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King tide is a colloquial term for an especially high tide, such as a perigean spring tide. "King tide" is not a scientific term, nor is it used in a scientific context. Use of the term "king tide" originated in Australia, New Zealand and other Pacific nations to refer to an especially high tide that occurs only a few times per year. The term has now come to be used in British Columbia and the United States as well.
King tides are simply the very highest tides. Conversely, the low tides that occur at this time are the very lowest tides. They are naturally occurring, predictable events.
Tides are actually the movement of water across Earth's surface caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of Earth which manifest in the local rise and fall of sea levels. Tides are driven by the relative positions of the Earth, Moon, and Sun, the elliptical orbits of the celestial bodies, land formations, and relative location on Earth. In the lunar month, the highest tides occur roughly every 14 days, at the new and full moons, when the gravitational pull of the Moon and the Sun are in alignment. These highest tides in the lunar cycle are called spring tides.
The proximity of the Moon in relation to Earth and Earth in relation to the Sun also has an effect on tidal ranges. The Moon moves around Earth in an elliptic orbit that takes about 29 days to complete. The gravitational force is greatest when the Moon is at perigee — closest to Earth — and least when it is at apogee — farthest from Earth — about two weeks after perigee. The Moon has a larger effect on the tides than the Sun, but the Sun’s position also has an influence on the tides. Earth moves around the Sun in an elliptic orbit that takes a little over 365 days to complete. Its gravitational force is greatest when the Earth is at perihelion — closest to the Sun in early January — and least when the Earth is at aphelion — farthest from the Sun in early July.
The king tides occur when the Earth, Moon and Sun are aligned at perigee and perihelion, resulting in the largest tidal range seen over the course of a year. So, tides are enhanced when the Earth is closest to the Sun around January 2 of each year. They are reduced when it is furthest from the Sun, around July 2. 
The predicted heights of a king tide can be further augmented by local weather patterns and ocean conditions.
- Tidal Variations — The Influence of Position and Distance NOAA ocean service education
- oceanservice.noaa.gov — NOAA National Ocean Service
- Tides and water levels — NOAA tides tutorial
- King tides photo initiative — British Columbia, Canada
- King tides photo initiative — Washington State, USA
- King tides photo initiative — Oregon, USA
- King tides photo initiative — Long Island Sound, USA
- King tides photo initiative — Barnegat Bay, USA
- King tides photo initiative — Australia
- King tides photo initiative — New South Wales, Australia
- www.thesinkingoftuvalu.com — King Tide / The Sinking of Tuvalu