Clearest Lake in Montana: Top 25 by Secchi Depth

Big Therriault Lake in Lincoln County is the clearest lake in Montana by Secchi depth, at 37.3 ft. Secchi depth is measured by lowering a black-and-white disk until it disappears from view; a deeper reading means clearer water. Big Therriault Lake is not rated: the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity.

They span 17.8 ft to 37.3 ft of Secchi depth, with a median of 21.1 ft. Big Therriault Lake leads by 4.2 ft. Flathead County has 13 of the 25; the rest are spread across 3 other counties. 3 of the 25 carry an A–F grade; the other 22 are not rated because their recent sampling does not support a letter.

Water clarity is measured by Secchi depth, named after the 19th-century Italian astronomer Pietro Angelo Secchi who developed the method. A standardized black-and-white disk is lowered into the water on a rope; the depth (in feet or meters) at which the disk disappears from view is recorded. Greater Secchi depth means clearer water, usually a sign of low sediment, low phosphorus, and limited algae growth. The figure for each lake here is the median of its readings.

Lake clarity is driven by three factors: low suspended sediment (clay, silt), low nutrient load (especially phosphorus, which fuels algae), and low chlorophyll-a (active algae biomass). Deep lakes with rocky basins and forested watersheds tend to be clearest because they take in little soil runoff and few nutrients. Shallow lakes in farmed watersheds are typically less clear because they receive more sediment and fertilizer runoff.

Not always. Clarity (Secchi depth) is measured and shown; the letter grades nutrients and algae (total phosphorus and chlorophyll-a) against that lake’s EPA ecoregion classes. A lake can be very clear but still have elevated phosphorus that just hasn't bloomed yet. See our companion cleanest-lakes ranking for Montana for the nutrient-and-algae list.