Clearest Lakes in Iowa — Water Quality Rankings (2026)

The single clearest lake in Iowa is Blue Pit Lake in Cerro Gordo County, with a Secchi depth of 17.1 feet — meaning a Secchi disk lowered from the surface disappears at that depth. Blue Pit Lake holds an overall water quality grade of A. The 25 clearest lakes in Iowa are ranked below by 2026 measurements.

How clear are Iowa's clearest lakes?

Across the 170 Iowa lakes with Secchi clarity readings in our 2026 dataset, the 25 clearest range from 4.8 to 17.1 feet of visibility. Blue Pit Lake sits at the top, 1.4 ft clearer than the next lake on the list, well above the 6.4-foot median of this leading group — a spread that shows how quickly clarity falls off once you move past a state's handful of truly pristine waters.

The clearest water is not spread evenly. Scott County alone accounts for 3 of the 25 top-ranked lakes, with the rest drawn from 19 other counties. That clustering follows the terrain: deep, glacially carved basins ringed by forest take in little sediment or fertilizer runoff, so phosphorus stays low and the water stays transparent.

Clarity and overall health usually align here: 18 of these 25 lakes also earn an A- or B-range overall grade. Where the two diverge — a clear lake carrying a weaker grade — the cause is typically elevated phosphorus that transparency alone can't reveal, which is why our grade combines phosphorus and chlorophyll-a with Secchi depth rather than relying on clarity by itself.

Rank	Lake	County	Secchi Depth	Grade	Max Depth	Area
1	Blue Pit Lake	Cerro Gordo	17.1 ft	A	-	-
2	West Okoboji Lake	Dickinson	15.7 ft	A	-	-
3	Terra Park Lake	Polk	11.5 ft	A	-	-
4	Nine Eagles Lake	Decatur	11 ft	A	-	59 acres
5	Yenruogis Pond	Keokuk	9.9 ft	A	-	-
6	Dale Maffitt Reservoir	Dallas	9.8 ft	A	-	230 acres
7	Slipbluff Lake	Decatur	9 ft	A	-	18 acres
8	Rudd Lake	Floyd	8.1 ft	A	-	-
9	Lambach Lake	Scott	7.9 ft	C	-	9 acres
10	Lake Wapello	Davis	7.5 ft	B	-	287 acres
11	Lacey Keosauqua Lake	Van Buren	7.1 ft	A	-	23 acres
12	Ada Hayden Heritage Park Lake	Story	6.8 ft	A	-	-
13	Sand Lake	Marshall	6.4 ft	A	-	-
14	Mormon Trail Lake	Adair	6.2 ft	B	-	33 acres
15	Poll Miller Park Lake	Lee	5.9 ft	B	-	15 acres
16	Little Sioux Park Lake	Woodbury	5.7 ft	A	-	-
17	Arrowhead Lake	Sac	5.6 ft	B	-	110 acres
18	Black Pit Lake	Cerro Gordo	5.6 ft	F	-	-
19	Big Spirit Lake	Dickinson	5.5 ft	C	-	-
20	Railroad Lake	Scott	5.1 ft	C	-	29 acres
21	Mariposa Lake	Jasper	5.1 ft	B	-	19 acres
22	Oldham Lake	Monona	5.1 ft	C	-	16 acres
23	Lake of the Hills	Scott	5.1 ft	C	-	657 acres
24	Kent Park Lake	Johnson	4.8 ft	B	-	29 acres
25	Cold Springs Lake	Cass	4.8 ft	D	-	16 acres

Where to stay near Iowa's clearest lakes

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Clearest lakes across all states →Cleanest lakes in Iowa (combined score) →All Iowa lakes →

Frequently Asked Questions

What is the clearest lake in Iowa?

Blue Pit Lake is the clearest lake in Iowa by Secchi depth, with water transparency of 17.1 feet. Secchi depth is measured by lowering a black-and-white disk until it disappears from view — deeper readings mean clearer water. Located in Cerro Gordo County, Blue Pit Lake has a water quality grade of A (Excellent).

How is water clarity measured?

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.

What makes a lake clear?

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 glacial lakes with rocky basins and forested watersheds — common in northern Iowa — tend to be clearest because they have little soil runoff and limited nutrient input. Shallow agricultural-watershed lakes are typically less clear because they receive more sediment and fertilizer runoff.

Is the clearest lake the cleanest?

Not always. Clarity (Secchi depth) is one of three indicators we use; the others are total phosphorus and chlorophyll-a. A lake can be very clear but still have elevated phosphorus that just hasn't bloomed yet, or chlorophyll-a from a recent bloom that's still settling. The most reliable measure of overall lake health is the combined grade, which weights all three signals. See our companion cleanest-lakes ranking for Iowa for the combined-score list.

How does Iowa compare to other states?

Iowa's clearest waters compete with the most pristine lakes in our 38-state dataset, where top-tier lakes typically register Secchi depths of 15–25 feet. Nationally, the gold standard is Crater Lake in Oregon (~100 ft) and Lake Tahoe (~65 ft).

Source: EPA National Aquatic Resource Surveys + state DNR Secchi sampling, 2026.