Minus tides at Calais

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 Calais

A twenty-foot tide uncovers a very great deal, and Cobscook Bay at low water is most of a mile of flats. Clamming is the traditional use and it is licensed town by town; the bay is also one of the best places on the coast to watch the water simply move, because at this range it runs like a river.

Treat the flats with more caution than they seem to need. The water comes back fast enough to cut off a route that was dry twenty minutes earlier, and mud that holds your weight on the way out may not on the way back.

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.