Water quality

How to read your lake’s water clarity

October 24, 2026 · Eric Barrett

Last reviewed: October 24, 2026. Public data sources and links change; verify on the linked MPCA and DNR pages.

Almost every lake association board gets asked the same question at the annual meeting: “Is the lake getting better or worse?” Water clarity is one of the simplest ways to answer it, and the data is free and public. This post explains where the numbers come from, how to read them without overreacting, and what to do when your lake doesn’t have much data.

What a Secchi reading is

A Secchi disk is an 8-inch white disk on a marked rope. A volunteer lowers it over the side of a boat until it just disappears, then notes the depth. That depth is the Secchi reading. A deeper reading means clearer water.

In Minnesota lakes, lower clarity usually means more algae in the water, and algae growth is tied to nutrients like phosphorus. So clarity is a useful early signal, even though it doesn’t tell the whole story on its own.

Where the numbers come from

Volunteers. For more than 50 years, volunteers in the MPCA’s Volunteer Water Monitoring Program (long known as the Citizen Lake Monitoring Program, or CLMP) have taken Secchi readings at least twice a month from May through September. For many lakes, those volunteer readings are the only long-term data that exists. You can look up your lake’s volunteer station and its reports on the MPCA’s volunteer monitoring search.

State and local agencies. The MPCA, counties, soil and water conservation districts and others also collect samples. That data shows up in the MPCA’s surface water data search, sometimes along with phosphorus and chlorophyll results.

Satellites. The University of Minnesota uses satellite images to estimate late-summer clarity for more than 10,000 Minnesota lakes, going back to the 1970s and 1980s. You can see these estimates in the Minnesota LakeBrowser. DNR LakeFinder pulls together MPCA clarity data and links to the satellite-based estimates, so it’s a good first stop.

How to read a trend without overreacting

A few habits make clarity data much more useful:

• Look at summer averages, not single readings. One murky reading after a storm or during an algae bloom doesn’t mean much. A pattern over many summers does.
• Compare like with like. Clarity naturally drops as summer goes on. A July reading should be compared with other July readings.
• Look for 10 years or more. Short records bounce around. A real trend usually takes many seasons to show.
• Remember that “clear” isn’t always “healthy.” Zebra mussels filter the water and can make a lake clearer while changing it in other ways. Clarity is one piece of the picture.
• Know your lake’s type. Shallow lakes and lakes with tea-colored water from wetlands naturally read lower than deep, clear northern lakes. Compare your lake with its own history first.

Why older readings still matter

Board members sometimes dismiss readings from the 1990s as “too old to count.” They’re the opposite. Older readings are your baseline, the only way to know what “normal” looked like for your lake. If a volunteer stopped monitoring years ago, those readings are still valuable, and restarting monitoring now adds to that record.

When your lake doesn’t have much data

Plenty of lakes have gaps: a few years of volunteer readings, a long break, then nothing. Satellite estimates can help fill in, since they cover most lakes of 20 acres or more. They’re estimates from satellite images taken on clear days, though, so treat them as a rough guide rather than a measurement from your boat.

If your lake has gaps, the most useful thing your board can do is find a volunteer. The MPCA provides the Secchi disk and rope, and anyone with a boat can do it.

What a Free Lake Snapshot shows

Our Free Lake Snapshot is a one-page summary of public MPCA and DNR information on your lake. Where the data allows, it includes:

• Water clarity readings over time, with volunteer and satellite data noted separately
• A plain-language read on whether clarity looks steady, improving or declining
• Invasive species listed for your lake
• What data is missing or out of date

Some lakes have limited or older public data. When that happens, the snapshot says so plainly. It’s meant to give your board a clear starting point, not a verdict.

If you’d like one for your lake, you can request it on our website or email [email protected]. There’s no commitment.

Sources

• MPCA, Volunteer water monitoring (Citizen Lake Monitoring Program)
• MPCA, Volunteer water monitoring stations search (look up your lake’s volunteer data)
• MPCA, Lake water quality monitoring
• MPCA, Surface water data search
• Minnesota DNR, LakeFinder (clarity data, lake reports and links to satellite clarity)
• University of Minnesota, Minnesota LakeBrowser (satellite-based lake clarity)
• University of Minnesota, Remote Sensing of Water Resources, Lake clarity