Mayfly hatches are a trial of summer for upper Midwest residents along the Mississippi. The summer hatches can be so thick that residents clean up mayfly bodies with leaf blowers and counties actually call out snowplows to clear the roads of the dead and dying insects. Last Sunday’s hatch was huge, perhaps because it was delayed by cool spring temperatures. The event actually showed up on National Weather Service radar, creating a bow echo on the radar that looked just like a summer rainstorm.

Mayfly larva live in mud at the bottom of lakes and rivers, then emerge to take wing and breed, usually living only about 24 hours as winged adults before dying and clogging roads and sidewalks throughout the region. Clouds of mayflies hamper visibility and carcasses on the roads make surfaces slick. This hatch was blamed for at least two car accidents, one in Minnesota and one in Wisconsin.

Disgusting and even dangerous as the summer hatches may be, Mark Wagner, education director of the National Mississippi River Museum and Aquarium in Dubuque told USA Today they are a reassuring sign that the Mississippi is not too polluted.

“Even though we hate the mayflies when they’re out and they’re all over our cars and under street lights and in parking lots, if you remember it’s a good sign of the river’s health, then it’s good to know that,” he says.