Minus tides: the best days for tidepooling and clamming

A minus tide is a low tide that drops below zero on the tide chart — below MLLW, the long-term average of the lower low water each day. Zero already sits near the bottom of the normal range, so a low of −1.5 ft uncovers rock, eelgrass, and sandflat that stay submerged through an ordinary month. That band of rarely-exposed shore is where the good stuff lives: anemones, sea stars, octopus dens, and the clam beds that only daylight minus tides make reachable.

When minus tides happen

Minus tides are spring tides taken one step further, so they cluster around full and new moons— and the deepest ones arrive when a spring tide coincides with the moon's closest approach to Earth (the same alignment that produces king tides; a big minus tide is a king tide's mirror image, and they usually share a week). On the US West Coast the best daylight minus tides bunch near the summer solstice, when the low of the day lands in the morning; midwinter serves them up too, but often before dawn. Every station's annual tide table on Perigee lists its full minus tide calendar for the year — find your station from the state directory and open its yearly table.

Reading the window

The water doesn't sit at the bottom of the curve for long. Plan around the predicted low time: arrive an hour to ninety minutes early while the water is still falling, work the lowest zone at the turn, and start moving up the beach as the flood begins. The flats flood faster than they drained — the middle two hours of a rising tide move roughly half the day's water (the rule of twelfths, explained here) — and channels behind sandbars fill first, which is how people get cut off.

A short field checklist

Check the observedwater level against the prediction before you go — onshore wind or low pressure can keep a promised −1.0 ft closer to flat. Know the low time, know when you'll turn around, and keep an eye on the route back rather than the route out. Step on bare rock and sand, not on the animals; put rocks back the way you found them; and check local regulations before harvesting anything.

Finding the next one

Every Perigee station page answers “when is the next minus tide here?” in its FAQ, seven days out, and the annual tide table lists the whole year, month by month. If the coming week has nothing below zero, don't force it — a −0.2 ft tide is a pleasant walk, but the −1.5 ft days are when the shore actually opens up.