Minus tides at Port Renfrew

What a minus tide is

Tide heights are measured from a datum — here Chart Datum — 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 Port Renfrew

Botanical Beach at Port Renfrew is one of the finest tidepool shores on the Pacific coast, and it works only on a low tide — the sandstone shelf holds pools deep enough to keep urchins, stars and anemones between tides. A tide below about four feet is what the park recommends.

Further north the same applies at Tofino's reefs and the ledges at Ucluelet. Everything in the tidepools is protected; look rather than collect, and watch the swell, which arrives on this coast without much warning.

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.