LA CROSSE, Wis. (WKBT) -

A sign of a healthy river -- although not necessarily liked -- is approaching: it's mayfly hatch season.

Every year around this time, thousands of mayflies emerge from the Mississippi River for about two days. Mayflies can only reproduce in areas with high-quality water, so it's a good thing to see all of them.

UW-La Crosse biology professor Roger Haro expects this year's hatch to come in the middle of July, the same time as last year.

"It could come earlier or later, but given the cold weather we've had, I think that's probably a good guess when they're coming out," said Haro.

Some smaller hatches of the harmless bugs could come after the first one.