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Viral tweet incorrectly identifies clearest lake in the world

A viral post claimed Montana’s Flathead Lake is the clearest lake in the world. That’s false. These are the clearest lakes in the U.S. and world.

In early July, a Twitter account devoted to science and nature posts tweeted out a picture that it said was taken from Flathead Lake, Montana, which it claimed had “the clearest water on Earth.” The tweet has since been liked about 400,000 times.

Many of the most popular replies were from people either posting their own favorite clear body of water, or debating the page’s claim with suggestions that somewhere else had the clearest water on Earth.


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



This is false.

No, Flathead Lake does not have the clearest water of any lake on Earth. Its water isn’t even the clearest among American lakes.

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

The U.S. lakes confirmed 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 states, federal agencies and tribes 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 every lake in the U.S., and includes data on water clarity.

Scientists research water clarity using an object called a Secchi disk. Water clarity is measured by dropping a Secchi disk 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 it disappears from view at the surface is called a water body’s Secchi depth, and is how water clarity is measured. A similar method is used with an all-black disk to measure water clarity horizontally in shallower bodies of water.

Many states and nonprofit organizations also conduct their own Secchi depth reviews separate from the EPA survey. Secchi depth results can be variable based on the available light and day-to-day changes in the water clarity, so measurements are typically recorded based on averages.

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

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Flathead Lake

The EPA’s latest lake survey from 2017 found that Flathead Lake has a Secchi depth of nearly 55 feet. According to Flathead Lakers, a nonprofit that works to protect the water quality of the Flathead watershed, Secchi disk readings at Flathead Lake from 1977 to 2018 were typically between 30 and 50 feet.

It is the largest natural freshwater lake west of the Mississippi in the lower 48 states. It’s located within a glacier-carved valley of the Rocky Mountains, and the lake’s southern half is inside of the Confederated Salish and Kootenai Tribes Flathead Reservation.

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

Credit: Frank - stock.adobe.com
Flathead Lake in Montana.

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Clearest lake in the world:

Blue Lake

The clearest lake on earth is thought to be New Zealand’s Blue Lake. Although it’s not deep enough to record a Secchi depth vertically, research by New Zealand’s National Institute of Water and Atmospheric Research (NIWA) found that the horizontal clarity of the country’s Blue Lake is typically between 230 and 262 feet. 

NALMS says the theoretical maximum Secchi depth, which would be recorded in absolutely pure water, is 230 to 262 feet. A 260 foot depth was recorded in the ocean near Antarctica, and a 216 foot depth was recorded in the Sargasso Sea, which is a part of the Atlantic Ocean off the southeast coast of the United States.

That means, at least when looking through the water horizontally, the lake is as clear as scientists believe it’s possible for water to be.

"The theoretical visibility in distilled water is about 262 feet, as estimated from the best available instrumental measurements in the laboratory," said Rob Davies-Colley, a scientist who participated in the research. "So Blue Lake is a close approach to optically pure water.”

Blue Lake, also called Lake Rotomairewhenua, is located in Nelson Lakes National Park within the country’s Southern Alps. To reach it requires a four to seven day hike from the village of St. Arnaud, the region’s development agency says, and the lake must be treated with care because it is sacred to the local Māori people.

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

Credit: www.nelsontasman.nz
Blue Lake (Lake Rotomairewhenua) in New Zealand.

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Clearest lakes in the U.S.: 

Snow Lake

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. There isn’t available data on Snow Lake’s average Secchi depth. 

It’s a small lake at an altitude of over 4,000 feet in the Cascade Mountains, and is located just within the eastern boundary of King County, where Seattle is located.

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

Credit: kellyvandellen - stock.adobe.com
Snow Lake in Washington state, Mount Rainier National Park

Crater Lake

Crater Lake is well-known as one of the clearest lakes in the U.S., although it was not part of the EPA’s most recent survey in 2017.

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

The lake, which formed within the crater of a collapsed volcano, has the record for deepest Secchi depth ever recorded within a lake, depending on the source. According to NALMS, a Secchi depth of 144 feet recorded at Crater Lake is the record holder. This record would not include lakes measured horizontally, such as Blue Lake.

Crater Lake’s clarity is credited to both a lack of mineral and sediment deposits, and a lack of organic material like algae. The U.S. Department of the Interior says the lake’s water comes directly from snow or rain and there are no inlets to feed the lake, which means there’s nothing to carry minerals and sediments into the lake. The Crater Lake Institute says there is even less organic material absorption near the surface of the lake than there is in some of the clearest, cleanest ocean waters.

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

Credit: aiisha - stock.adobe.com
Crater Lake National Park in autumn.

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Waldo Lake

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

DEQ gives Waldo Lake the edge over Crater Lake for the Secchi depth record, stating 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 altitude of 5,400 feet. It’s within the Willamette National Forest, north of Crater Lake, between Eugene and Bend.

Credit: mjlennonjr - stock.adobe.com
Waldo Lake in Oregon.

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Charlton Lake

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 in the survey. 

Charlton Lake is much like Snow Lake in that it is a small lake more well-known for nearby hiking trails than it is for water clarity. There is no average Secchi depth data available for Charlton Lake.

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

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

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