The Winter Solstice this year put on two spectacular shows, one in the sky, the other on earth. The water level in the North Basin — the cove between the park and the Berkeley mainland — went from overflowing the seawall at 10 a.m. to laying bare acres of mudflats at 5 p.m. The water dropped more than eight feet during those seven hours. On the 23rd, the drop is a shade under nine feet in just over six hours, from a high of 7.23 feet at 11:18 a.m. to a low of minus 1.71 feet at 6:28 p.m.
Everybody learned in school that the moon causes the tides. Circling around the earth, its gravity pulls at our globe and anything loose on it, like for example the ocean, will try to rise.
But it isn’t so simple. On Solstice afternoon shortly before 5 p.m., the moon rose over the Berkeley hills, very plainly visible. You’d think that this would be the peak of the high tide, with all that lunar gravitation pulling the water up. Think again. At the moonrise the water level hit its lowest level of the day. Earlier in the day, when the tide peaked, the moon was off somewhere on the other side of the globe. How come?
This kind of problem has agitated great minds for thousands of years. A large body of science has grown up around it. The sun’s gravity also plays a role, and then there’s the lag time that it takes the ocean to respond, and there’s the bump on the far side of the earth from the bump on the moon side, and then there’s ocean depth, current, shoreline shape, wind, atmospheric pressure, the Coriolis effect, and much else.
The moon’s gravity pulls not only on the water, but also on the land. The earth consists of a broken shell of thin crusts over a liquid core. At the equator, lunar and solar gravity can pull up and drop the earth’s surface by as much as 22 inches. That doesn’t matter for hiking, but it plays havoc with ground-based astronomical instruments. It also moves the sea bottom, affecting the water level. I highly recommend the article in Wikipedia as an introduction to this fascinating topic.
The area between the peak of the high tide and the bottom of the low tide is called the intertidal zone. In our North Basin, the intertidal zone consists almost entirely of mud. This mud isn’t smooth and level. It has big and little patterns. In the southeast corner of the cove, Schoolhouse Creek has carved a winding channel. Everywhere along the shore, there are shallow dips of various shapes. Many of these depressions are round; a fisher told me once that skates make them. Of course, these round, flat fish depart for deeper water when the tide drops.
The exposed mud forms a buffet for shore birds. The mud is a source of nutrients for many kinds of wet creatures like worms and little clams and crabs. Shorebirds come equipped with different lengths and shapes of beaks, and use different techniques, for getting at these buried proteins in the mud. On Solstice and on the next day, I saw more than a dozen Snowy Egrets on the mud, along with Mallards, a Great Egret, some Bufflehead, a Marbled Godwit, a Spotted Sandpiper, a Willet, a couple of Black Oystercatchers, a Ring-billed Gull, a few American Wigeons, Ruddy Ducks, Brown Pelicans, and a beautiful little flock of American Avocets with their striking black and white plumage.
Visiting the park is good whenever you have time to do it. You may see more and learn more if you time your visits with the tides. There are several apps, besides the official NOAA website, that give this information. The very best time of year to witness the fascinating game that moon, sun, earth, water, worms, birds, and people play comes on the days of the Winter Solstice.