46 WILD / READING THE WILD PACIFIC / 03

Why the Pacific Has Two High Tides

The oyster ground was underwater this morning. It is underwater again tonight—but the water does not reach quite the same place. Why twice, and why different?

The Moon sets a rhythm

The Moon’s gravity pulls more strongly on Earth’s near side than its far side. Those differences in pull help create the tidal pattern; the Sun contributes too. In a simplified ocean-covered Earth, the pattern has two broad high-water regions. As Earth rotates relative to the Moon, a place passes through both.

The real Pacific is more interesting

Continents interrupt the water, and ocean basins, seafloor shape and the changing positions of the Moon and Sun reshape the response. Along much of the Pacific Northwest the pattern is mixed: usually two unequal highs and two unequal lows in a lunar day of about 24 hours 50 minutes. It is not two identical highs in every calendar day.

Willapa Bay adds its own shape

Water must move through the bay entrance, channels and shallows. Tide times and heights depend on location. Use a prediction for the place you plan to visit, not a convenient table for another harbor. The lowest low exposes more working flats, but exposed ground is not automatically public access.

Height is not current

A tide table predicts water height. A current prediction describes flow. Slack current need not occur at high or low water, especially near river mouths. Wind, pressure and river flow can also push actual water levels away from astronomical predictions.