The Susquehanna Swarm

It might seem like a freak of nature, a bizarre phenomenon set off by the latent effects of the Three Mile Island nuclear accident 50 miles (and more than 46 years) upstream. But it’s actually a natural phenomenon and a sign of environmental health.

Workers at Conowingo Dam, on the Susquehanna River, were recently awed by a swarm of mayflies so thick they could hardly see through it, according to Amir Hammad, the hydroelectric plant manager for Constellation. Hammad shared the photos with this post; they record the Susquehanna Swarm of July 22.

These aquatic insects hatch from their nymphal stage in the river, swim to the surface, then fly off in search of a mate.

Seeing mayflies in sporadic rises on a river is not unusual. Seeing them swarm like honey bees is.

But apparently the Susquehanna Swarm occurs every summer.

“Our staff says this is the Hexagenia genus,” reports Gregg Bortz of the Maryland Department of Natural Resources. “Swarmers on the Susquehanna are known to be Hexagenia bilineata, but [DNR] can’t make a positive ID from the photos.”

My iNaturalist app suggested Hexagenia bilineata.

“That does appear to be a particularly large swarm,” Bortz added. “But if their numbers are greater this year, it could be a sign of healthier water quality, which should be good for fishing too.”

For those who fish, especially fly anglers who target trout in cold rivers and creeks, the sight of mayflies emerging in the surface is quite exciting. It’s the height of fly fishing. It means that trout might start rising to the surface for a mayfly meal. And that means the angler gets to tie a dry fly imitation to his leader and have a go at fooling fish.

But a swarm like this?

I’ve never seen one in Eastern trout streams. They sometimes happen in the big creeks out west.

And, apparently, they happen on the mighty Susquehanna.

“The bugs spend a day or two in their new stage before molting one more time into their final form,” wrote Adam Miller in a post on the Susquehanna Swarm for the Alliance For The Chesapeake Bay. “They breed, and the female lays her eggs on the water, starting the cycle all over again. . . . The increased abundance of mayfly hatches on the Susquehanna over the last few years indicates that water quality is on the mend.”