The Maritime Report: Tides

The Maritime Report: Tides

Dave Bloch

High and low tide at the Marina Village Yacht Harbor. Notice the height of the concrete post and the steepness of the ramp; remember that it is the DOCK that has moved up and down. Photos by Dave Bloch.

Did you know our Island rises up and then sinks down several feet twice every day?

Of course it doesn't. But if you stand in one place and look out on the water that surrounds us, it may look like it is. Of course, it's actually the Bay that is rising and falling while Alameda stays where it is.

Do you ever notice these changes? They happen so slowly that they're easy to miss. But living on a boat, it really does look like it's the land around us that's changing its altitude. The concrete posts that hold our docks steady get longer and shorter. The ramp up to the shore goes from almost horizontal to a pretty steep climb!

121015TidesAtBayviewDrive

High and low tide at the bird sanctuary east of Robert Crown Beach.

Where do these changes come from? The gravity of the moon pulls the water of the Pacific Ocean toward it when the Earth is aligned that way. As the moon revolves around us, and the Earth rotates, the moon moves away and releases our area of the ocean, and the tides drop. When it does that, it pushes and pulls millions of gallons of water through the Golden Gate, creating tidal changes and powerful currents everywhere around us. This happens twice a day, every single day, in a completely predictable way; you can pick up a little book at any boat supply shop or look it up online. (See link at the end of this post.)

The best way I know to enjoy the tides is from one of the viewing platforms on the southwest corner of the Island. Walk down Robert Crown Beach to its far eastern end to find the first one; the second is about a quarter-mile walk along the Bay Trail, behind the Bayview Drive duplexes. Your reporter used to live in one of these.

You're walking along a tidal marsh, one of very few left in the Bay Area. This area is a bird sanctuary, and the birds are the best reason to visit. Depending on when you arrive, you may find water lapping the shore within a few yards of the trail. Or you may be looking out across mud flats that reach nearly all the way to the shore of Bay Farm Island. You can choose a spot to look at where the water meets the mud, and it will take only a very few minutes until you see the water's edge has moved, in or out.

The birds see that, too. As the tide goes out and high spots of mud begin to show, the birds start to arrive. They circle, find a dry spot, and land. Some go fishing, diving from high above to places that fish get trapped in shallow water. (Easy pickings!) More mud is revealed; more birds arrive. On some days, they come by the thousands.

It will take about five and a half hours to go all the way out; the same amount of time to come back to all water once again. You'll watch the water encroach on the birds' resting places; you can almost hear them shrug in resignation as they have to take to the air again.

I have no idea where they go. I just know they'll be back in a few hours.

121015MidTideBayviewDrive

Mid-tide, and birds are crowding into a small bit of exposed land out in San Leandro Channel off of Bayview Drive.

Take a couple of hours sometime, and experience the tides! Use a website like the one below; pick a time when the tide is dropping if you want to see the birds arrive, or rising if you'd like to watch them squeeze into smaller bits of land before giving up and flying off.

Just another way to experience something easily taken for granted, here around our Island.

TIDES AND CURRENTS FOR ALAMEDA
http://tidesandcurrents.noaa.gov/data_menu.shtml?stn=9414750%20Alameda,%...