BOULDER, Colo. - Nothing says summer quite like a swim beach, and nothing says summer in Colorado this year like yet another closed swim beach.On the hottest day of the year so far, families hoping to bear the heat in Boulder Re servoir had a toasty day on the sand before packing up.The swim beach closed after toxic blue-green algae was discovered on the swim beach side of the reservoir.Denver7 met Melinda Reilly Suzanne Numan at the reservoir during their visit to Boulder."I was like, 'Oh wait, there's algae in the lake," Reilly said, "Let's see if we can still at least kayak. And yes, we can."A few locals we talked to didn't even realize the beach was closed."The nice man that told me, you may not want to jump in there because there's a huge algae bloom, which I have no idea about," said Kelsie Cady. Watch Danielle Kreutter's report in the video below: Beach at Boulder Reservoir closed due to algae outbreakDenver7 noted a sign posted at the park entrance window; however, community members said it would be helpful to have signs at the beach too.Crews were hopeful Monday morning when their first test showed no algae, but by 9am it was a different story."We determined that it was more than likely blue-green algae, and that's what caused our closure,' said Stacy Cole, Boulder Reservoir Manager.It's not common for Boulder Reservoir to have large blooms like that. The first recorded bloom in modern history was in 2024, followed by another in 2025, both showing up in September."This year it's a bit earlier, probably because of the hot temperatures," Maggie Spangler, Water Quality Engineer, City of Boulder.That same heat made for a pretty tough blow to the community to lose their beach."I set up like a little kiddie pool, so I'm gonna sit poolside at my house with my golden retriever," Cady said.For updates, visitors can check BoulderRez.org or sign up for BPR texts by texting "BPRReservoir" to 888-777. ...read more read less