Minus tides at Deception Pass

What a minus tide is

Tide heights are measured from a datum — here MLLW — which is roughly the average of the lower low waters. Most low tides sit a little above it. A few times a year one falls below, and the height is printed as a negative number: a minus tide.

They happen on the same days as the king tides, and for the same reason. When the sun and moon pull together the water comes up further than usual, and it also goes out further than usual. The big high and the big low are six hours apart on the same day.

A foot below datum does not sound like much. On a shallow shore it can mean a few hundred yards of ground uncovered that nobody sees the rest of the year.

What to do at low water at Deception Pass

A minus tide in the Sound uncovers a great deal of ground. The flats at Dungeness, the beaches at Alki and Golden Gardens, and the whole of Padilla and Skagit bays come out far enough to walk on, and the summer daytime lows are the ones people plan around.

Clamming and oystering are managed beach by beach, with seasons and biotoxin closures that change through the year. Seattle Aquarium and the state park naturalists run low-tide walks on the biggest summer lows, which is the easiest way to see what is out there without disturbing it.

Going out safely

The water comes back. Work out when the tide turns before you walk out to anything, and allow more time than the return trip took — a rising tide fills channels behind you before it reaches where you are standing.

Wet rock is slippery, and what is exposed at a minus tide has been underwater long enough to be covered in weed. Anything living out there is usually protected by state or local rules; look it up before taking any of it home.