Tidal Currents Background

Ocean Tides


Types of

Tides in
Ocean Basins

Tidal Currents


Open-Ocean Tides

Ebb and flood tide in a harbor.
Ebb and flood tide in a harbor.

Alternating horizontal movements of water accompanying the rise and fall of astronomical tides in coastal areas are called tidal currents. Along the boundaries of an ocean basin (the location of the antinodes) tidal ranges and hence, tidal currents are at their maximum. Irregulari­ties along the coast modify the rotary motion of tide waves so that tidal currents move more directly into and out of rivers and harbors. Tidal currents flow in one direction during part of the tidal cycle and in the oppo­site direction during the remainder of the tidal cycle. When tidal currents are directed toward the land, water levels rise in harbors and rivers; these are called flood tides. Tidal currents flowing seaward with falling sea levels are called ebb tides. Between flood and ebb tides are slack water periods (little or no horizontal movement).

In some coastal areas where the tidal range is relatively large and the flood tide enters a narrow bay or channel, a tidal bore forms and moves upstream in a river or shallow estuary. A tidal bore is a wall of turbulent water, usually less than a meter in height. Tidal bores are well known at the mouth of the Amazon River in Brazil, on the Severn River in England, and in Turnagain Arm off Cook Inlet, Alaska.

Adapted from DataStreme Ocean and
used with permission of the
American Meteorological Society.