Negative low tides and tide pooling

Why the water drops below zero

Tide tables measure height from a datum, and US tables use Mean Lower Low Water. On an ordinary day the lower low lands near zero. On a spring tide, when the sun and moon pull in line, the whole range widens: highs go higher and lows go lower, and the low falls below the average. Add the moon's perigee, its closest point to Earth each month, and the low drops further. That is all a minus tide is: an ordinary low on a day of big range. The spring and neap tides guide shows how the range follows the moon.

How low it goes depends on the coast

The bigger the station's range, the deeper its minus tides. Homer reaches -5.3 ft on its lowest day, Seattle -3.9 ft, San Diego -2.0 ft, Myrtle Beach -1.4 ft and Galveston -1.4 ft. On the Gulf coast, a strong north wind behind a cold front often pushes the water lower than any prediction, while an onshore wind can cancel a predicted minus tide altogether.

When the minus tides come in daylight

The time of day matters for tide pooling, and it is not the same on every coast. In Washington and Oregon, the deepest lows of the month fall around midday from late spring through summer and late in the evening in winter, which makes May to July the season for daytime beach walks around Puget Sound. In Southern California it is the reverse: the winter lows come in the afternoon, in daylight, and the summer ones before dawn. On the Atlantic, where the two daily lows are of similar size, deep lows come in daylight in every season on some days of each spring tide. Each station page lists its six lowest lows with date and time; the low tide planner lists daylight windows below any level.

Planning a tide pool visit

Arrive an hour or more before the low and follow the water out, so you are moving back toward shore as it turns. The lowest band of the shore is uncovered only for an hour or so around the low. Watch the sea, not only the pools: on rocky coasts, waves come in sets, and a quiet ten minutes can end with a surge over the bench. Wear shoes with grip; rock covered with weed is slick at any tide. Leave animals where they are and turn rocks back the way you found them. Many tide pool areas are protected reserves with their own rules, posted at the access points.

What lives in the lowest band

Rocky shores are zoned by how long each level stays dry. Near the high-water line, barnacles and periwinkles tolerate hours of sun. Lower down, mussels, rockweed and limpets dominate. The band that only a minus tide uncovers belongs to animals that cannot take much air: on the Pacific coast, sea stars, anemones, sea urchins, chitons and nudibranchs; in New England, the kelps and the creatures sheltering under them. That is why a low of minus one or two feet shows a different shore from an ordinary low, even though the difference in height is small. The animals there are stressed by every minute of exposure, especially on a hot midday low, which is one more reason to look and not lift.

Sand bars, wrecks and the long walk out

On flat sandy coasts the lowest lows reveal bars, shoals and sometimes the ribs of old wrecks. On big-range coasts the walk out can be long: at Homer or on the Maine coast, a minus tide can put the water's edge hundreds of yards from the high-tide line. The return is the risky part. Channels between bars refill first, often from behind, and the rate of rise is fastest in the middle of the flood, two to three hours after the low. Mark the time of the low, set a turnaround time well before the water is due back, and stay off mudflats entirely where the silt is soft.

Minus tides for boats and ramps

The same days that delight tide poolers can strand boats. A minus tide drops channels, harbors and the end of ramps below their usual low-water depth. A boat that floats at a normal low can touch bottom on a minus tide, and a ramp that works most days may leave the trailer short of the water. The boat ramp planner shows the hours when the water stays above the depth you need.