There are several different kinds of currents including oceanic, river, and wind-driven; all with their own driving force. This page addresses only the tidal currents.
As mentioned before, tidal currents (a horizontal motion) are a result of the rise and fall of the water level due to tides (a vertical motion). The effects of tidal currents on the movement of water in and out of bays and harbors can be substantial.
A Small Primer on Water
We are so familiar with water that we tend to forget some of its basic physical properties.
Water is a viscous fluid and exactly how water flows is a function of its viscosity. No matter how you move water around, it will always take time to move any distance due to its own viscosity, or the interaction of its viscosity with its surroundings.
As an illustration of the effects of the viscosity of water, consider this: No matter how fast you pour out a bucket of water, it will always take some amount of time to empty the bucket. Always.
Now apply this idea to the volume of water in a bay or river!
An Illustration to Bring It All Together
Imagine a large, long, narrow bay on the coast. We position one person on the ship anchored at the opening to the sea (lower right) and another at the distant white light, a point on the bay as far from the sea as he can get. We assume the tide is low and there are no tidal currents in the bay.
The tide comes in and reaches high tide at 11 am so the person at the mouth of the bay reports high tide at 11 am. Meanwhile the person inland is still watching the water level rise until, at 1 pm, he announces high tide where he is. That's a difference of two hours between high tide in the two locations.
Let's look at what actually happens throughout the cycle. As the tide comes in, the water entering the bay has to overcome slow water to move forward into the bay (viscosity) so this change is not seen at the other end of the bay immediately. The tidal currents in the bay are now in flood flow.
When the tide is highest at the entrance of the bay, the tide is at high stand in that location, but there is still a flood flow into the bay because the high stand has not been reached further into the bay yet. A while later, half way into the bay (the red light), the water also reaches its high stand, but there's still a flood flow because the high stand has not yet been reached further in.
Finally the high stand is reached all the way inside the bay at the white light and the current stops. It doesn't reverse; it stops. This is called slack water. Even though the tide may have started going out at the bay's entrance, the current in the bay stops, like a ball that has been thrown up in the air stops at the apex of its flight before falling back to earth.
As the tide starts going out, the same thing happens in reverse. The water level once again changes first at the bay's entrance while the water further in the bay may still be at high stand. The current in the bay, though, is now in ebb flow.
When the ocean is at low tide at the entrance of the bay, the water is at its low stand. Further into the bay, low stand has not yet been reached so the ebb flow continues. Finally low stand is reached all the way inside the bay and once again slack water occurs in the bay.
To summarize, we can list the sequence of events at any point in the bay, but the time at which these events occur will be different between any two points at different distances from the sea. The sequence is as follows (starting at low tide):
The same applies to rivers flowing into the sea, but with some important differences. The water flowing from the river will tend to hinder the movement of water into the river, hence causing the flood current to be less swift. On the other hand, the ebb flow currents can be extremely swift because water leaving the river at low tide is augmented by water flowing from the river. Add to that the possibility of rain and/or snow runoff inland that has caused the river to swell, and ebb currents can be even faster.
In some waters, even the maximum current is so swift that less powerful boats must wait for slack water to navigate them.