E. coli is a type of bacterium that lives in our intestines and in all warm-blooded animals. Most varieties are harmless, but some can lead to serious illness. High levels of E. coli, coming from leaking infrastructure, pet waste, or runoff from streets, are common in the South Platte River. In warm summer months—when the water is the most inviting—it creates a conundrum for public health officials and city planners: How do you engage people to care about a river they can't touch? For the city of Denver, one solution is hiding in plain sight.