The California Delta waterways are comprised of fresh water, but the waterways depths are controlled by tidal action from the Pacific Ocean. There are approximately two high tides and two low tides every day. The difference between the high and low tides can be six feet and more. A boat that might be comfortably anchored at high tide, can find itself hard aground at low tide. Water depths also can change the vertical clearance at drawbridges and fixed bridges. Anglers also conclude that fishing often is at its best at tide changes.  Below are several links to help you to find tidal information.