Prepare to be disgusted. Mayfly hatch underway.

As you slip on dead mayflies over the next few days, try to remember that it’s indicative of a river’s health.

The annual massive mayfly hatch on the Mississippi is finally underway, a little later than usual.

Alex Lamers, a National Weather Service meteorologist in the south, posted this radar loop, which picked up the hatch last night in La Crosse, Wisconsin.

There are millions of mayflies on buildings and roadways now, National Weather Service La Crosse meteorologist Tim Halbach tweeted.

It’s not the most fun time for filling up the gas tank:

Or grabbing some cash:

Update 3:29 pm: Or quenching a thirst:

Or doing just about anything else:

Last year, the Minnesota Department of Transportation had to call out snowplows to remove the dead mayflies from the highways.

From the archive: