By Mary Swift

My first dramatic recognition of tidal effect was in 1980, watching a 50’ race boat, in town for the Big Boat Series, trying to get by Pier 39 heading to South Beach Harbor. It had a tall mast, over 65’, with Spinnaker, Blooper, winged out main, crew positioned to maximize air flow, and yet it was going backwards, slow but steadily being pulled toward the Golden Gate. That was an impressive amount of sail power all drawing and full, still the boat could not make “head way”. It could not go forward against the San Francisco Bay tidal forces. A boat out away from the shore in the middle of the channel was going by this big boat with only basic white sails up. We looked, as did those aboard the big racer, and nope, no engine. So how did that work? Tides and the current that move them.

The San Francisco Bay Model, in Sausalito, helped me understand these forces. The Army Corp of Engineers, who run it, emptied then refilled the model then set in motion a mock tidal cycle strategically dropping in dyes to simulate an oil spill for disaster recovery planning. It was great to walk around the acre and a half model of the three major bays that link together to form San Francisco Bay waters. Seeing the Bay without water added to help to make sense of why some places have stronger current than other places just a few feet away.

You can see these forces at work. Color, lines and surface texture are three key indications. Let’s first look at color.

Blue-green in the Central Bay indicates the flood of incoming ocean water. The tan-gray color is caused by silt in the water stirred by the inland waters pushing their way to sea. Have you ever noticed, when near a tidal area shoreline, perhaps crossing a bridge or riding a ferry, large areas of two colors pushing each other separated by a white line? That’s where a flood is meeting an ebb. New tide pushes in through the Golden Gate, acting against all the Northern California streams and rivers combined.

Have you ever seen spots of color popping up in the middle of an expanse of another color? Tides flow in and out of the Golden Gate but they also start in estuaries and mud flats because of the effect of the moon. The first few inches trickle down from shore to the deeper water. Then, successively, the first foot of water, then two feet of water, and so on, builds on the one before.
In the Central Bay the average depth is 15 feet and yet there are spots that range from 1 foot by Berkeley to 300 feet under the GG Bridge around Angel and Alcatraz Islands it is 90 to 125 feet deep. Deeper sections take longer to turn the tide. In an area of uneven depth. like the area between Richmond and Marin south of the Richmond/San Rafael Bridge, sometimes the water looks like a Dalmatian dog with spots of tan-gray showing through the blue/green.

Out for a sail this Opening Day, there were color bands reaching toward the shore of the blue-green ocean waters. In the deeper sections there were stripes of tan-gray mixed between bans of blue-green. All along the shore of the Islands Angel and Alcatraz was tan-gray and out toward the city was a large expanse of blue-green.