Because of the many rivers and sloughs throughout the Delta there are many bridges. Some open, some don't. The ones that open are referred to as drawbridges. The vertical clearance and the hours of operation of these drawbridges are extremely important to boaters exploring the Delta. It is almost routine for a boater to make a long cruise to some intended destination, only to find that a drawbridge of modest vertical clearance blocks their route, and that the bridge's hours of operation are past and that the bridge tender has gone home.
Boaters need to become apprised of the bridges on their intended route, their vertical clearance (including allowance for the tide) and their operational schedule. This information is contained on some Delta maps, in a booklet free from the Coast Guard Bridge Section in Alameda, in some Tide Tables, and in other publications.
You signal a drawbridge to open with one long and one short toot of the boat horn; or by calling the Bridge tender on Channel 9 on the Marine Band VHF Radio.