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A Tidal Primer

The tide comes in, the tide goes out. But what are tides and how do they work?

Tide pool hikers viewing many armed starfish at Fitzgerald Marine Reserve, image
Photo caption
The Fitzgerald Marine Reserve harbors a rainbow of creatures—especially at low tide.

Tides are the alternating rise and fall of sea levels caused mainly by the earth’s rotation and the gravitational pull of the sun and the moon. Most coastlines experience roughly two high and low tides each day, at ever shifting times and levels. During new and full moons, when the sun and moon are aligned, their combined pull creates tides of exceptional highs and lows.

Tide measurement begins with the average low tide for a particular location, called the zero tide height. “Minus tides,” which are generally best for tide pooling, occur when the low tide does not reach the zero level. While there are minus tides throughout the year along the West Coast, the most significant summer ones often happen after dark; in the winter, they occur during more convenient daylight hours. In Hawaii, which experiences little tidal fluctuation, the lowest daylight tides are in late spring.

The timing and level of tides are specific to a location. To find out predicted high and low tide times and heights for a particular place, consult a tide table for the nearest given location. You’ll find tables for more than 400 West Coast locales at

Photography by David H. Collier

This article was first published in March 2011. Some facts may have aged gracelessly. Please call ahead to verify information.