San Francisco Bay Bathymetry

Methods

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In the example sequence shown below, the first step in the preparation of regular grids displayed on this web site begins with irregular hydrographic survey data (soundings) that have been corrected to a common datum (1).

The National Oceanic and Atmospheric Administration (NOAA) is the primary resource for obtaining these original soundings. Other agencies, including the US Army Corps of Engineers, California Department of Water Resources, US Bureau of Reclamation, and the USGS, have contributed local studies. Once the soundings are in hand they are contoured, and shoreline and marsh perimeters are added and combined into a geographic information system (GIS) (2).

All data layers must be adjusted to a common horizontal and vertical datum and all depths must have the same orientation and units. At this point a grid can be generated.

Quality control is an iterative process, performed on the resulting grid by comparing it with the original soundings (3, 4).

Errors are computed, plotted and repaired when appropriate. Errors are usually a result of incorrect unit tags on the source data or digitizing mistakes, but some are due to gradients in bathymetry that cannot be resolved by a single grid cell.

The final grid (5) can be adjusted to a different tidal datum using an adjustment grid.

This grid is produced by assigning tide levels observed at shore stations to co-tidal lines from the TRIM-2D model (6, 7).

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Geostatistics

Using this 100m grid cell representation of the Bay we can compute some primary geomorphic features of the basin--such as surface area and volume--for a given tidal datum, and compare these and other statistical properties in the sub-basins of San Francisco Bay.

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Full Bay 

Properties based on the Mean Sea Level grid

Bathymetry Change

As described in our METHODS section, a continuous surface representation of each bathymetric survey was created using Topogrid, an Arc/Info module that utilizes sounding and contour information to create a hydrodynamically correct surface. Input data was a combination of point soundings and hand-drawn depth contours (see table below). Once a bathymetric surface has been created for each hydrographic survey, the surfaces are adjusted to a common datum and we compute change or difference grids. These new ‘change’ surfaces identify areas of erosion and deposition.

Here is an example difference map of San Pablo Bay (1856-1887). During this period there was massive sediment accumulation related to hydraulic gold mining.

The data supporting historical change analysis is quite extensive. The following tables summarize the survey dates, digitized soundings, and contours used to produce the bathymetric surfaces and difference maps for San Francisco Bay.

Official Publications

• San Pablo Bay Historical Analysis
  USGS Open-File Report 98-759
  Sedimentation and bathymetric change in San Pablo Bay, 1856-1983
   
• Suisun Bay Historical Analysis
  USGS Open-File Report 99-563
  Sedimentation and Bathymetry Changes in Suisun Bay: 1867-1990
   
• Central Bay Historical Analysis
  USGS Open-File Report 2008-1312
  Sediment Deposition, Erosion, and Bathymetric Change in Central San Francisco Bay: 1855–1979
   
• South Bay Historical Analysis Part 1
  USGS Open-File Report 2004-1192
  Deposition, Erosion, and Bathymetric Change in South San Francisco Bay: 1858-1983
   
• South Bay Historical Analysis Part 2
  USGS Open-File Report 2006-1287
  Sediment Deposition and Erosion in South San Francisco Bay, California from 1956 to 2005

Animations of change for North Bay

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By linear interpolation, we can compute sedimentation maps for years between surveys and combine the maps to produce an animation of sedimentation for the North Bay. This animation gives an overall view of the system in time and space. We can see that, in the more active channels of Suisun Bay, surface sediment is deposited and erodes quickly in response to changing flows (floods/drought) and modifications (such as dredging the southern channel or long term mooring of the mothball fleet).

We assume:

1. the sediment deposited in North San Francisco Bay between 1856 and 1887 was dominated by hydraulic mining debris;
2. erosion observed in subsequent surveys was not re-deposited locally; and
3. material deposited after 1887 was not mining debris.

Making these assumptions, we can predict the location and thickness of the original hydraulic mining debris. It is especially notable that the mercury employed in gold mining in the Sierra Nevada was refined liquid quicksilver or elemental mercury; this is a form of mercury much more likely to foster net methylation than is cinnabar, the form of mercury in most mercury mines. Approximately 10,000 tonnes of refined mercury were lost to the watershed during the Gold Rush mining era. Much of the mercury consumed by gold mining could have been incorporated into the 12 billion cubic meters of sediments extracted by the mining activities and released to the rivers of the Bay-Delta watershed. The mercury-laced hydraulic mining debris was ultimately transported to the bay-delta; it is known that large deposits of hydraulic mining debris remain in bay sediments. These wastes formed marshes, islands, or filled or diked marsh, or were deposited in shallow waters. Under the right circumstances this mercury contamination is transported through the food chain and concentrated in some commercial and sport fish. Human consumption of fish caught in the Bay is already restricted because of mercury contamination. Specifically, adults are advised to limit consumption of sport fish from the Bay to two times a month; pregnant or nursing women and children 6 or under should limit consumption to one time a month. Large shark and striped bass from the Bay should not be consumed at all. As we study the feasibility of restoration of marshes that were sinks for mining debris, the possibility of releasing mercury to the Bay must be considered.

Animations of mining debris deposition and subsequent erosion

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Sources/Usage: Public Domain. View Media Details