Is Black Hollow Reservoir safe to swim?

As of July 20, 2026, Black Hollow Reservoir in Weld County, Colorado is clear to swim. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Rainfall over the past week totaled 0.0 in, which can wash bacteria and nutrients into the water.

How this read is built

Black Hollow Reservoir is a reservoir in Weld County, Colorado covering about 0.6 sq mi, the 82nd-largest of the 107 lakes SwimZone monitors in Colorado. Satellite imagery detected no blue-green algae bloom as of July 19, 2026.

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, Black Hollow Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 2,307,932 cells/mL in July 2022.

Is it safe to swim in Black Hollow Reservoir right now?

Black Hollow Reservoir is clear to swim today. Satellite imagery detected no blue-green algae bloom as of July 19, 2026. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Black Hollow Reservoir have blue-green algae (cyanobacteria)?

No blue-green algae bloom is detectable at Black Hollow Reservoir as of July 19, 2026 (below the satellite's roughly 10,000 cells/mL detection limit). Cyanobacteria can still appear within hours in warm, calm, nutrient-rich water, so look before you swim.

Can dogs swim in Black Hollow Reservoir?

No bloom is detected at Black Hollow Reservoir 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 Black Hollow Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Black Hollow Reservoir has reached blue-green algae advisory levels in 6 of them, most recently in June 2026. This is a lake that blooms most summers. Its highest recorded reading was 2,307,932 cells/mL in July 2022.

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 Black Hollow 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 Black Hollow Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.