sentences:  There used to be a natural passage between the lagoon and the San Francisco Bay through the southern spit.The passage was spanned by a drawbridge.From the late 19th century, the drawbridge would be raised twice a year, once in the Spring to allow pleasure boats of the Corinthian Yacht Club to exit the lagoon, where the boats wintered, and go to their Summer moorings, and once in the Fall, to allow the boats to return.