Minus tides at Ship Island

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 Ship Island

These are boat-access islands and the tide governs the trip: where you can land, how far you have to wade, and when you need to be away. Ship Island has a ferry and a schedule; the others do not.

Low water uncovers sand flats that run out a long way, and the shelling on the Gulf side of Horn and Petit Bois is as good as the coast gets, precisely because so few people are standing on it. The islands are national seashore — look rather than collect where the rules say so.

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.