Is Williams Fork Reservoir safe to swim?

As of July 20, 2026, Williams Fork Reservoir in Grand County, Colorado carries a swim caution. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but 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

Williams Fork Reservoir is a reservoir in Grand County, Colorado covering about 2.4 sq mi, the 24th-largest of the 107 lakes SwimZone monitors in Colorado. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but 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, Williams Fork Reservoir has reached blue-green algae advisory levels in 2 of them, most recently in September 2021. Blooms here have been occasional rather than annual. Its highest recorded reading was 143,174 cells/mL in September 2020.

Is it safe to swim in Williams Fork Reservoir right now?

Williams Fork Reservoir carries a swim caution today. Satellite imagery measured 29,238 cells/mL of blue-green algae on July 19, 2026, elevated but 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 Williams Fork Reservoir have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 29,238 cells/mL at Williams Fork Reservoir 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 Williams Fork Reservoir?

Use caution. Blue-green algae at Williams Fork Reservoir are elevated (29,238 cells/mL on July 19, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does Williams Fork Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Williams Fork Reservoir has reached blue-green algae advisory levels in 2 of them, most recently in September 2021. Blooms here have been occasional rather than annual. Its highest recorded reading was 143,174 cells/mL in September 2020.

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 Williams Fork Reservoir'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 Williams Fork Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.