The rise and fall of the sea level is called a tide. The tide is caused by a giant wave that is thousands of kilometers long. As the crest of this wave approaches the shore, the sea level rises, causing high tide. As the trough of the wave approaches the shore, the sea level falls, causing low tide.
Tides are caused by the interaction of gravity between the Earth and the moon. The moon’s gravity exerts a strong pull on the Earth and the oceans respond to this pull. This pull results in high tide. As the earth rotates, the high tide travels across the ocean. The sun also affects the Earth’s tides. During spring tides, the earth, moon, and sun are in alignment. This causes the high tides to become higher and the low tides become lower. During neap tides, the sun, moon, and earth form a right angle, causing the high tides to become lower and the low tides are a bit higher.
As the Earth rotates, different locations on the Earth’s surface pass through the high and low positions. A regular pattern of one high and one low tide each day is a diurnal tide. Some coasts such as the Atlantic and Pacific coasts experience two high tides and two low tides each day. This is known as a semi-diurnal tide. One high-tide-low-tide cycle takes 12 hours and 25 minutes. A cycle of two tides takes 24 hours and 50 minutes. Most shorelines have a tidal range between 1 to 2 meters. Other shorelines such as the Bay of Fundy in Nova Scotia have a tidal range as large as 15 meters.
In some areas, a rising tide will enter a shallow narrow river from a wide area of the sea and cause a wave called a tidal bore. A tidal bore can have a breaking crest or it can be a smooth wave. Tidal bores tend be found in places with large tidal ranges.