Surficial Characteristics of the Bay Floor of South San Francisco, San Pablo, and Suisun Bays, California by John L Chin1 and H

DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Surficial characteristics of the bay floor of South San Francisco, San Pablo, and Suisun Bays, California by John L Chin1 and H. Edward Clifton1 Open-File Report 90-665 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1 U.S. Geological Survey, Menlo Park, California 94025 1990 INTRODUCTION San Francisco Bay is the largest estuary on the Pacific Coast of the United States. Although the hydrology and biology of the bay have been extensively studied, almost no studies have been conducted on the surficial characteristics and composition of the bay floor, and how physical processes both form and modify it into different morphologies. Rubin and McCulloch (1979) conducted such a study in central San Francisco Bay. This study was designed to study the southern and northern parts of the bay in a manner analogous to that of Rubin and McCulloch (1979). Our reconnaissance investigation characterizes the surficial morphology of the bay floor as revealed by side-scan sonar imaging and high-resolution bathymetry and deduces the general nature of sedimentation, bedload sediment transport directions, and areas of deposition versus erosion. Our results should apply to current issues involving sedimentation, dredging, pollution, and the disposal of dredge spoils in the San Francisco Bay system and the highly developed urban areas that border it. PHYSICAL SETTING The San Francisco Bay estuary is a large, complex system comprised of broad intertidal and subtidal flats and narrow, relatively deep tidal channels. In this report, the shallow embayments within the bay are referred to as South Bay, Central Bay, and North Bay (San Pablo and Suisun Bays) (Fig. 1). The average depth of the bay is 6 m at mean lower low water and the volume of the bay is 6.66x109 m3 with a tidal prism of 1.59x109 m3 (Conomos, 1979). Tides in the bay are mixed and semidiurnal in nature. The highs in each cycle are usually quite different in height as are the lows (Conomos, 1979). Ninety percent of the fresh water that enters the bay is derived from the Sacramento and San Joaquin Rivers. This combined riverine system receives runoff from 163,000 km2 of drainage basin, which is about 40% of the land area of the State of California. Basic flow patterns within the bay are principally tidally induced and fairly consistent throughout the year. Tides within the bay generate reversing currents that are strongest in the tidal channels and weakest over the shoal areas. Nontidal currents can be generated by winds and river flow although their velocities are typically only 10% that of tidal currents (Conomos, 1979). North and Central Bays exhibit an estuarine circulation due to the ebb dominance of surface water and the flood dominance of bottom water (Peterson, et al., 1975). This circulation system is characterized by a landward flowing density current and occurs in parts of the bay where a significant freshwater inflow is present. The South Bay, with only a small amount of freshwater inflow from local streams, has no definite estuarine circulation system. McCulloch et al. (1970), however, suggested that South Bay may have a reverse estuarine circulation system that is generated during periods of peak delta outflow. Their theory is that low salinity water moves southward from North Bay into South Bay, which displaces the more saline water of South Bay northward. Conomos (1979) further suggests that a sluggish, transient three-dimensional circulation exists in South Bay that is apparently both density- and wind-induced. Climate over the bay is variable; near San Francisco the climate is dominated by the Pacific Ocean yielding warm winters, cool summers, and small seasonal temperature changes. In contrast, inland areas have a more continental climate with warmer summers and cooler winters. During summer, winds dominantly blow from the west or northwest while during the winter they mostly blow from the east or southeast except during storms when they blow from the south. Strong winds generate waves where fetch is sufficient and thus resuspend sediment as well as mix and oxygenate the water (Conomos, 1979). The largest waves generated by summer winds have wave periods of 2 to 3 seconds and heights of 1 m plus. Winter storms, where fetch is significant, may generate waves with 5-second periods (Conomos, 1979). METHODS Field work for this study was conducted using the USGS Water Resources Division Research Vessel Polaris. Side-scan-sonar profiles were collected using a Klein system operated at a frequency of either IOO kHz (South Bay) or 500 kHz (North Bay). This system is comprised of a graphic recorder and towfish that can be operated at varying frequencies and slant ranges. In general, slant ranges of IOO m (200-m swath) or 150 m (300-m swath) were used for optimum resolution of bottom features on the bay floor. Towing speed averaged 7.4 km/hour, although this varied greatly with tidal current speed and direction as well as local wind regime. No attempt was made to acquire overlapping side-scan sonar images. The side-scan sonar has a theoretical resolution on the order of about 1 m in range scale and 10 to 20 cm in vertical scale (ripples were therefore not resolved by the sonar). The side-scan sonar emits a high frequency (100 or 500 kHz) sonic signal that insonifies the sea floor, producing a continuously recorded oblique view of it on both sides of the vessel. The intensity of the sound scattered back to the sonar varies as a function of several parameters, the most important of which are the bottom roughness (morphology, relief) and the angle of incidence. Thus, areas with distinct relief, such as areas floored by bedforms, typically produce dark patterns that are readily discernible from areas with little to no relief. The texture of the sea-floor sediment is important as it greatly influences the bottom roughness and hence the darkness/lightness of the image produced on the side-scan profile. The coarser and rougher the sea floor the darker the image produced on the graphic recorder tends to be. An Innerspace fathometer was used to acquire high resolution bathymetry. This system operated at a frequency of 200 kHz with a hull-mounted transducer. The bathymetry was not corrected for tidal elevation. Navigation was obtained with a shipboard Raytheon precision radar. Fixes were plotted every 5 min. and correspond to the event marks on the side-scan sonar and fathometer records. INTERPRETATIONS Tracklines were plotted on overlays of the 1:40,000-scale National Ocean Survey bathymetric maps of San Francisco Bay (Sheets 1, 2, 3). In general, patterns on the side-scan profiles correlate well with bottom relief. In some instances, however, the correlation is not good; for example, patterns produced by the bay floor off Hunters Point in South San Francisco Bay (Fig. 2; Sheet 1). There the bay floor appears as a patchwork of alternating light and dark tonal patches, whereas the corresponding bathymetric profile shows an irregular bottom with relief up to 0.5 m but with no consistent or regular bottom morphology as seen in other areas where the side-scan record shows distinct patterns. A second exception is the occurrence of lineations in South San Francisco Bay on the side-scan record where no discernible bottom morphology occurs on the bathymetric profile (Fig. 3). Both of these cases will be discussed in subsequent sections. Mapping the distribution of bedforms by the remote imaging of large areas of sea floor leads to a better understanding of the hydraulic regime that created these bottom features (and most likely continues to modify them) and to inferring the direction of bedload sediment transport. Both of these issues are of great importance to the present and planned usage of this estuary by the adjoining communities. Usually, bedload sediment is transported normal to the crestline of transverse bedforms such as sand waves (Belderson, et al., 1972; Rubin and McCulloch, 1979; Knebel, 1989) and, conversely, net bedload sediment transport is parallel to the crest/trough orientation of longitudinal bedforms such as furrows and lineations (Belderson, et al., 1972; Flood, 1983). The asymmetry of transverse bedforms indicates the net direction of bedload sediment transport (these have been reported as "ebb" or "flood"). Rubin and McCulloch (1979) demonstrated that bedload transport directions inferred from sand wave orientations were generally within 15% of the directions measured by current meters of the strongest near-bottom currents in Central San Francisco Bay. We thus feel that bedload sediment transport directions inferred from bedforms yield a fairly accurate measure of tractive sediment transport in the bay. In this report the arrows depicting bedload sediment transport indicate bi-directional transport if the arrow is equidimensional about the locality indicator (Sheets 1, 2, 3) or net (dominant in one direction) if the arrow is longer in the flood or the ebb direction (Sheets 1,2, 3). Based on the available data, we cannot establish whether and how bedforms in the bay respond to individual tidal cycles because our profiles represent only one discrete interval of time. The direction of bedload sediment transport does not however infer that suspended sediment is transported in the same direction. As Rubin and McCulloch (1979) demonstrated, some ebb-dominated parts of Central Bay are characterized by opposite directions of tractive versus suspended sediment load. SURFICIAL BOTTOM TYPES Five surficial bottom types can be delineated on the floor of South and North Bay by correlating distinctive patterns on side-scan-sonar and bathymetric profiles.

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