbor) into the Monterey Bay National 

Marine Sanctuary following a storm. Findings regarding sediment 

dispersal and deposition from dredge disposal projects need to be put into 

the context of such natural processes. 

Without a good understanding of these 

pathways and processes, it will be 

impossible to predict the potential effects 

of natural or human changes to the 

supplies of sediment to coastal waters. 

Demonstration Projects 

Two recent demonstration projects, 

conducted by the Pacific Coastal and 

Marine Science Center of the U.S. Geological Survey (USGS), with other Federal, 

State, and local agencies, characterized the 

dispersal of fine-grained sediment in California coastal waters. These projects, near 

the mouth of the Tijuana River near San 

Diego and in Santa Cruz Harbor, focused 

on the pathways and rates of transport of 

fine-grained sediment from dredging and 

beach nourishment. Both projects utilized 

sediment with greater amounts of finegrained sediment particles—40 and 70 

percent by weight—than are traditionally 

allowed by regulatory agencies during 

these types of activities. The goals of the 

two projects were to describe the patterns 

of water turbidity and sedimentation of the 

sea floor caused by these sediment sources 

and to evaluate the effects of environmental parameters like waves, currents, and 

seabed morphology on these sedimentation 

patterns. A host of sampling techniques 

were utilized, including water and sediment 

sampling by traditional physical techniques, 

use of optical and acoustic technologies to 

sample water and seabed-sediment properties, and remote sensing to characterize 

turbidity plumes. 

The general findings of these two 

projects were similar even though the 

studies were conducted in different coastal 

settings. Fine-grained sediment was found 

to disperse quickly from the release sites 

and move alongshore and cross-shore 

to locations away from nearshore rocky 

habitats. Although turbidity could be 

extensive during sediment placement, 

it dissipated within hours to days. 

Accumulation of fine-grained sediment on 

the seabed was negligible in the shallow 

waters around both project sites. The 

pathways and rates of sediment movement 

were related to the size and direction of the 

waves, strength and direction of the coastal 

currents, and vertical settling properties 

of the sediment. Details of these findings 

and data collected for these projects are 

available in published reports (see the 

“Further Reading” section). 

USGS Role

The USGS led the development and 

implementation of the science plans for both 

the Tijuana River and Santa Cruz Harbor 

demonstration projects, and USGS scientists 

worked closely with project partners to 

design monitoring plans and led all project 

sampling. The USGS has been solely 

responsible for the data collection, analysis, 

and publication, and USGS scientists are the 

primary authors of project results. 

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November 5, 2009

Sediment concentration (g/L)

Santa Cruz

Harbor

Arana Gulch

San

Lorenzo

River

Pacific Ocean
The Tijuana River Demonstration 

Project was conducted in 2008 and 

2009 between the mouth of the Tijuana 

River, California, and the U.S.-Mexico 

international border. This project utilized 

over 34,000 cubic meters (over 40,000 

cubic yards) of sediment from a debris 

basin within Border Field State Park that 

consisted of approximately 40 percent 

fine-grained sediment by mass. This 

sediment was placed directly within the 

intertidal sandy beach over an interval of 

several weeks. It was observed to disperse 

quickly in nearshore waters but was 

strongly influenced by surf zone currents, 

which delivered sediment more than 1 

kilometer (0.6 miles) up- and downcoast 

from the emplacement site. Sedimentation 

in the project area was limited to a zone 

immediately within and offshore of the 

project site, and the majority of sediment 

moved far offshore of the project site in 

response to waves and currents.

A Few Key Questions about Fine-Grained Sediment

•	What are the pathways and processes of fine-grained sediment

in coastal waters?

•	How does coastal setting influence these pathways and

processes?

•	What are the coastal-ecosystem effects of fine-grained

sediment?

•	Does sediment released by natural processes disperse in

fundamentally different ways than sediment released by human

activities?

•	How do the impacts of fine-grained sediment discharged to the

coastal ocean by humans compare to the impacts of fine-grained

sediment delivered by natural processes such as river discharge

and coastal erosion?

Left, a series of water samples retrieved near the mouth of 

the Tijuana River. Darker bottles contain more sediment, and 

samples like these allowed scientists to map patterns of sediment 

concentration along the coast. Far left, a dump truck adds sediment 

to the beach near the mouth of the Tijuana River as part of the 

demonstration project. 

Above, scientists deploy a tripod with instruments to measure the effects of demonstrationproject sediment and environmental conditions on ocean water turbidity. 

Top, mouth of San Lorenzo River and Santa Cruz Beach 

Boardwalk. Right, Santa Cruz West Breakwater Light 

and Yacht Harbor. Photos by Kenneth and Gabrielle 

Adelman, copyright 2002–2013, California Coastal 

Records project, www.CaliforniaCoastline.org. 

Tijuana River Demonstration Project
Implementing the Findings

For monitoring these demonstration 

projects, the USGS developed new tools, 

field methods, and analytical techniques 

that will be applicable to similar studies 

elsewhere. As part of these efforts, threedimensional physics-based numerical 

models of sediment movement developed 

by Deltares and the USGS were used to 

evaluate the applicability and transferability 

of the results. The models adequately 

described directions and rates of fine-grained 

sediment movement both at the mouth of 

the Tijuana River and in Santa Cruz Harbor 

as long as the dominant processes of waves 

and currents were included in the models 

and as long as field data on these processes 

were collected during the demonstration 

projects to calibrate and validate the 

models. The field data were also paired 

with historical climate data to determine if 

the short-term measurements made during 

and after sediment emplacement accurately 

characterized the dynamics of the study site. 

These findings suggest that future projects 

to manage and beneficially reuse sediment 

along the coast of California can utilize these 

integrated field, laboratory, and numericalmodeling techniques to predict sediment 

movement and environmental effects under a 

wide range of coastal conditions and settings. 

This should allow for better and more 

efficient sediment management by Federal, 

state, and local agencies along the broader 

U.S. west coast.

For More Information Contact:

Chief Scientist, USGS Pacific Coastal and Marine 

Science Center

400 Natural Bridges Dr., Santa Cruz, CA 95060

E-mail: pcmsc_chief@usgs.gov (831) 460-7401

http://walrus.wr.usgs.gov/index.html

This Fact Sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2013/3023/

Further Reading

Cronin, K., van Ormondt, M., Storlazzi, 

C.D., Presto, M.K., and Tonnon, P.K., 

2011, Nearshore disposal of finegrained sediment in a high-energy 

environment–Santa Cruz Harbor case 

study: Coastal Sediments, v. 1, 

p. 616–627.

Farnsworth, K.L., and Warrick, J.A., 

2008, Sources, dispersal, and fate 

of fine-grained sediment for coastal 

California: U.S. Geological Survey 

Scientific Investigations Report 

2007–5254, 86 p. Available at http://

pubs.usgs.gov/sir/2007/5254/.

Lescinski, J., Van Ormondt, M., 

and Warrick, J.A., 2011, Beach 

nourishment with a high percentage 

of fine sediments—A modeling 

study of the Tijuana Fate and 

Transport Project, California, in

Kraus, N.C., and McDougal, W.G., 

eds., Proceedings of the Seventh 

International Symposium on Coastal 

Engineering and Science of Coastal 

Sediment Processes, v. 1, p. 43–56.

Storlazzi, C.D., and 9 others, 2011, 

The dynamics of fine-grain sediment 

dredged from Santa Cruz Harbor: U.S. 

Geological Survey Open-File Report 

2011–1045, 110 p. Available at http://

pubs.usgs.gov/of/2011/1045/.

Project Research Partners 

Border Field State Park

California Coastal Conservancy

California Coastal Sediment Management Workgroup 

California Department of Boating and Waterways

California Ocean Protection Council

Deltares 

Monterey Bay National Marine Sanctuary 

Ocean Imaging, Inc. 

Santa Cruz Port District

Scripps Institute of Oceanography

Southwest Wetlands Interpretive Association

Tijuana River Estuarine Research Reserve

U.S. Army Corps of Engineers

University of California, San Diego

University of California, Santa Cruz

Warrick, J.A., and 7 others, 2012, 

Observations of coastal sediment 

dynamics of the Tijuana Fine Sediment 

Fate and Transport Demonstration 

Project, Imperial Beach, California: 

U.S. Geological Survey Open-File 

Report 2012–1083, 29 p. and data files. 

Available at http://pubs.usgs.gov/

of/2012/1083/.

Warrick, J.A., 2013, Dispersal of fine

sediment in nearshore coastal waters: 

Journal of Coastal Research, v. 29, no.3. 

Jonathan A. Warrick

Curt D. Storlazzi 

Edited by Claire M. Landowski

Layout by Jeanne S. DiLeo