If the amount of water changes to vary its level there must be a flow back and forth between different areas;these flows are tidal currents. Water movements are little noticed on the high seas, but they become critical along coasts and in bays, estuaries, and rivers; particularly in fog or thick weather. The horizontal movement of the water toward the land is called flood current,and the horizontal movement away from the land is called ebb current. Between these two, when the current changes direction, there is a brief period when no horizontal motion can be detected; this is slack water.
Offshore, where the direction of flow under tidal influence is not restricted by any barriers, the tidal current is rotary; it flows continuously, with the direction changing through all points of the compass during the tidal period. Due to the effect of the earth's rotation, the change is clockwise in the northern hemisphere, and counter clockwise in the southern hemisphere. The speed usually varies throughout the tidal cycle,passing through two maximuns in opposite directions and two minimums. One thing about rotary currents is that it has no time of slack water. In rivers, bays, and straits, where the direction of flow is more or less restricted to certain channels, the tidal current is called a reversing current; that it flows alternately in opposite directions, with a short period of little or no current, called slack water, at each reversal of the current. During the flow in each direction, the speed varies from zero at the time of slack water to the maximum flood or ebb about midway between the slacks. The reversing currents is affected in certain areas by the land and/or the natural flow of a river. In the navigation of vessels, it happens at times that the effect of tidal current is of more importance than the height of tide; in any body of water subject to tidal action, both must be considered. Mariners will sometimes speak of "the tide" as ebbing or flooding; this is incorrect terminology and should not be used.
Along a straight coast with shallow indentations there is usually little difference between the time of slack water and high or low tide, but where a large bay connects with the ocean through a narrow channel, the tide and current may be out of phase by as much as seven hours.
The effect of the tide in causing currents may be thought of by two things:
Where there is a small tidal basin connected with the sea by a large opening.
Where there is a large tidal basin connected with the sea by a small opening.
In the first case, the velocity of the current in the opening has its maximum value when the height of the tide within is changing at a time about midway between high and low water. The water in the basin keeps at approximately the same level as the water outside. The flood current corresponds with the rising of the tide and the ebb current with the falling of the tide.
In the second case, the velocity of the current in the opening has its maximum value when it is high water or low water outside the basin, for then there is the greatest difference in levels, and the maximum for producing motion. The flood current generally begins about three hours after low water and the ebb current about three hours after high water, slack water occurring about midway between the tides.
Along most shores not much affected by bays, tidal rivers, ete., the current usually turns soon after high water and low water.
The swiftest current in straight portions of tidal rivers is usually in the middle of the river, but in curved portions the most rapid current is toward the outer edge of the curve, and here the deepest water will generally be found.
