Mayfly hatch coming, good predictor of river water quality

Nature could tell us any day now how healthy our rivers are.

The mayflies are getting close to hatching. The largest hatches generally occur within a week of July 4.

“When you have highly polluted water, you lose oxygen and insects like the mayflies, can’t survive,” Hallie Rasmussen with the Upper Mississippi River National Wildlife and Fish Refuge said. “So when we see large mayfly hatches, we know that the water quality of the river is doing well.”

Rasmussen adds the mayflies are a great source of food to both animals on land and underwater.

“Huge food source,” she said. “They’re harmless but might cause a mess when they die off. Overall, they’re very good.”

Rasmussen says a good way to check to see the progress of the mayfly hatch is to look to the south, as mayflies are born based on water temperature.

“The water warms up further south before it warms up here,” she said. “You’ll see hatches kind of working their way up the river as that water warms up.”