Is Spruce Run Reservoir safe to swim?

As of July 20, 2026, Spruce Run Reservoir in Hunterdon County, New Jersey is not recommended for swimming. Satellite imagery measured 458,124 cells/mL of blue-green algae on July 19, 2026, above the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.3 in, which can wash bacteria and nutrients into the water.

How this read is built

Spruce Run Reservoir is a reservoir in Hunterdon County, New Jersey covering about 2.1 sq mi, the 4th-largest of the 15 lakes SwimZone monitors in New Jersey. Satellite imagery measured 458,124 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, Spruce Run Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in August 2024. Its highest recorded reading was 1,042,949 cells/mL in July 2020.

Is it safe to swim in Spruce Run Reservoir right now?

Spruce Run Reservoir is not recommended for swimming today. Satellite imagery measured 458,124 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 Spruce Run Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 458,124 cells/mL of blue-green algae (cyanobacteria) at Spruce Run Reservoir 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 Spruce Run Reservoir?

No. Blue-green algae at Spruce Run Reservoir is above the safe-contact threshold (458,124 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 Spruce Run Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Spruce Run Reservoir has reached blue-green algae advisory levels in 4 of them, most recently in August 2024. Its highest recorded reading was 1,042,949 cells/mL in July 2020.

How many New Jersey lakes and beaches are unsafe for swimming today?

Today, 4 New Jersey waters are above a safe-contact threshold and 106 more warrant caution, out of 467 monitored waters SwimZone tracks in the state.

How is Spruce Run 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 Spruce Run Reservoir, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.