Is Shadow Mountain Lake safe to swim?

As of July 20, 2026, Shadow Mountain Lake in Grand County, Colorado is clear to swim. Satellite imagery measured 19,655 cells/mL of blue-green algae on July 19, 2026, well below the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Shadow Mountain Lake is a lake in Grand County, Colorado covering about 2.1 sq mi, the 27th-largest of the 107 lakes SwimZone monitors in Colorado. Satellite imagery measured 19,655 cells/mL of blue-green algae on July 19, 2026, well below the 100,000 cells/mL advisory threshold.

Blue-green algae (cyanobacteria) blooms are most common from midsummer into early fall, when the water is warm, calm, and rich in nutrients from runoff. They can produce toxins that make people sick and can be rapidly fatal to dogs, so skip water that looks green, scummy, or like spilled paint, and keep children and pets out of it.

Over 7 summers of satellite monitoring since 2020, Shadow Mountain Lake has reached blue-green algae advisory levels in 5 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 259,766 cells/mL in August 2021.

Is it safe to swim in Shadow Mountain Lake right now?

Shadow Mountain Lake is clear to swim today. Satellite imagery measured 19,655 cells/mL of blue-green algae on July 19, 2026, well below the 100,000 cells/mL advisory threshold. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Shadow Mountain Lake have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 19,655 cells/mL at Shadow Mountain Lake on July 19, 2026, below the 100,000 cells/mL advisory threshold. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in Shadow Mountain Lake?

No bloom is detected at Shadow Mountain Lake today, but dogs are much more vulnerable to blue-green algae than people, and blooms can form within hours. Always keep dogs away from water that looks green, scummy, or discolored, and rinse them off after swimming.

How often does Shadow Mountain Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Shadow Mountain Lake has reached blue-green algae advisory levels in 5 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 259,766 cells/mL in August 2021.

How many Colorado lakes and beaches are unsafe for swimming today?

Today, 26 Colorado waters are above a safe-contact threshold and 3 more warrant caution, out of 113 monitored waters SwimZone tracks in the state.

How is Shadow Mountain Lake's water quality measured?

SwimZone builds this read from EPA CyAN satellite imagery for blue-green algae, official beach and fecal-bacteria advisories where a program monitors Shadow Mountain Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.