The millions of mayflies that lit up the La Crosse, Wisc., weather radar on July 20 are a sign that the water of the Mississippi River is markedly cleaner than just a few decades ago.

In the 1980s, the Mississippi was so polluted that the bugs virtually disappeared according to the Minnesota Pollution Control Agency:

For the delicate mayfly — which is sensitive to chemical pollutants, increases in sediment and decreases in oxygen levels in the water — pollution ensured the collapse of their populations. By the 1980s, mayfly hatches had disappeared from rivers and streams in Minnesota. The collapse of mayflies from the aquatic food chain also meant the disappearance of stoneflies, caddis flies and even some species of fish from the Mississippi River.

The mayflies suffered a similar fate in the 1950s and 1960s in portions of the Great Lakes. Lake Erie, the smallest and shallowest was in the worst shape.

Erie was often described as “dead” with piles of stinking, dead fish regularly found along its shores as green, algae scum covered large parts of the lake and weeds choked many areas along the shore. Commercial and recreational fishing nearly disappeared as the algae and other pollutants killed pike, whitefish, sturgeon, and other fish.

When Lake Erie’s regular spring and early summer mayfly swarms vanished, few missed them. At the time most people saw may flies as no more than a nuisance. Decomposing piles of mayflies smelled like rotten fish. Cars ran over piles of mayflies on roads, making the roads slick.

Few realized that mayflies, which are about an inch long, are part of the food web for both fish and many birds.

When mayflies emerge from the water for a day or so of life they are too busy mating to bite or sting. In fact, adult mayflies don’t even have mouths or digestive tracts. After depositing eggs on the water the flies die.

The naiads that hatch from eggs on the lake bottom spend four years or five years there growing before leaving for their day or two of swarming and mating.

By the late 1960s concern about Lake Erie’s sad shape was increasing when on June 23, 1969 oil and other waste floating on the Cuyahoga River caught fire in Cleveland. The flames that leapt as high as a five-story building into the air made stunning television images and headlines around the world.

The Cuyahoga incident helped spur major cleanup efforts in the U.S. and Canada that eventually brought more life to all of the lakes, including to Lake Erie’s mayflies that begin swarming again in the 1990s.

Put-In-Bay, Ohio, now holds a yearly mayfly festival, which was held this year on June 21.

Just as mayflies were detected on weather radar in LaCrosse,  a swarm was also picked up on radar over Lake Erie on June 13 this year.

Unfortunately, levels of phosphorus in some parts of the Great Lakes are increasing and fish die-offs occur from time to time in the lakes. The health of mayflies continues to bear watching, not only in the Great Lakes, but also in rivers and other lakes that mayflies call home.

“[N]ext time you see a cloud of mayflies, remember that even though it may look like a scene out of a horror movie, their healthy population means improved water quality…” concludes the Minnesota Pollution Control Agency.

(Jason Samenow contributed to this post)