Figure captions for U.S. Geological Survey Circular 1259 Cover: San Francisco Bay is one of the world's finest natural harbors and a major center for maritime trade. Since the 1850's, the margins and floor of the bay have been greatly modified by human activity. Several submerged bedrock knobs in the central bay protrude through the sediment of the bay floor and rise to within about 40 feet (12 m) of the water surface. These rocks have been lowered by blasting several times in the past, but they remain a potential hazard to shipping because cargo vessels transiting the bay increasingly have deeper drafts. This panoramic view from the Marin Headlands shows a large container vessel passing beneath the Golden Gate Bridge and entering west-central San Francisco Bay. The San Francisco- Oakland Bay Bridge, in the distance to the left of the San Francisco skyline, connects to Yerba Buena Island, Alcatraz Island (back cover) is just to the left of the northern pylon of the Golden Gate Bridge, and the low-relief feature just above Alcatraz is Treasure Island. (USGS Photograph by Michael F. Diggles.) Frontispiece: A large container ship traveling in the eastbound vessel-traffic lane in the vicinity of submerged Blossom Rock as it begins to pass between Treasure Island and downtown San Francisco on its way to the Port of Oakland. (USGS Photograph by Michael F. Diggles.) Figure 1. San Francisco Bay, one of the world's finest natural harbors, consists of three subembayments-north bay (San Pablo and Suisun Bays), central bay, and south bay- each characterized by a central area of open water surrounded by intertidal mudflats and marshes. This map shows the bay region as it was in the mid-1850's, before development-areas in yellow-green along the margins of the bay are "baylands" (lands that lie between the elevations of high and low tides) that were present then. The population of the region has grown rapidly since the 1850's and now exceeds 7 million people. The bay is an urbanized estuary, perhaps the major estuary in the United States most modified by man's activities. This explosive development has increasingly changed the estuary and its environment. Inset map shows the 40 percent of California that drains through the Sacramento and San Joaquin Rivers into San Francisco Bay. (Modified from Goals Project, 1999; original map from Bay Area EcoAtlas _1999 San Francisco Estuary Institute.) Figure 2. Bathymetric map and shaded-relief image of west-central San Francisco Bay generated from geographically corrected data obtained on 1997 U.S. Geological Survey (USGS) multibeam survey (95 kHz), with the artificial illumination from the northwest. This image was digitally merged with a shaded-relief image for land areas derived from a USGS digital elevation model (DEM) having 30-m resolution. Inset shows setting of this area within central bay. Red bold lines are major bridges. Bathymetric contours in meters, contour interval 10 m (about 33 feet; 1 meter=3.281 feet). Red dots indicate bedrock knobs. Figure 3. Historical changes to the bay margins in the City of San Francisco. Light purple, tidal marshlands that have been filled since 1847. Dark purple, open water areas filled since 1847. (Modified from Dow, 1973; base map is USGS San Francisco Quadrangle 1:100,000-scale topographic map, 1978 edition.) Figure 4. The Gold Rush of the 19th century caused a population explosion in California and led to the first major manmade changes to the region's landscape. This photograph shows high-pressure water cannons (monitors) washing down gold-bearing gravel at the Malakoff Diggings in the foothills of the Sierra Nevada. Such destructive hydraulic mining methods, later outlawed in California, sent enormous quantities of sediment down the rivers and into San Francisco Bay, where it reduced the water depth over large areas. (Photograph by Carleton E. Watkins, courtesy of the Bancroft Library, University of California, Berkeley.) Figure 5. Bathymetry (water depths) over Harding, Shag, and Arch Rocks. Depths in meters corrected to MLLW (mean lower low water; the average height of the lower of the two daily low tides); contour interval is 1 m (1 meter=3.281 feet). Areas shallower than 20 m (66 feet) shown in brown shades. Data from 1997 U.S. Geological Survey multibeam (95 kHz) survey. Figure 6. Sketch of Arch Rock (with the Golden Gate in the background) around 1900, before it was blasted away as a hazard to navigation. The opening through the rock was visible only at low tide, and navigating small boats through it provided a challenge to daring souls. The death of one such adventurer helped seal the rock's fate. (Image courtesy of U.S. Army Corps of Engineers.) Figure 7. Blasting of Arch Rock in 1901 is seen in this photograph taken from San Francisco. Alcatraz Island is in the right middle ground. (Photograph courtesy of U.S. Army Corps of Engineers.) Figure 8. The U.S. Geological Survey shaded-relief image of the floor of west-central San Francisco Bay (fig. 2) was created from data obtained in 1997 using a multibeam swath bathymetry system, which is a variety of sidescan sonar. In this multibeam system, diagramed here, a hull-mounted transducer sends sound energy toward the sea floor and receives back reflected sound through multiple narrow beams. As the ship moves forward (toward the lower left in the diagram), it maps a swath of sea floor. The record or image created on the shipboard recorder has the ship's direction as one axis and the distance between the transducer and the bottom ("slant range") as the other axis. Strong returned signals ("high backscatter") form dark areas on the image; weak signals ("low backscatter") form light areas. The first signal returned is from directly below the ship, and its slant range is equal to the water depth below the transducer. Traditional sidescan- sonar systems produce similar images (see figs. 13 and 15- 18), but because the transducer is towed behind the ship, data are not precisely referenced as to location and water depth. Uncorrected images produced by any sidescan-sonar system contain distortions of scale and shape, both because of variations in the ship's forward speed and because of the varying angles of the slant-range distances. Image- processing software can remove these distortions and produce a geographically correct image, such as that in figure 2. The 1997 survey was operated from the C&C Technologies vessel Coastal Surveyor (seen in the inset photograph with the semicircular multibeam sound transducer raised above water-during operations the transducer is lowered into the water). Figure 9. Sidescan-sonar surveys in the 1970's delineated areas of the bottom in west-central San Francisco Bay characterized by bedrock, boulders, flat sediment, and bedforms. Bedforms are mainly sand waves and dunes. Red lines are bridges-see figure 2. (Map modified from Rubin and McCulloch, 1979.) Figure 10. Shaded-relief image of west-central San Francisco Bay (see fig. 2), showing borrow areas used during 1936Ð38 to provide dredged material for constructing Treasure Island. Note that the topographic depression in the present bay floor southwest of Angel Island largely coincides with the western part of the 1936Ð38 borrow site on Point Knox Shoal. (Multibeam shaded-relief image of bay floor by U.S. Geological Survey, 1997; borrow areas from Cruickshank and Hess, 1975.) Figure 11. Aerial photograph of Treasure Island and the 1939 Golden Gate International Exposition. Treasure Island was built up entirely by filling in a shallow area of San Francisco Bay north of Yerba Buena Island (right). The eastern half of the San Francisco-Oakland Bay Bridge in the background leads to Oakland and the East Bay. The Golden Gate International Exposition celebrated the completion of the Bay Bridge in 1936 and the Golden Gate Bridge in 1937. Part of Treasure Island served as Pan American Airways' seaplane terminal for its trans-Pacific Clipper service (below right), and the intention was that after the close of the Exposition in 1940, Treasure Island would become an international airport for San Francisco. However, in World War II the U.S. Navy took over the island, and San Francisco had to build its international airport elsewhere. (Photographs courtesy of the San Francisco Museum history collection.) Figure 12. Bathymetric profile and map of borrow area set aside for dredging of fill material in San Francisco Bay adjacent to Bay Farm and Alameda Islands. This borrow area encompassed roughly 5.6 million square yards (4.7 million m2) and served as a source of sediment for construction fill on Bay Farm Island. It likely was excavated by a stationary dredge and in places was 26 to 33 feet (8-10 m) deeper than the surrounding area. (U.S. Geological Survey data collected in 1985 using 200-kHz sonar.) Figure 13. U.S. Geological Survey sidescan-sonar image (100 kHz; uncorrected) collected in January 1998 showing dredge-related features in the disturbed zone on Point Knox Shoal. Features visible in this image include elongate troughs (depressions), rims, and circular to semicircular depressions. Compare this to figure 15. (For explanations of slant-range scale and light and dark areas in image, see figure 8.) Figure 14. Approximate locations of lease areas for sand extraction in west-central San Francisco Bay as of June 1999, based on data from the California State Lands Commission (CSLC) Boundary Unit. Base map is composed of USGS shaded-relief images (see fig. 2). Figure 15. U.S. Geological Survey sidescan-sonar image (100 kHz; uncorrected) of recent dredge-related features on Point Knox Shoal collected in January 1999, shortly after a hopper dredge was observed operating over this part of the shoal. Note that elongate depressions cross each other several times. Compare this figure to figure 13. (For explanations of slant-range scale and light and dark areas in image, see figure 8.) Figure 16. Elongate and circular to semicircular depressions (within outlined areas) on the periphery of the Point Knox Shoal disrupted zone. Image is an enlarged portion of the USGS shaded-relief image of west-central San Francisco Bay generated from the 1997 multibeam survey (see fig. 2). High backscatter appears dark, low backscatter appears light (for explanation, see figure 8). Note the similarity of these features to those shown in figures 13, 15, and 17. Figure 17. U.S. Geological Survey sidescan-sonar image (100 kHz; uncorrected) of recent dredge-related features on Presidio Shoal collected in January 1999, shortly after a hopper dredge was observed operating over this part of the shoal. The dredge-related features cut across natural sand waves (the light and dark stripes in the lower part of the image), and some of the elongate depressions cross over each other several times. Note the similarity of these dredged features to those in figures 13, 15, and 16. (For explanations of slant-range scale and light and dark areas in image, see figure 8.) Figure 18. U.S. Geological Survey sidescan-sonar image (100 kHz; uncorrected) over Shag Rock collected in January 1999, showing the rocky debris that covers its surface. The debris includes boulders and blocky rubble of various sizes and shapes. On upper left side of figure are sand waves that terminate against the flank of Shag Rock. (For explanations of slant-range scale and light and dark areas in image, see figure 8.) Figure 19. The evolution of vessel draft through time. Image at far left represents a sailing ship from the Gold Rush era; the other images show successive generations of ever-larger container ships. The evolution of oil tankers has paralleled that of container vessels, with increasing vessel size and draft over time. The bedrock knobs in San Francisco Bay that were artificially lowered to about 40 feet below mean sea level in the first half of the 20th century are clearly once again a potential hazard to the largest vessel (see fig. 20). Diagram modified from Pisani (1989); original schematic copyright by Port of Oakland, September 1989. Figure 20. Traffic lanes for large vessels (1,600 gross tons or greater) in west-central San Francisco Bay, as designated by the U.S. Coast Guard's Vessel Traffic Service San Francisco (more information at http://www.uscg.mil/D11/vtssf). Lane boundaries are shown by dashed white lines, and white arrows indicate directions of vessel traffic. Two-way arrow marks the deep-water route for vessels of draft greater than 45 feet (13.7 m). Note that several of the bedrock knobs (red dots) are in the traffic lanes, and Harding Rock is even close to the deep-draft lane. Base map is composed of USGS shaded-relief images (see fig. 2). Figure 21. A large container ship unloading cargo at the Port of Oakland. (Photograph courtesy of National Oceanographic and Atmospheric Administration.) Figure 22. Large, modern oil tankers, when fully loaded, ride low in the water. However, as this combination photograph/diagram shows, the hull of the ship extends several tens of feet below the surface. (Image courtesy of Petroleum Association of Japan.) Figure 23. Evolution of the bay floor in the area of the Alcatraz Disposal Site. Oblique images show the topography of the bottom in 1894, during the period 1960-80, and in 1997. The 1997 image is derived from 1997 U.S. Geological Survey multibeam survey (95 kHz) of west-central San Francisco Bay. Images for earlier times constructed from available data. Cross section shows how dumping of dredged material in the Alcatraz Disposal Site transformed a bay- floor depression into a mound reaching within 33 feet (10 m) of the water surface. Modified from Chin and others (1998). Figure 24. Vast changes have occurred to the wetlands around the San Francisco Bay and Delta estuary as a result of development and population increase. The maps show the distribution of various types of wetlands around 1800, before any significant development (names on map are Native American tribal regions), and what had happened to those areas by 1998. Modified from Bay Area EcoAtlas (1999). Shaded relief by Graham and Pike (1997).