The life cycle of the little sulphur mayfly has been changed, due to the increase in the release of cold bottom-water.

Of all the topics that I’ve written about for the River Reporter, none has received more attention than those about the little sulphur mayfly, Ephemerella dorothea.

From a fly fishing standpoint, no other mayfly hatch causes more consternation among anglers than this fly. In fact, my old friend, Bill (Willie) Dorato, had more imitations of sulphurs in his fly box than anyone else I knew.

Why? When sulphurs are hatching, the trout take the emerging flies as well as duns. As a result, fishing the dry fly can be most frustrating. Nevertheless, we anglers keep flailing away.

When I checked the various hatch charts for this species, I found that normal emergence is projected between the last week of May and the first week of July. Sulphurs hatch at dusk on Catskill freestone streams such as the Beaver Kill and Willowemoc. Emergence on the colder tail waters is a completely different story.

Once the new water release regulations went into effect on the East Branch of the Delaware in 1977, the summer flow was increased from 19 to 70 cubic feet per second (cfs). At that time, the sulphur hatch began right around 6:30 p.m., about two hours earlier than on the freestone rivers.

These hatches were huge, with thousands of flies on the water and fish rising everywhere.
In fact, there were so many flies hatching, that hooking a trout was almost impossible. There were just too many flies.

I found some success by picking one rising fish and repeatedly casting to it.

On the positive side, those trout weren’t particularly selective, provided your fly was the next fly.

In those days, if I landed one trout 14 inches or larger, I considered it a successful outing.
If it was a hot, humid day, a mist would form over the water, a function of the moist air condensing over the cold river. Under those conditions, it was impossible to see rises. So we would cast toward the “plop-plop”—the noise the trout made while feeding. We fished completely blind, by feel and with our ears.

The new summer flow of 70 cfs on the East Branch lasted several years before regulating authorities began the process of adjusting the summer flows upwards.

Most recently, the flow has been based on the Flexible Flow Management Plan. This new release structure was based on reservoir storage and rainfall. Once the new flow regimen was implemented, the release of almost twice as much cold bottom water caused the sulphurs’ hatches to begin at 1 p.m. in the upper reaches of the East Branch.

Those hatches lasted for years before we heard the first grumblings that sulphur hatches were on the decline. That was during the summer of 2016.

Since that time, the sulphur hatch has been a mixed bag on the upper river. The 1 p.m. hatches we saw for several years appeared to be an event of the past.

Then in 2021, I found a very large hatch of sulphurs on our camp water, beginning around 7 p.m.
The following season, there were very few sulphurs.

This year, on July 26 and again on August 2, there were very large sulphur hatches on that same water, beginning at 6:30 p.m. and extending past dusk.

What we’ve observed on the upper East Branch of the Delaware is the impact that the different levels of the release of cold bottom water have had on the evolution of the biology of sulphur mayflies. At 70 cfs, the sulphur hatches began at 6:30 p.m.; at 140 cfs, they started at 1 p.m., where they hatched for several years. Now the hatches are beginning at 6:30 p.m. and last several hours. In addition, hatches that normally began in June and ended in early July are beginning in late July and lasting well into August.

To better understand all the changes we’ve observed in the sulphur hatches on the upper East Branch, I spoke with an aquatic entomologist at a well known research facility. What he told me was that sulphurs normally hatch when water temperatures reach 60 degrees Fahrenheit. Since the water temperature in the upper East Branch never reaches that level, sulphur hatches are retarded, hatch later in the season and last longer. That is all because of the changes in volume of the release of cold water from the Pepacton Reservoir.

So what we’ve witnessed is how the life cycle of sulphur mayflies has adjusted over the 46 years since the implementation of the water release legislation in 1976.

In summary, we can say that the increase in the flow of cold bottom water has had a positive impact on the trout population in the East Branch. At the same time, it has had a major impact on the life cycle of the sulphur mayfly hatch. It has caused the sulphurs to adjust and evolve over a period of many years.

On the positive side and for us as fly fishers, the hatches start later in the season and last much longer than on freestone rivers. So in essence, the sulphur hatch is now an anomaly, in that the flies no longer hatch in a historic manner.

Now at dusk, when the night falls, and the mist rises, we’ll wait to hear the “plop-plop” as the trout begin their feed on these little mayflies.