The vertical rise and fall of the tides — driven by the gravitational forces of the moon and sun acting on the oceans’ waters — also causes motion of water called currents within bays, harbors and estuaries.
In general, as the tides rise there will be a current flowing from the oceans into the bays, harbors and estuaries; this is termed a flood current. As the tides fall there will be a current flowing towards the oceans; this is called an ebb current.
There are also periods when there is little or no horizontal motion of the water. This is called slack water.
Most of us boaters and people who live along the coast know this basic stuff, but what we really want to know is the speed and direction of current at a given time of day when we will be recreating, whether it be swimming, surfing, boating or fishing.
Problem is, Ma Nature complicates the matter like a mixer complicates a bowl of batter. In the ocean, the metaphorical mixer is wind, which drives currents that can be at odds with tidal currents.
Direction of currents are also warped as water flows around points of land or encounters prevailing gyres, such as the Santa Barbara Channel Gyre, which circulates water around and around within the channel. Wind also pushes upwelling which is like a vertical current.
Many professional and recreational users of tide and tidal current information have a rule of thumb to assume a relationship between the times of high/low tides and the times of the currents. That the times of slack water will be at the same time as the high and low tides, and that the flood and ebb current will occur between the high and low tides.
Unfortunately, this assumed “rule of thumb” does not hold for all locations.
The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three base case conditions, according to the oceanic current gurus at National Oceanic and Atmospheric Administration (NOAA).
The first is a standing wave type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid-way between the high and low tides.
The second is a progressive wave current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide.
The third case is a hydraulic current. In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same.
Progressive currents are most common at the oceanic entrance to many bays and harbors. Standing wave conditions are most common at the head (most inland point) of larger bays and harbors.
Most areas of the coast will fall somewhere in between a progressive and standing wave current. The exact relationship between the times of high and low tides and the maximum current or slack water is unique to each location and cannot be determined from a generic rule of thumb, sadly.
Because horizontal tidal currents are created by the same forces which cause vertical tides, the currents can be predicted in much the same way as the tides but remember that there are other factors (such as wind) that drive currents.
Observational data on the currents at a location can be analyzed using the same methods employed to analyze tides, and the results of that analysis can be used to generate predictions of tidal currents.
- Tide predictions provide the times and heights of the tides.
- Tidal current predictions provide the times and speed of maximum current and times of slack water.
That is what NOAA does daily. However, because the relationship between tides and tidal currents is unique to each location, tide predictions and tidal current predictions are generated separately:
All this book-learnin’ is great, but after my lengthy lifetime of trying to understand this stuff and predict the currents to catch more fish and keep my passengers and crew safe, I can pretty much guarantee that you, me and the professional predictors will all get it wrong quite often.
That is why all savvy skippers coming out of harbor look first at the sea state and wind and then look for buoys, kelp paddies and other things that will tell us the direction and the approximate velocity of the current. We use that information to re-evaluate our fishing game plan and take advantage of what we see.