Minus tides at Arcata

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 Arcata

The rocky shore from Crescent City to Trinidad holds some of the richest tidepools on the Pacific coast, and a winter minus tide is when the lower zone comes out. Palmer's Point, Luffenholtz and the rocks below Patrick's Point uncover a world of anemones, chitons and sea stars.

Clamming and mussel gathering are both done here in season, and both are closed at times on biotoxin grounds. Anything taken is regulated; check the current rules before collecting.

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.