Is Brock Slough safe to swim?

As of July 20, 2026, Brock Slough in Kidder County, North Dakota carries a swim caution. Satellite imagery measured 34,663 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 0.1 in, which can wash bacteria and nutrients into the water.

How this read is built

Brock Slough is a lake in Kidder County, North Dakota covering about 0.6 sq mi, the 167th-largest of the 216 lakes SwimZone monitors in North Dakota. Satellite imagery measured 34,663 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, Brock Slough has reached blue-green algae advisory levels in 6 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,508,075 cells/mL in July 2024.

Is it safe to swim in Brock Slough right now?

Brock Slough carries a swim caution today. Satellite imagery measured 34,663 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 Brock Slough have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 34,663 cells/mL at Brock Slough 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 Brock Slough?

Use caution. Blue-green algae at Brock Slough are elevated (34,663 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 Brock Slough have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Brock Slough has reached blue-green algae advisory levels in 6 of them, most recently in July 2025. This is a lake that blooms most summers. Its highest recorded reading was 1,508,075 cells/mL in July 2024.

How many North Dakota lakes and beaches are unsafe for swimming today?

Today, 167 North Dakota waters are above a safe-contact threshold and 4 more warrant caution, out of 241 monitored waters SwimZone tracks in the state.

How is Brock Slough'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 Brock Slough, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.