The entrance to the Golden Gate Channel is guarded by an area of shallow water called The Fourfathom Bank. It is a sand/mud bank built up over millennia by the tidal outflow from The Bay and the rivers draining from the Sierra. The Bank is notorious because of the lumpy seas that it produces, even in mild conditions. When there is a big swell running, The Bank becomes a spectacle with 20 plus foot waves breaking for miles out to sea. Many sailors have come to grief on and around the Fourfathom Bank.
The depths in the middle of the bank are less than 30 feet even four and five miles offshore. A channel passes through the semicircle of shallow water guarding the Golden Gate allowing commercial shipping and the rest of us to pass through.
On a fine day in February, I headed out planning to meet my family at Bolinas. As often happens, I left a little late so abandoned the original plan and sailed an erratic path out and over the Fourfathom Bank. The wind was blowing barely 10 knots and the ocean swell was small. Nonetheless the waves over the bank were steeper and closer together than one would expect given the benign conditions (see video above). You can imagine what it is like out here when things are really pumping!
Did you see the porpoise? I have been trying to photograph a Harbor Porpoise for years, but their fast movements defy getting a good shot. By coincidence I caught a porpoise surfing behind the boat in the video - if you missed it see the edited version below (look at the top left of the video).