Tidal currents occur when water flows in and out of bays or rivers due to the rise and fall of ocean levels. The sun and moon create tides due to the gravitational pull, which is caused by the mass of the planets. Their gravity causes the ocean's water to be pulled toward the planets as the moon and Earth move around the sun. Since the sun and moon are in different locations each day, there are typically two high and low tides on a daily basis.
As the tide rises, called a flood tide, the height of the rising water will cause a flow into bays, rivers or other lower areas. When the tide effect reaches a maximum height, this is called slack tide, and there is no movement of water in either direction. The tide will then begin to lower, or ebb, and the tidal currents will be outward toward the ocean until the low tide occurs, when another slack tide occurs.
These tidal changes occur about every six hours, moving from flood to slack to ebb, but also vary due to the position of sun and moon. When the sun and moon line up on the same or opposite sides of the Earth, the gravitational forces are stronger. The high tide will be a greater height than at other times, and is called a spring tide. Spring tides can be predicted from looking at the moon at night, because they occur at the same time as a full or new moon.
At times, the sun and moon create a lower tide than normal, which is called a neap tide. They occur when the moon is at first or third quarter phase, and the moon and sun are at an angle to each other, reducing the gravitational effect. These moon phases occur halfway between the new and full moons.
Historically, sailing ships required high ebb tides to leave ports of call to permit the tidal currents to carry the ship to sea. Navigators used the moon phases to predict best times of arrival and departure into ports. This was particularly important in rivers or bays affected by tides. Sailing ships did not have engines before the steam engine was invented, and could not not navigate in narrow rivers or in zero wind conditions. They relied on the tides to enter and depart ports of call using currents alone.
Calculations of tidal currents are complex due to the interaction of the sun and moon. In addition, the amount of tidal flow can vary greatly from place to place due to the shape of the river or bay inlet and other factors. Various government and private groups publish tidal tables for many locations around the Earth, allowing mariners access to information that can prevent groundings or low water ship damage.
Beginning in the 20th century, there was a growing interest in using tidal currents to generate electricity. Some areas of the world have very high tidal flow rates, and installing generators under the water might create electricity except at slack tides. The advantages of using tidal currents include not having to build dams or interrupt movement of ships and wildlife. Research was continuing into the 21st century to develop commercial systems that could generate enough electricity to make them useful.