Is Black Hawk Lake safe to swim?

As of July 20, 2026, Black Hawk Lake in Sac County, Iowa is not recommended for swimming. Satellite imagery measured 721,279 cells/mL of blue-green algae on July 19, 2026, above 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

Black Hawk Lake is a lake in Sac County, Iowa covering about 1.4 sq mi, the 11th-largest of the 42 lakes SwimZone monitors in Iowa. Satellite imagery measured 721,279 cells/mL of blue-green algae on July 19, 2026, above 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, Black Hawk Lake has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in June 2021.

Is it safe to swim in Black Hawk Lake right now?

Black Hawk Lake is not recommended for swimming today. Satellite imagery measured 721,279 cells/mL of blue-green algae on July 19, 2026, above 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 Black Hawk Lake have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 721,279 cells/mL of blue-green algae (cyanobacteria) at Black Hawk Lake on July 19, 2026, above the 100,000 cells/mL advisory level. Avoid swimming and keep children and pets out of the water.

Can dogs swim in Black Hawk Lake?

No. Blue-green algae at Black Hawk Lake is above the safe-contact threshold (721,279 cells/mL on July 19, 2026). Cyanobacteria toxins can be rapidly fatal to dogs, which swallow it while swimming and lick it from their fur. Keep dogs out of the water and away from any scum along the shore.

How often does Black Hawk Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Black Hawk Lake has reached blue-green algae advisory levels in 7 of them, most recently in July 2026. This is a lake that blooms most summers. Its highest recorded reading was 1,272,047 cells/mL in June 2021.

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

Today, 31 Iowa waters are above a safe-contact threshold and 1 more warrant caution, out of 46 monitored waters SwimZone tracks in the state.

How is Black Hawk 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 Black Hawk Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.