Is Thief Valley Reservoir safe to swim?

As of July 20, 2026, Thief Valley Reservoir in Union County, Oregon is not recommended for swimming. Satellite imagery measured 207,025 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

Thief Valley Reservoir is a reservoir in Union County, Oregon covering about 1.2 sq mi, the 72nd-largest of the 115 lakes SwimZone monitors in Oregon. Satellite imagery measured 207,025 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, Thief Valley Reservoir 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 3,064,947 cells/mL in July 2025.

Is it safe to swim in Thief Valley Reservoir right now?

Thief Valley Reservoir is not recommended for swimming today. Satellite imagery measured 207,025 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 Thief Valley Reservoir have blue-green algae (cyanobacteria)?

Yes. Satellite monitoring measured 207,025 cells/mL of blue-green algae (cyanobacteria) at Thief Valley 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 Thief Valley Reservoir?

No. Blue-green algae at Thief Valley Reservoir is above the safe-contact threshold (207,025 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 Thief Valley Reservoir have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Thief Valley Reservoir 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 3,064,947 cells/mL in July 2025.

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

Today, 31 Oregon waters are above a safe-contact threshold and 1 more warrant caution, out of 213 monitored waters SwimZone tracks in the state.

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