Popular Northern Colorado lake closed to swimming amid toxic blue-green algae bloom

The Town of Windsor has closed access to the swim beach at Lake Windsor after testing confirmed the presence of a harmful blue-green algae bloom, marking another summer in which the popular Northern Colorado recreation area has been impacted by water quality concerns.

The closure comes during one of the hottest stretches of the summer and only weeks after the town reversed an earlier decision to prohibit motorized boating for the season because of low water levels.

Town officials said the closure took effect immediately after testing confirmed the presence of harmful algae blooms, also known as cyanobacteria. Swimming, tubing and water skiing are prohibited until further notice. Nonmotorized boating is allowed at the user's own risk, though officials strongly recommend staying out of the water.

Sean Porter said he and his daughter Vivian arrived at the lake expecting to cool off before discovering the beach was closed.

"We just came down to Windsor Lake. We were wanting to go cool off with some friends and let the girls splash around and play. Unfortunately, we're a little bummed because bacteria levels are up again," Porter said.

Signs posted around the lake warn visitors that the water may contain toxic levels of blue-green algae.

"It's hot. Summertime. Kids want to go out and have fun splashing in the water, and now we have to change our plans and go elsewhere," Porter said.

The lake has experienced recurring harmful algae blooms in recent years. In 2024, the town installed four LG Sonic MPC buoys designed to combat algae growth by using ultrasonic waves to disrupt algae in the water column. Town officials said data from the devices has shown improvements in bacteria levels, but the blooms continue to occur.

"When it's closed like this, it's a little bit of a bummer," Porter said.

Town officials advise visitors not to enter the water while the advisory remains in effect. Catch and release fishing is still permitted. Recreating around the lake, including using the volleyball court and the 2.5-mile trail, remains safe.

Officials said harmful algae blooms are becoming increasingly common across Colorado lakes and are fueled by prolonged hot weather, stagnant water and nutrient pollution carried by stormwater runoff. Excess nitrogen, phosphorus and potassium from fertilizers can accelerate algae growth, while polluted stormwater is considered a leading source of water pollution, the town said.

Porter said his family will look elsewhere to beat the heat until conditions improve.

"Yeah, there's still other options in the area, but it's just a little bit further of a drive, and it's just a little bit more time consuming. But it's still summertime, you got to let the kids have fun. But yeah, hopefully bacteria levels go down, and we'll be able to come back and let the girls play in the water, hopefully," Porter said.

The town said it will continue monitoring water quality throughout the summer in coordination with the Colorado Department of Public Health and Environment. The lake will remain closed to swimming until testing shows the water is safe.