Montana’s Flathead Lake not clearest water in world

A viral post claimed that Flathead Lake in Montana is the clearest lake in the world. That’s wrong. These are the clearest lakes in the US and the world.

In early July, a Twitter account dedicated to science and nature tweeted a Image allegedly taken from Flathead Lake, Montana, which has been said to have “the clearest water on earth.” The tweet has since been liked around 400,000 times.

Many of the most popular responses have come from people either posting their own favorite clear body of water or debating the suggestion site’s claim that the clearest water on earth is somewhere else.


Does Flathead Lake, Montana have the clearest water on earth?



That's wrong.

No, Flathead Lake doesn’t have the clearest water of any lake on earth. Its water isn’t even the clearest of America’s lakes.

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There are at least four lakes in the US that are clearer than Flathead Lake. The clearest lake in the world is Blue Lake in New Zealand, which researchers say has visibility similar to pure water.

The US lakes that are clearer than Flathead Lake are:

  • Crater Lake in Oregon
  • Waldo Lake in Oregon
  • Snow Lake in Washington State
  • Charlton Lake in Oregon

Every few years, the Environmental Protection Agency (EPA) works with state, federal, and tribal agencies to survey the water quality of American lakes and streams. The most recent survey, from 2017, collected data on over 5,000 lakes, which is still not all lakes in the US, and includes water clarity data.

Scientists study water clarity using an object called the Secchi disk. Water clarity is measured by dropping a Secchi disc into the water. The North American Lake Management Society (NALMS) describes a secchi disk as an 8-inch disk with alternating black and white quadrants. The point at which it disappears from view on the surface is called the Secchi depth of a body of water and is the measure of water clarity. A similar method is used with an all-black disc to measure water clarity horizontally in shallower bodies of water.

Many states and nonprofit organizations also conduct their own in-depth Secchi audits independently of the EPA survey. Secchi depth results can be variable depending on available light and daily changes in water clarity, so measurements are usually recorded based on averages.

The average secchi depth measured in American lakes is less than 11 feet, according to NALMS.

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The EPA’s most recent lake survey in 2017 found that Flathead Lake has a Secchi depth of almost 55 feet. According to Flathead Lakers, a nonprofit organization dedicated to protecting the water quality of the Flathead watershed, Secchi disc readings at Flathead Lake typically ranged from 30 to 50 feet from 1977 through 2018.

It is the largest natural freshwater lake west of the Mississippi River in the lower 48 states. It lies in a glacier-carved valley of the Rocky Mountains, and the southern half of the lake is within the Flathead Reservation of the Confederate Salish and Kootenai tribes.

Flathead Lakers say the lake is so clear because it’s relatively low in nutrients, phosphorus, and nitrogen, which promote algae growth.

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Clearest lake in the world:

The clearest lake on earth is considered New Zealand’s Blue Lake. While not deep enough to record a secchi depth vertically, research from New Zealand’s National Institute of Water and Atmospheric Research (NIWA) has found that the country’s Blue Lake’s horizontal clarity typically ranges from 230 to 262 feet.

According to NALMS, the theoretical maximum secchi depth that would be measured in absolutely pure water is 230 to 262 feet. A depth of 260 feet has been measured in the ocean near Antarctica, and a depth of 216 feet has been measured in the Sargasso Sea, which is part of the Atlantic Ocean off the southeast coast of the United States.

That means, at least when looking horizontally through the water, the lake is about as clear as scientists think water can be.

“Theoretical visibility in distilled water is about 262 feet, as estimated from the best laboratory instrumental measurements available,” said Rob Davies-Colley, a scientist who was involved in the research. “So Blue Lake is a close approximation of optically pure water.”

Also known as Lake Rotomairewhenua, Blue Lake is located in Nelson Lakes National Park in the country’s Southern Alps. Reaching it requires a four to seven-day trek from the village of St Arnaud, says the region’s development agency, and the lake must be treated with care as it is sacred to local Māori.

“Please respect the people of Ngāti Apa ki te Rā Tō and the purity of the water in this sacred place by not washing yourself, your clothes or your dishes in the lake,” says the Nelson Regional Development Agency.

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Clearest lakes in the US:

Snow Lake in Washington state was the clearest lake surveyed by the EPA in 2017, although its recorded Secchi depth was not as deep as the average Secchi depth of two Oregon lakes. Snow Lake mean Secchi depth data is not available.

It is a small lake at over 4,000 feet in the Cascade Mountains and is just inside the eastern border of King County, where Seattle is located.

The lake is more popular for its nearby hiking trails and fishing than for the clarity of its water.

Crater Lake is known as one of the clearest lakes in the United States, although it wasn’t part of the EPA’s most recent investigation in 2017.

The Oregon Department of Environmental Quality (DEQ) determined that the average Secchi depth at Crater Lake from 1978 to 2019 was about 102 feet. Some Secchi disk measurements at Crater Lake have exceeded 130 feet, DEQ data shows.

The lake, formed in the crater of a collapsed volcano, has the record for the deepest Secchi depth ever measured in a lake, according to sources. A secchi depth of 144 feet recorded at Crater Lake is the record holder, according to NALMS. This record would not include horizontally measured lakes like Blue Lake.

Crater Lake’s clarity is attributed to both the lack of mineral and sedimentary deposits and the lack of organic material such as algae. The US Department of the Interior says the lake’s water comes directly from snow or rain and there are no tributaries to feed the lake, meaning there is nothing to carry minerals and sediment into the lake. The Crater Lake Institute says even less organic matter is absorbed near the surface of the lake than in some of the clearest, cleanest ocean waters.

The lake is in the Cascade Mountains at the southern end of Oregon, south of Eugene and Bend and north of Medford. It is at an elevation of over 6,000 feet – more than a mile high.

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The 2017 EPA survey measured a secchi depth of just over 70 feet at Waldo Lake, but the lake’s average secchi depth from 1986 to 2019 was about 107 feet, according to the DEQ.

DEQ gives Waldo Lake the edge over Crater Lake for the Secchi depth record, stating that a 1938 measurement recorded a Secchi depth of 157 feet at Waldo Lake.

Waldo Lake is also in the Cascade Mountains and sits at an elevation of 5,400 feet. It is located in the Willamette National Forest, north of Crater Lake, between Eugene and Bend.

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Charlton Lake is also an Oregon lake. The 2017 EPA survey recorded a Secchi depth of 64 feet at Charlton Lake, the third deepest Secchi reading on the survey.

Charlton Lake is similar to Snow Lake in that it’s a small lake better known for nearby hiking trails than its clear waters. Secchi depth average data is not available for Charlton Lake.

It is less than two miles east of Waldo Lake, still at the southern end of the Cascade Mountains. It sits slightly higher than Waldo Lake at 5,692 feet, meaning all three Oregon lakes listed are over a mile tall.

There are no year-round surface currents that flow into Charlton Lake, says the Atlas of Oregon Lakes; Water instead flows into the lake from snowmelt runoff and intermittent, temporary streams.

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