Minus tides at Biddeford Pool

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 Biddeford Pool

The sand beaches from Ogunquit to Old Orchard are wide already and a low tide doubles them. Clamming is done in the back bays and the river mouths — the Webhannet, the Ogunquit, the Scarborough marsh — under town licences that differ from one to the next.

The rocky ends at Kittery, Nubble and Cape Neddick hold pools that come out on the bigger lows, and with a nine-foot range those lows are substantial.

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.