River tide

Article By:

Gartner, Jeffrey W. Water Resources Discipline, U.S. Geological Survey, Tucson, Arizona.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.591200

A tide that occurs in a river with an open connection to a tidal estuary or ocean. River tides are highly modified from coastal ocean tides as they propagate landward into shallow water, often through a brackish estuary and then into a freshwater river. Ocean tides form in response to the gravitational forces exerted by the Sun and Moon acting on the water masses on the surface of the rotating Earth. Within estuaries or rivers, water masses are small enough that tidal motions are not generated directly from lunar and solar forces; rather, the motions occur because of tide-generating forces at the estuary entrance or river mouth. However, variations in the tide-generating forces in a coastal ocean produce corresponding variations in tides within the upland estuary/river system. In addition to the well-known diurnal or semidiurnal occurrences of high and low waters, numerous other important cycles occur that are necessary to describe tidal oscillations in estuaries and tidal rivers. Tides have their greatest amplitude when the Sun, Moon and Earth are in alignment during full and new moon and the tidal forces of the Moon and Sun reinforce each other. These tides are referred to as spring tides. Alternatively, neap tides occur when the moon is in quadrature (the solar and lunar forces oppose rather than reinforce each other) and tidal amplitudes are much smaller; that is, higher low tides and lower high tides occur during neap periods. In addition to the spring/neap cycle that has an approximately fifteen-day period, many other astronomical cycles occur. Two of the more important cycles are the ones responsible for diurnal inequalities between the heights of the two high and two low waters experienced daily in some locations and the effect on tidal amplitude that results from the moon's varying distance from earth. See also: Estuarine oceanography; River; Tide

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