Examples of Lake Clarity Variation Over Time

Quote of the Day

Put down everything that comes into your head and then you’re a writer. But an author is one who can judge his own stuff’s worth, without pity, and destroy most of it.

— Sidonie-Gabrielle Colett


Figure 1: Pattern on Freshwater Secchi Disk.

Figure 1: Pattern on Freshwater Secchi Disk. (Source)

I am having my cabin built on a small lake in northern Minnesota. At the same time that the cabin is being built, a friend is in the process of locating an an existing cabin for purchase on a nearby lake – there are dozens of lakes within a few miles of my building site. He has been asking questions about the clarity of the water in these lakes. Fortunately, the state of Minnesota has an excellent web page with all sorts of technical data on lake water, including clarity measurements. Professional lake monitors are also used. On a regular basis, they gather technical information on the lakes: chemistry, fish populations, presence of invasive species, etc. The lake water clarity data historically has been measured using a Secchi disk (Figure 1) and volunteer lake monitors. In recent years, satellites have been tasked with clarity monitoring as well. So there are now two sources of lake clarity information that can be used to cross-check one another.

Lake clarity is important because it gives you an idea as to the health of the lake, and the lake's health has a major influence on the value of your property. For me, a healthy lake supports a significant fish population, has no algae blooms, and has no significant weed issues, etc. However, low clarity does not necessarily mean an unhealthy lake. For example, a bog is the source of water for my lake. This gives the water a tea-color (referred to as "bog stain"), which reduces its clarity. However, the lake itself is quite healthy. In summary, clarity is just one of a number of useful measures of lake health.

In this post, I will be focusing on the Secchi disk data gathered by the volunteer lake monitors over the years on three lakes in Itasca County. The data acquisition process is straightforward:

  • lower the Secchi disk into the water.
  • record the depth at which you lose sight of the disk.
  • send the data to the Minnesota Department of Natural Resources (MnDNR).
  • For each lake, the MnDNR generates a CSV file containing data from both their volunteer program and their professional lake monitors. The file can be downloaded via the Internet, which is how I have configured Excel.

Lake clarity is just one parameter that volunteers gather. Another important parameter is lake level. It turns out that lake level and clarity are somewhat related. If you have have a substantial amount of rain (i.e. very clean water), the lake clarity will improve and your lake level will also rise. One issue with the volunteer program is that there sometimes are gaps in the data because volunteers are not always reliable data gatherers. Overall, I have to say the data gathering program is a great success.

For those who are interested, my workbook is available here.


My friend is strongly considering the purchase of a cabin on Johnson Lake, and I want to help him gather information for making an intelligent choice. So I grabbed the CSV files for Johnson Lake and two other nearby lakes, Eagle and Ruby, so he would have a basis for comparison.

Here is how I view these lakes.

  • Ruby Lake

    Ruby is sometimes called a "gem" of a lake because of its water quality and the small number of cabins on the lake. When I first saw Ruby Lake 20 years ago, I was very impressed with its clarity – you could clearly see the bottom. It is also has limited boat traffic because there are not many cabins on the lake. One reasons for the small number of cabins is because Ruby Lakes abutts a national forest, which means that part of the lake will never have cabins on it.

  • Johnson Lake

    Johnson is a good general use lake. The lake is near a major county road,which means access is easy in both winter and summer. Of course, this has the downside of being somewhat noisy. It has a good-sized gamefish population for those who like to fish and is large enough for those who like watersports, like waterskiing.

  • Eagle Lake

    Eagle is a small lake that is bog-stained. One side of the lake can be accessed by a major county road (i.e. easy access) and the other side of the lake has relatively poor access. It is not considered a major fishing lake, though kids love to fish on it because it has large numbers of sunfish and crappies. However, this is nice for those of us who do not fish and like our lakes quiet (i.e. me). It is large enough for watersports, but most folks just cruise around on their pontoon boats. There are relatively few cabins on the lake because much of the shoreline is owned by a few families.

Figure 2 shows the Secchi disk data for all three lakes. We can make several observations about this data:

  • Johnson Lake (green rectangles in Figure 2) has had very stable lake clarity for a long period of time. This indicates that the lake's health is stable.
  • Ruby Lake (blue triangles in Figure 2) had very good clarity back in the early '90s. No one gathered Secchi data during the early 2000s, but recent data shows that the clarity has declined down to the level of Johnson Lake. I would like to know more about why this change has occurred.
  • Eagle Lake (red circles in Figure 2) has very limited data. The data we have shows that the clarity can be low but is highly variable. This data seems to correlate with Eagle's Lakes level. Eagle has a variable level because its outflow is regularly blocked by beaver dams.
Figure 2: Excel Chart of Clarity of Three Lakes Over Time.

Figure 2: Excel Chart of Clarity of Three Lakes Over Time.


I am not sure if my friend will buy a cabin on Johnson Lake. The data shows that the lake is stable and not undergoing any stress.

Just for fun, I also plotted the data using ggplot2 (Figure 3).

Figure 3: Same Data as Figure 2 Plotted Using ggplot2 and Default Settings.

Figure 3: Same Data as Figure 2 Plotted Using ggplot2 and Default Settings.

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