It’s tough to escape from Alcatraz. During its three decades as a prison, only five men got away without recapture, and it’s likely that some or all of them drowned in the frigid waters of San Francisco Bay.
Those waters have also held another kind of captive — sand and silt dumped near Alcatraz Island.
Alcatraz Island. Credit: National Oceanic and Atmospheric AdministrationRunoff and other activities deposit huge amounts of these sediments into the bay each year. They can pile up around the ports and in navigation channels, making it unsafe for ships. So these parts of the bay are scooped out and the sediments are dumped at other sites.
Starting in 1894, one of those sites was just south of Alcatraz, between the island and downtown San Francisco. Measurements showed there was a depression in the bay floor there. Modern studies say it was more than 150 feet deep.
Officials thought that the outgoing tides would carry away much of the material dumped into this hole. But in 1982, they discovered that this wasn’t happening. The sediments had formed a giant mound that reached to within about 35 feet of the surface. So the material dredged to make the bay safe for navigation was itself becoming a hazard.
After a long debate, federal and local agencies agreed on a management plan. The amount of material that can be dumped near Alcatraz was limited, and the plan encourages projects to re-use dredged materials. So far, these limits have kept the waters around Alcatraz safe for navigation — even while much of the sediment remains locked up near Alcatraz.
