Do This Before You Let The Kids Swim
February 19, 2026
source: Flickr
How many times have you looked at a boat and seen a black and white circle attached to a rope? There’s no need to be an expert in reading a circle to understand what a Secchi disk represents. Yet for generations, Secchi disk water clarity tests have allowed us to “see” our lakes in ways we never thought possible. No Apps, No Screens, just a steady hand, a marked line and your eyes.
What a Secchi Disk Measures
A Secchi Disk measures the clarity of the water by determining the average of two measured depths (the depth where the disk disappears and the depth where it reappears). That is known as the Secchi Depth. This simple test is used in many lake monitoring programs. It is popular due to its low cost, ability to be repeated multiple times and relative ease of use.
An Old Idea that Still Works
The method was first proposed by Angelo Secchi in 1865, while he was conducting studies in sea water. He developed the method to measure the visibility of light through the water. As the years went by, the color and style of the disk evolved from a single color to a black and white disk with a marked quadrant. This change helps measure visibility under various lighting and water conditions. What is interesting is that although the basic method has been in use for over 100 years, the underlying principles have remained the same.
The Reason Why it Feels Like the Good Old Days
This method is a part of a simpler time and technology such as paper maps and hand tools. When using a Secchi Disk there are no “taps to measure.” Instead, you stand at the side of the boat and take notice of your surroundings. It is this aspect of the method that allows users to experience something that they may never have noticed before. For example, greenish looking water after a storm, hazy looking water during late summer or clearer looking water after a few days of calm weather.
Additionally, it encourages all to participate. Children can call out the measurements while adults assist by holding the rope steady. Regardless of who assists in the process, everyone seems to insist that the disk “still visible” for another inch. It becomes a fun little family activity every time the Secchi Disk is used.
What the Number Can Tell You
When the Secchi Disk measures shallow waters it indicates that the water is clouded with particles which block light and usually means high levels of algae, suspended sediments or post-storm runoff. On the other hand, when the Secchi Disk measures deeper waters it indicates that the water is clearer and that light is able to penetrate the water further. While a single measurement does not provide enough information to make conclusions, consistent readings can indicate changes in trends. They also indicate when a closer examination is warranted.
A Small Circle with a Big Memory
We tend to get nostalgic for the memories of past summers, rather than getting nostalgic for the data collected. Using a Secchi Disk to measure the clarity of water fits well within the picture of a warm summer day. The sun may be glinting off of the waves, the boat swaying slightly in the water and all members of the party leaning in to see when the disk will disappear from sight. It is very easy to do, uses a manual technique, and provides a quiet satisfaction. Therefore, the Secchi Disk is the type of “old days” tool that continues to earn its place in the boat.