The south end of San Francisco Bay has a number of areas that are encircled with walls that allow the water to be cut off. These are used to dry out salt beds for harvesting. The sun evaporates the water and the salt is left when all the water is gone. After harvesting, the tide can be allowed to flood the beds again and the process repeated. One cool thing about these beds is that, presumably as a result of algae, they can turn some interesting colors as the water evaporates. I was flying in to SFO for work and we turned right over the beds on to the approach. With the sun out, the colors looked excellent.