Ready for more about sediment? Great.
The San Francisco Bay Area has a deep and bizarre relationship with mobile sediments. It's a shallow, turbid bay, where sediments slosh in-and-out with the tides and arrive by way of small creeks and the Sacramento-San Joaquin Delta. (N.B., the Delta system drains 2/3rds of the state of California, and the sediment therein.) The Bay's coastline is muddy, and its ecosystems intertwined with the state of sediment on the floor or in the water. That it sits at the center of one of America's urban hubs makes it all the richer.
The anthropogenic part of the sediment story can start with the Gold Rush. From 1852 to 1884 (when it was outlawed), hydraulic mining was used to blast the Sierras with water to free up gold. The scale of this was insane. So much sediment entered the river system (that drains the Western Sierras to California's central valley, then to the SF Bay and the ocean) that it took until 1999 for it all to get pushed out. Over 100 years, dear reader! Marshy shorelines are very effective at catching and trapping sediment as it comes downstream—the coastline within SF Bay accreted dramatically as this plug of sediment worked its way through the system. (I've heard, anecdotally, that the geographic zone we now call Martinez didn't exist as dry land until after hydraulic mining.)
The population explosion with the gold rush increased demand for land, and wetland "reclamation" (I hate this word) became popular. Reclamation largely entails dumping sediment on shallow wetlands, to turn them into dry land. At the time, the consequences of this (loss in marshland ecosystem services, seismic instability) were less clear and sediment was plentiful. So, we filled in the majority of tidally-inundated lands in the Bay and put farmland and houses on it.
We then started the major Californian public works to control the rivers of this same sediment transport system. Dams, flood control, and water routing reconfigured the system and started to prevent sediment from moving downstream, into the Bay.
Suddenly, in the 21st century, we find ourselves with a dearth in sediment: not much coming down through the rivers because of dams and diversions, and we no longer have the big stock of hydraulic mining debris to erode. Additionally, the marshes that are so good at keeping sediment around have been extirpated. In short, SF Bay is eroding.
We keep pushing sediment around regardless. "Beneficial Re-Use" is the buzzword for taking sediment that is dredged for some operational purpose (typically, to keep shipping channels deep enough for big boats, which happens at SF's Embarcadero, at the Port of Oakland, and up the Sacramento deep water channel) and using it elsewhere, often to make new wetlands or coastal levees. Here's an example of where Port of Oakland sediments go. It's called "re-use" because the alternative is the surprisingly-common strategy of just dumping it in deep water offshore.
This, however, comes intertwined with some nasty legacy issues, namely, we've been awful about what we've dumped into the Bay. San Francisco Bay sediments suffer from high levels of mercury (above already high natural baseline conditions), PCBs, and radioactive material. Any movement of contaminated sediment smears the contamination around may impose additional health risks to flora and fauna and triggers a heap of paperwork.
At this point, the sediment system that the San Francisco Bay represents is a full manifestation of anthropogenic forcing overloading, then underloading the natural hydrology. Most landscape processes work when the parts are connected, but we've disconnected wetlands from the tides, and dammed up the rivers between their source and their end. To keep the whole sediment budget balanced, there is a ton of work to do.
Caked in mud,
Lukas
The San Francisco Bay Area has a deep and bizarre relationship with mobile sediments. It's a shallow, turbid bay, where sediments slosh in-and-out with the tides and arrive by way of small creeks and the Sacramento-San Joaquin Delta. (N.B., the Delta system drains 2/3rds of the state of California, and the sediment therein.) The Bay's coastline is muddy, and its ecosystems intertwined with the state of sediment on the floor or in the water. That it sits at the center of one of America's urban hubs makes it all the richer.
The anthropogenic part of the sediment story can start with the Gold Rush. From 1852 to 1884 (when it was outlawed), hydraulic mining was used to blast the Sierras with water to free up gold. The scale of this was insane. So much sediment entered the river system (that drains the Western Sierras to California's central valley, then to the SF Bay and the ocean) that it took until 1999 for it all to get pushed out. Over 100 years, dear reader! Marshy shorelines are very effective at catching and trapping sediment as it comes downstream—the coastline within SF Bay accreted dramatically as this plug of sediment worked its way through the system. (I've heard, anecdotally, that the geographic zone we now call Martinez didn't exist as dry land until after hydraulic mining.)
The population explosion with the gold rush increased demand for land, and wetland "reclamation" (I hate this word) became popular. Reclamation largely entails dumping sediment on shallow wetlands, to turn them into dry land. At the time, the consequences of this (loss in marshland ecosystem services, seismic instability) were less clear and sediment was plentiful. So, we filled in the majority of tidally-inundated lands in the Bay and put farmland and houses on it.
We then started the major Californian public works to control the rivers of this same sediment transport system. Dams, flood control, and water routing reconfigured the system and started to prevent sediment from moving downstream, into the Bay.
Suddenly, in the 21st century, we find ourselves with a dearth in sediment: not much coming down through the rivers because of dams and diversions, and we no longer have the big stock of hydraulic mining debris to erode. Additionally, the marshes that are so good at keeping sediment around have been extirpated. In short, SF Bay is eroding.
We keep pushing sediment around regardless. "Beneficial Re-Use" is the buzzword for taking sediment that is dredged for some operational purpose (typically, to keep shipping channels deep enough for big boats, which happens at SF's Embarcadero, at the Port of Oakland, and up the Sacramento deep water channel) and using it elsewhere, often to make new wetlands or coastal levees. Here's an example of where Port of Oakland sediments go. It's called "re-use" because the alternative is the surprisingly-common strategy of just dumping it in deep water offshore.
This, however, comes intertwined with some nasty legacy issues, namely, we've been awful about what we've dumped into the Bay. San Francisco Bay sediments suffer from high levels of mercury (above already high natural baseline conditions), PCBs, and radioactive material. Any movement of contaminated sediment smears the contamination around may impose additional health risks to flora and fauna and triggers a heap of paperwork.
At this point, the sediment system that the San Francisco Bay represents is a full manifestation of anthropogenic forcing overloading, then underloading the natural hydrology. Most landscape processes work when the parts are connected, but we've disconnected wetlands from the tides, and dammed up the rivers between their source and their end. To keep the whole sediment budget balanced, there is a ton of work to do.
Caked in mud,
Lukas