TLDR; Bad algae bloom. Archive: https://archive.today/w76mV From the post: >>The city of Boulder is telling people to stay out of the Boulder Reservoir because of a potentially harmful algae bloom. The algae, known as cyanobacteria, can produce toxins that can make people and pets sick, the city said. The 700-acre reservoir is a popular spot for dogs, and pet owners are being warned to keep their pets out of the water.