Blue-green algae, also called cyanobacteria are found in Colorado freshwaters and are especially common in lakes or reservoirs, like Bear Creek Reservoir. Elevated concentrations of cyanobacteria maybe harmful to the environment, animals and even human health. Blue-green algae sometimes appear as dense mats that can impede activities like swimming and fishing. These types of algae may cause taste, col-or or odor problems in drinking water.