Is Green Lane Reservoir safe to swim?

As of July 20, 2026, Green Lane Reservoir in Montgomery County, Pennsylvania is not recommended for swimming. Satellite imagery measured 1,013,778 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

Green Lane Reservoir is a reservoir in Montgomery County, Pennsylvania covering about 1.2 sq mi, the 10th-largest of the 24 lakes SwimZone monitors in Pennsylvania. Satellite imagery measured 1,013,778 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, Green Lane Reservoir 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 2,659,641 cells/mL in August 2022.

Is it safe to swim in Green Lane Reservoir right now?

Green Lane Reservoir is not recommended for swimming today. Satellite imagery measured 1,013,778 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 Green Lane Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 1,013,778 cells/mL of blue-green algae (cyanobacteria) at Green Lane 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 Green Lane Reservoir?

No. Blue-green algae at Green Lane Reservoir is above the safe-contact threshold (1,013,778 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 Green Lane Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Green Lane Reservoir 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 2,659,641 cells/mL in August 2022.

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

Today, 10 Pennsylvania waters are above a safe-contact threshold and 9 more warrant caution, out of 41 monitored waters SwimZone tracks in the state.

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