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 Northern San Francisco Bay.

See details of San Pablo Bay Historical Analysis
See details of Suisun Bay Historical Analysis

ANIMATIONS OF CHANGE FOR NORTH BAY

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. Click here to view the animation (1.4 MB animated GIF). 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:

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.

Click to view an animation of mining debris deposition and subsequent erosion (these animations are in review and are restricted to internal access):