Clearest Lake in Colorado 2026: Rampart Reservoir by Secchi

The single clearest lake in Colorado is Rampart Reservoir in El Paso County, with a Secchi depth of 28.5 feet — meaning a Secchi disk lowered from the surface disappears at that depth. Rampart Reservoir holds an overall water quality grade of C. The 25 clearest lakes in Colorado are ranked below by 2026 measurements.

How clear are Colorado's clearest lakes?

Across the 71 Colorado lakes with Secchi clarity readings in our 2026 dataset, the 25 clearest range from 8.2 to 28.5 feet of visibility. Rampart Reservoir sits at the top, 8 ft clearer than the next lake on the list, well above the 13.8-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. Lake County alone accounts for 3 of the 25 top-ranked lakes, with the rest drawn from 16 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: 3 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 or chlorophyll-a that transparency alone can't reveal. The letter grades nutrients and algae; Secchi is shown, not scored.

Rank	Lake	County	Secchi Depth	Grade	Max Depth	Area
1	Rampart Reservoir	El Paso	28.5 ft	C	-	300 acres
2	Standley Lake	Jefferson	20.5 ft	NR	-	1,222 acres
3	Lake Nighthorse	La Plata	18.7 ft	C	-	1,548 acres
4	Island Lake	Delta	16.7 ft	C	-	161 acres
5	Turquoise Reservoir	Lake	16.1 ft	D	-	1,788 acres
6	Crystal Reservoir	Montrose	15.4 ft	A	-	301 acres
7	Antero Reservoir	Park	14.8 ft	C	-	3,446 acres
8	Mountain Home Reservoir	Costilla	14.4 ft	NR	-	597 acres
9	Blue Mesa Reservoir W of Steuben Creek	Gunnison	14.1 ft	NR	-	-
10	Pearl Lake	Routt	14.1 ft	C	-	166 acres
11	Twin Lakes East	Lake	14.1 ft	C	-	-
12	Haviland Lake	La Plata	13.8 ft	D	-	61 acres
13	Lake John	Jackson	13.8 ft	A	-	612 acres
14	Elevenmile Reservoir	Park	13.1 ft	D	-	-
15	Skaguay Reservoir	Teller	12.5 ft	C	-	88 acres
16	Summit Lake	Jackson	12.1 ft	NR	-	-
17	Dillon Reservoir	Summit	11.6 ft	D	-	3,300 acres
18	Blue Mesa Reservoir	Gunnison	11.5 ft	D	-	-
19	Twin Lakes West	Lake	11.5 ft	C	-	-
20	Green Mountain Reservoir	Summit	11.5 ft	A	-	2,125 acres
21	Mcphee Reservoir	Montezuma	10.5 ft	NR	-	4,918 acres
22	Ridgway Reservoir	Ouray	9.8 ft	C	-	1,109 acres
23	Grand Lake	Grand	8.7 ft	C	-	-
24	Dowdy Lake	Larimer	8.2 ft	D	-	83 acres
25	North Delaney Lake	Jackson	8.2 ft	NR	-	258 acres

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Frequently Asked Questions

What is the clearest lake in Colorado?

Rampart Reservoir is the clearest lake in Colorado by Secchi depth, with water transparency of 28.5 feet. Secchi depth is measured by lowering a black-and-white disk until it disappears from view — deeper readings mean clearer water. Located in El Paso County, Rampart Reservoir has a water quality grade of C (Fair).

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 Colorado — 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 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 Colorado for the nutrient-and-algae list.

How does Colorado compare to other states?

Colorado's clearest waters compete with the most pristine lakes across US lakes with an A–F letter, 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).

More on water clarity

Clearest Lakes in America 2026: Ranked by Secchi Clarity

The clearest lakes in America, ranked by measured Secchi depth in feet from EPA monitoring data — and why the famously clear lakes are missing.

Water Clarity Explained: What Secchi Depth Tells You

What a Secchi depth reading measures, how many feet counts as good clarity, and why clear water is not always clean — with readings from graded lakes.

The Clearest Lake in Every County We Track

County-by-county Secchi champions: the clearest measured lake in each county, the murkiest county winners, and why a county title is not a national one.

Do Deeper Lakes Have Better Water Quality?

Surveyed maximum depth against A–F nutrient-and-algae grades: the pattern, the shallow lakes that still grade an A, and why land use beats bathymetry.

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