Minus tides at Savannah

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 Savannah

Eight feet of range takes a great deal of water off Tybee's flats, and the back river and the creeks behind the island empty into mud and oyster rake. Crabbing and casting for shrimp are both summer occupations worked on moving water.

Low water also makes the beach at the north end walkable round to the lighthouse, and uncovers shells and the occasional shark tooth. Little Tybee, which is boat access only, is a different proposition again — check the turn before you cross.

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.