When planning any boating trip, one of the first considerations in every passage plan is to take account of the tide.
If your passage includes a transit on a lake, however, things are a little different.
The tidal effect on a lake is so small that it can be ignored. Due to the relatively small size of lakes, the bulge of water created by the gravitational pull of the moon is simply too small to create a noticeable rise and fall.
Therefore, in the majority of cases, it is safe to say that there are no tides on lakes.
Although there are no tides, the water in a lake will still vary for a number of different reasons.
Most commonly, barometric pressure will cause the water level to rise and fall, tracking the change in pressure.
For example, if there is a high-pressure weather system, the water level will fall and if there is a low-pressure system it will rise.
The wind can also make it seem like there is tide on a lake. It will tend to blow the surface of the water towards one side of the lake, heaping it up in the same way as a tide.
Although the change in water level caused by barometric pressure and the wind can create a navigational challenge for boaters, it should not be treated the same as a tide.
Background Reading: Check out this article for a little more information: How Do Tides Affect Boats?
A tide is a predictable change in water level, caused by the gravitational attraction of the moon, and to a lesser extent, the sun.
According to that definition, the tides on a lake should be ignored, although they do still occur.
How do tides work on a lake?
Tides occur in any body of water that feels the gravitational attraction of the moon and the sun.
Under the influence of gravity, the water heaps up in the direction it is being pulled, creating a bulge.
When the bulge reaches a landmass, you get a high tide as the water level rises. At the other side of the ocean, where the water has been pulled away, you get a low tide.
The larger the body of water, the greater the tidal effect.
On a lake, the same tidal principles apply, just on a much smaller scale to those felt by the oceans.
Water is still pulled by the moon and the sun’s gravity towards one edge of the lake, creating a high tide.
Unlike the ocean, however, the typical size of a lake means that there is not the same mass of water being pulled, so the overall effect is reduced.
During a spring tide, the tidal effect is stronger but is still reduced in comparison to the tides on the ocean.
Find out more about spring tides in this article: What Is The Difference Between Spring And Neap Tides?
For example, the biggest lakes in the world might experience a 5cm tidal range during the biggest spring tides of the year.
The tidal range on a lake is so small that it is often overshadowed by other factors such as barometric pressure changes or the wind.
While the wind and pressure do not power the tide, they do power another phenomenon that is often mistaken for a tide. Seiches.
A lake tide might actually be a seiche
A seiche is a standing wave that forms in an enclosed body of water like a lake, harbour, swimming pool or even a small sea.
The wave is started by any effect that causes water to move: pressure changes; wind; landslides; tsunamis; etc.
As the wave travels across the surface of the enclosed body of water, it might continue to build or it might just keep running under its own energy until it reaches a boundary where it gets reflected back.
Assuming the original wave and the reflected wave are coincident, they will superimpose over each other, creating a standing wave.
It is the same principle as a standing wave on a piece of string that you will have seen in school science lessons.
When the wavelength of the standing wave is similar to the size of the body of water, the seiche acts like a tide.
First, let’s consider regular waves on a lake surface.
The image above shows a normal water surface with lots of small waves. Across the entire body of water, the surface appears flat.
If, however, the wavelength of the waves approaches the size of the water body, we get a seiche.
The peak at one side of the lake is part of the same wave that has its trough at the other side of the lake.
As the wave gets reflected off the side of the lake, the peak and trough will swap sides.
When the period of the wave is measured in hours, the seiche in a lake can be mistaken for a tide.
According to NOAA, on the Great Lakes, the time period between the high and low of a seiche may be between four and seven hours. This is so similar to an ocean tide that the two are often confused.
With a real tide, the period between successive high waters is around 12 hours, and the time between high water and low water is around 6 hours.
If any seiche on any body of water has a similar period, it is very easy to mistake it for a tide.
Do the Great Lakes have tides?
The Great Lakes have tides, but they are so small that the variation in water height is less than the typical fluctuations due to meteorological effects.
According to NOAA, the largest spring tides experienced on the Great Lakes are less than 5cm.
Of all the tides, spring tides have the greatest tidal range, so we know that all other tides will be even less perceivable.
This is why most people simply state that the Great Lakes do not experience tides.
Typical meteorological effects such as barometric pressure and the wind will act to change the height of water across the Great Lakes more than a spring tide will.
Other factors, such as rainfall and seiches further negate the small movement caused by the moon and the sun’s gravity.
It is technically correct to say that the Great Lakes experience tides, but from a navigational perspective, they are negligible.
Do ponds have tides?
As with all bodies of water, ponds experience tides.
Due to their small size, however, the tidal effects caused by the gravitational attraction of the moon and the sun are imperceivable.
With a pond, other factors such as rainfall and the wind will have a much greater impact than the tidal range.
Although it is technically correct to state that ponds do have tides, the reality is that for all practical purposes, the tides are negligible.