Under Bay—A New View of the Floor of West-Central

Suisun This poster features a new Bay 2. Dredged Sediment: Where to put it? San Pablo view of the floor of west- Bay Oblique view of three rock Tiburon central San Francisco Bay Sand and mud must be dredged from parts of San Francisco Bay and surrounding harbors to keep channels and berths deep enough for pinnacles on the floor of San Peninsula Angel Island —the deep part of the bay, ships and other vessels. This dredging has gone on since the first shipping channel was created in the 1850’s. Some areas are dredged Francisco Bay that could Harding Shag Rock pose a threat to deep-draft where ocean-going vessels every few years, others every year, depending on how fast sediment builds up. Rock CALIFO Area Dredging keeps harbors and waterways open but also creates the problem of where to put the dredged sediment, which not only takes vessels (see text below). of poster San traverse the shipping lanes Heights are exaggerated in Francisco up space but commonly contains pollutants such as droplets of oil and paint chips from ships’ hulls. Strict standards set by the Arch Rock San Francisco and strong tidal currents this image, so that summits RNIA Bay Environmental Protection Agency (EPA) are used to determine which sediment gets dumped where. The most contaminated sediment is Pacific Ocean appear higher and slopes sculpt the bay floor. The data dumped on land. Cleaner sediment is dumped at sites in the open ocean and in the bay. used to create it provide the steeper than they really are. 0 10 20 Kilometers One disposal site in San Francisco Bay—currently in limited use—is just south of Alcatraz Island. Before dumping began there in This vertical exaggeration 0 10 20 Miles first look at such a large area 1894, the site was a hole on the bay floor 50 meters (165 feet) deep. By the mid-1980’s, the dumped sediment (more than 6 million cubic helps viewers see subtle var- of the bay floor in such detail. meters, or 8 million cubic yards) had formed a mound that rose to within 10 meters (33 feet) of sea level and began to pose a threat to iations in the topography of deep-draft vessels (see images below). The mound has also raised questions that apply to any sediment dumped in the bay: the bay floor. A reference • Will the sediment be eroded? mesh has been added to pro- This new view of the floor of San Francisco Bay was made from data gathered by the U.S. • Will currents carry the sediment to other parts of the bay? vide a scale for measuring Geological Survey (USGS) and the National Oceanic and Atmospheric Administration (NOAA) • Will pollutants in the sediment harm bay ecosystems? distance. using a state-of-the-art mapping system (see “High-Resolution Multibeam Mapping,” below). The data used to create this poster will help identify sites where dredged sediment will stay put and not drift back into channels or Outline of rocks shallower The new data from the floor of San Francisco Bay are helping to answer challenging sensitive bay ecosystems. than 17 meters (55 feet) questions, such as: Reference mesh of Some rocks on San Francisco Bay floor 50-yd (46 meter) squares 1. How much sediment is on the bay floor, and how do water that pose hazards to navigation Vertical exaggeration: 10x currents move the sediment? Alcatraz 1894 Alcatraz 1997 2. Where can sediment dredged from harbors and shipping Island Island channels be safely disposed of? 3. How much rock must be trimmed from pinnacles on the 3. Rock Pinnacles: How much to remove for safer shipping? bay floor to ensure that ships do not run aground on them? West-central San Francisco Bay is a geographic bottleneck that ships must navigate on their way This poster describes the ways in which the USGS has started answering these questions. to ports, harbors, and waterways in the bay area (see map below). This part of the bay has several N N rock pinnacles that rise from the bay floor to within 11 meters (36 feet) of the water surface. Of the Outline of Alcatraz disposal site Outline of Alcatraz thousands of ocean-going vessels that travel yearly through the west-central bay, more than 200 have Reference mesh of disposal site drafts in excess of 12 meters (39 feet). When loaded, the bottoms of these deep-draft vessels are 100-yard (91meter) Reference mesh of squares 100-yard (91meter) deeper than the tops of the rock pinnacles. 1. Sediment on the Floor of San Francisco Bay: squares Vertical exaggeration: Taking Harding Rock as an example (see image above), USGS scientists estimate that Reconstruction of San Francisco Bay Underwater: 10x San Francisco Bay Vertical exaggeration: approximately 38,000 cubic meters (50,000 cubic yards) of rock would have to be removed to lower Island above sea level Where is it and how does it move? floor of 1894. Maximum depth is floor near Alcatraz Underwater: 10x (brown): 2x Island above sea level the pinnacle to a depth of 17 meters (55 feet). Shag Rock and Arch Rock pose similar hazards. 50 meters (165 feet). disposal site. (brown): 2x Currently under study is a plan to lower rock pinnacles in the bay in order to promote safe ship travel Understanding sediment on the floor of San Francisco Bay—how much is present and where and prevent oil spills that might occur if tankers or other vessels hit the rocks. and how fast it is moved by currents—is central to understanding virtually every other issue Oblique views of the floor of San Francisco Bay south of Alcatraz Island. On the left is a view of the bay floor in 1894, reconstructed from sparse affecting the bay. USGS scientists have combined new water-depth data from the west-central bay historical data. On the right is a view of the bay floor in 1997, constructed from the new USGS data. Dredged sediment dumped since 1894 has turned a depression in the bay floor into a mound that could pose a threat to deep-draft vessels. Heights are exaggerated in these images, so that with data used to measure depth to bedrock in the bay during the 1960’s. The result is the summits appear higher and slopes steeper than they really are. This vertical exaggeration helps viewers see subtle variations in the topography sediment-thickness map shown below. Most maps of the bay floor are bathymetric maps—they of the bay floor. A reference mesh has been added to provide a scale for measuring distance. show the depth from the water surface to the bay floor. This sediment-thickness map shows the depth from the bay floor to bedrock, also called “basement.” The map shows that more than 90 0.5 0 0.5 Kilometer meters (295 feet) of sediment underlies some parts of the bay floor. In other areas, sediment is 0.50 0.5 Mile absent and bedrock is exposed. Tiburon Peninsula Tiburon Sediment Thickness Based on Sediment Thickness Area 1 100 meters Peninsula 1967-1968 Acoustic Profiling and 90 Three-Dimensional Image of 80 70 1997 USGS Bathymetry 60 50 Tiburon Peninsula 30 40 Angel 0 20 30 1 the Floor of West-Central 0 20 0 0 4 40 1 20 Island 3 10 0 5 600

60 0 7 50 Rocks near Alcatraz San Francisco Bay Sausalito Island shallower 40 20 0 than 15 meters 2 3 Angel 0

3 40 Sausalito 0 Island 0 60 5

10 5 0 20

0

4 30 30 Harding Rock 0 3 Shag 40 NE 30 5 70 Rock 0 1 0 0 20 4

7 (2-way) Arch 0 6 0 0 Deep-water Traffic 3 Precautionary 6 Rock 0 c 80 450 0 0 Area 40 3 3 Westbound Traffi 0 5060 30 40 20 20 0 50 6 1 30 0 50 40 Alcatraz Island 4 Harding Rock 10 3 0 0 0 7 1 0 Shag Rock 60 50 Alcatraz 30 50 Eastbound Traffic 40 0 Dump Site 2 Blossom 20 30 Alcatra 3 Arch 0 0

0 Rock 3 80 20 6 10 0 0 9 5 0 Rock 10 4 30 0 7 0

3 2 0 Alcatraz z Golden

1 60 Island30 20 0 Gate 20 30 10 0 5 1 40 0 San Francisco 50 60 0 4 Blossom 1 40 0 0 5 0 60 0 6 Rock 4 0 0 4 0 5 40 3 7 View of the floor of west-central San Francisco Bay with shipping lanes (approximate) 0 0 0 4 3 3 5 0 0 40 superimposed. Shipping lanes were approximated from U.S. National Oceanic and Fort SW Atmospheric Administration National Ocean Survey Chart 18649, “San Francisco Golden Point Gate 0.5 0 0.5 Kilometer Entrance,” 48th edition, 1981. Bridge San Francisco 0.5 0 0.5 Mile

SW NE High-resolution seafloor maps support studies like those described here and (Fort Harding (Angel Point) Rock Island) many others, including studies of offshore earthquake faults, seafloor habitats of Sea Level 0 fish and shell fish, and saltwater intrusion into coastal aquifers. Making such maps 50 SEDIMENT BASEMENT V.E. = 5x is part of the work of the U.S. Geological Survey’s Coastal and Marine Geology Depth (m) 100 01000 2000 3000 4000 5000 6000 Program, which conducts scientific research along coastlines and in Distance (m) adjoining ocean waters. USGS research aims to reduce the risk from coastal Map showing thickness of sediment on the floor of west-central San Francisco Bay (see text). Most of the geologic hazards and to support the sound management of the Nation’s coasts and sediment in the bay is sand, silt, or mud. It rests on solid rock, called “bedrock” or “basement.” Vertical profile seas. To learn more about the program, visit http://marine.usgs.gov/. below map runs from Fort Point through Harding Rock to Angel Island, along arc shown on map.

Currents have shaped much of the sediment on the bay floor into sand waves, which are Island underwater sand dunes. Some of the largest sand waves—for example, east of Tiburon Peninsula For More Information: (see Area 1 on 3-dimensional image at right)—are up to 3 meters (10 feet) high and 30 to 40 http://TerraWeb.wr.usgs.gov/projects/SFBaySonar/centralbay.html meters (100 to 130 feet) from crest to crest. Strong tidal currents form and move these sand “San Francisco Bay Multibeam Bathymetry Data: West-Central Bay Area” waves. The waves’ asymmetric shapes reveal the direction of sand movement—currents sweep sand up the gently sloping side of a sand wave, and the sand tumbles down the steep side. http://walrus.wr.usgs.gov/outreach/sfbay/ “Sediment and Pollutant Transport in Central San Francisco Bay”

http://sfbay.wr.usgs.gov/access/mapping/multibeam/ “Multibeam Data and Socio-Economic Issues in West-Central San Francisco Bay” AngelAngel Island Island USGS Open-File Report 98-139

http://walrus.wr.usgs.gov/pacmaps/ “Pacific Sea Floor Mapping Project”

http://walrus.wr.usgs.gov/seds/ “USGS Bedform Sedimentology Site”

Bathymetry and selected perspective views of central San Francisco Bay, , 2000, by James V. Gardner, Peter Dartnell, Larry A. Mayer, John E. Hughes Clarke, and J. Christopher Stone: U.S. Geological Survey Water-Resources Investigations Report 00-4164, 2 map sheets.

Contributors: David A. Cacchione, Paul R. Carlson, Pat F. Chavez, Jr., John L. Chin, Peter Dartnell, James V. Gardner, Helen Gibbons, David M. Rubin, Miguel Velasco, and Florence L. Wong

Editors: James W. Hendley II and Peter H. Stauffer

Graphic Design: Sara Boore, Susan Mayfield, and Stephen Scott

TTiiburonburon Peninsula Peninsula Cooperating Agencies: Use 3-D glasses, red lens over left eye, to view this 3-dimensional image of the floor of west-central San Francisco Bay. In this image, the San Francisco Bay floor data have been combined National Oceanic and Atmospheric Administration Oblique view of sand waves between Angel Island and Tiburon Peninsula (from Area 2 on 3-dimensional with on-land data from a Panchromatic image taken on December 25, 1996, by the IRS-IC (Indian Remote-Sensing-IC) satellite. Both the bay-floor data and the on-land data are presented in U.S. Army Corps of Engineers image at right). The most recent movement of the sand waves was from right to left, or to the northeast, shaded-relief and stereoscopic (3-dimensional) view. Slopes look steeper than they are really are; vertical exaggeration is approximately 4x. (IRS-IC Panchromatic data used with permission, San Francisco Bay Harbor Safety Committee as shown by the asymmetry of the waves. © Space Imaging.)

High-Resolution Multibeam Mapping

High-resolution multibeam mapping is a variation on echo sounding, a Since its introduction in the early 20th century, the use of sound technique oceanographers have used since the 1920’s to measure water for seafloor mapping has become increasingly sophisticated. The depths. In the simplest form of echo sounding, a pulse of sound sent from a mapping system that collected the data used to create this poster is a ship travels to the seafloor and bounces back to a receiver on the ship. The multibeam echo sounder. Mounted on a ship’s hull, it sends out time lapse between sending the sound pulse and receiving its echo is used to approximately 3 pulses of sound per second. Returning echoes are calculate water depth. received by 60 narrow receive apertures, each of which records data from a precisely located point on the seafloor. The time an echo takes to return to the system is used to calculate depth to the seafloor, and the depths are combined to make a bathymetric map (see first image to the right). The strength of the returning echoes—called acoustic backscatter—provides information about what types of material are on the seafloor. Hard rock reflects more sound back to the ship than soft mud, for example, and the near sides of sand waves return more sound than the far sides. The second image to the right is an acoustic-backscatter map. This poster highlights bathymetric data. The data used to create the poster have a horizontal resolution of 4 meters (13 feet) and a depth resolution of 30 centimeters (12 inches). That means that any Sand waves, or underwater sand dunes, on the floor of San Francisco Bay east of the Tiburon Peninsula (see Area 1 on 3- A sunken vessel can be seen in this view of the floor of San Francisco Bay object larger than 4 meters long by 4 meters wide and 30 dimensional image, above). On the left is a shaded-relief image of the area created from bathymetric data. On the right is an northeast of Angel Island. The presence of this object was already known to centimeters high will be revealed (for example, see image of sunken image of the area created from acoustic-backscatter data. Bathymetric images can be manipulated to make them look much the U.S. Coast Guard, but its identity has not been determined. This image was vessel at far right). like photographs—by adding shadows, for example, as in the left image. Acoustic-backscatter images are harder to interpret, created from the recently collected bathymetric data (for the bay floor) and but they have the potential to tell scientists a great deal about the material at and just below the seafloor surface. These sand USGS topographic data (for the land). View is from the north, looking south Multibeam mapping system mounted on the hull of a research vessel collects waves are up to 3 meters (10 feet) high and 30 to 40 meters (100 to 130 feet) from crest to crest. toward San Francisco. data along a swath of seafloor.

The U.S. Geological Survey (USGS) mapped about 95 percent of the bay floor pictured on this poster, and the National Oceanic and Atmospheric Administration (NOAA) mapped about 5 percent (in a wedge-shaped area opening to the Map supplement to Chin, J.L., Wong, F.L., and Carlson, P.R., 2004, Shifting shoals and northeast from Alcatraz Island). The data were collected from December 1996 to February 1997 with a Simrad EM1000 95-kHz multibeam swath-mapping system. This system uses 60 independent beams in an arc to collect a swath of bathymetric and backscatter values. These values have been plotted on a 4-meter resolution grid of bathymetry and a 2-meter resolution grid of backscatter. All bathymetric values have been tide-corrected to mean lower low water shattered rocks—how man has transformed the floor of west-central San Francisco Bay: using measured tide values. The surveys were navigated using a differential Global Positioning System (GPS). The data were collected under contract by C & C Technologies, Inc., of Lafayette, Louisiana. The multibeam image strips were processed into digital mosaics by the USGS using a software package developed by the Ocean Mapping Group of the University of New Brunswick, Canada. The digital image mosaics were post-processed and enhanced using U.S. Geological Survey Circular 1259. USGS MIPS (Mini Image Processing System) software. (Not for navigational purposes. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.)

U.S. Department of the Interior Reprinted from U.S. Geological Survey U.S. Geological Survey Open-File Report 01-90, 2003.