As far back as the summer of 2016, anglers who fished the East Branch of the Delaware River below Pepacton Reservoir started to see significant changes in mayfly hatches. A great deal of discussion followed, with a variety of theories put forth as to the cause. Everything from climate change and pollution, along with the buildup of stream-bottom detritus from dead leaves, was debated. Yet despite all the speculation, there was no proof that any of those factors were the culprit. There was so much concern over the issue that I wrote about it for the 2019 Fish Magazine (“Another Silent Spring?,” River Reporter, March 28, 2019.) 

In that article, I explained some of the problems documented about negative changes in mayfly  populations in other rivers, including the famous chalk streams of Hampshire and Devon in England. Although it was not good news, researchers found that pesticide pollution in those two regions of England was the cause of mayfly population declines.

Finding the cause of the decline in mayfly populations in the East Branch has not been easy or straightforward. As it stands, we have no answers, but I have a theory that can be backed up by my observations as well as research conducted on other tailwater river systems.

When the Water Releases Legislation was signed into law by New York Gov. Hugh Carey in 1976, the New York State DEC prepared regulations that would provide additional releases to the East and West Branches of the Delaware River, as well as the Neversink River. Those regulations were reviewed and approved by the States of New Jersey, Pennsylvania and Delaware along with the New York City DEP and the federal government. 

After engineering studies were performed, biologists and engineers came up with the following new schedule of releases:

East Branch of the Delaware River: Summer—70 cubic feet per second (CFS); winter—50 CFS.

West Branch of the Delaware River: Summer—325 CFS; winter—45 CFS

Neversink River: Summer—45 CFS; winter—25 CFS.

While not huge, those increases in flow were significant enough to improve fly hatches and fishing dramatically. 

I can recall fishing the upper East Branch soon after the new releases went into effect, and found the sulphur mayfly hatch began at 6:30 p.m. and continued well after dark. There were thousands of flies on the water, making fishing challenging but exciting. 

Despite the improved flows and fishing opportunities, fisheries managers, some engineers and the public all wanted more water released. At the time, it made sense; the more water released, the further downstream temperatures would be favorable for trout. So over time, DEC staff, in cooperation with the DEP and others, worked to increase releases. 

By the mid-1990s, the release from Pepacton was increased to 90 CFS in summer and 70 CFS in winter. 

Then in 2007 and 2008, after some very intensive modeling studies, the Flexible Flow Management Plan (FFMP) was developed and adopted by the parties involved. The FFMP was designed to maximize releases based on the amount of storage in Delaware reservoirs, on the time of year and, I believe, on average annual rainfall. As a result, the summer release in the East Branch was increased to 140 CFS during the summer and about 80 CFS during the winter, in normal water-storage years. 

Everyone applauded, because we believed it would extend the downstream cold water environment and improve fishing, which it did—for a while. 

With the increase of summertime releases to the East Branch to 140 CFS, the summer temperature in the upper river close to the dam seldom if ever exceeded 52 degrees Fahrenheit. There were some interesting, positive side effects from the increased flow on some aquatic insect populations in this reach of river. For example, the sulphur hatch, which normally began around 6:30 p.m., suddenly started to show around noon.

Those hatches ran well into the afternoon, so anglers had several hours of excellent dry fly fishing opportunity. In addition, the hatches lasted from the middle of June well into August, well outside of the normal hatching range for this species of mayfly. As anglers, we looked forward to this hatch each season, because it provided such a unique angling opportunity. 

Then it suddenly just stopped. These days, if there is a sulphur hatch, it begins about 6:30 p.m. and is sporadic with not a lot of flies.

In addition to the decline in sulphurs from the upper East Branch, my observations revealed that only about six species of mayflies inhabit the upper reaches of the river. These include quill Gordons, Hendrickson/red quills, blue quills, pale evening duns, some sulphurs and blue winged olives. 

Comparatively speaking, this same East Branch from Shinhopple downriver supports good populations of the six species described, along with good hatches of March browns, gray foxes, green drakes, brown drakes, tricos, plus a variety of caddis. In effect, the upper reaches of the East Branch is a biological desert when it comes to the species diversity of mayflies. 

Based on my knowledge of the river and the research available, I believe the increase in flow to 140 CFS has had an extreme, negative impact on the aquatic insect community of the upper East Branch. This is what some of the research concludes: “Hypolimnetic releases are from the bottom of reservoirs. They are colder than natural river temperatures in summer and warmer than natural river temperatures in winter. These releases turn a naturally variable environment into a thermally constant environment. Many aquatic insects depend on temperature cues to fulfill life–history events, such as emergence and reproduction. Altered water temperatures can disrupt these cues, creating mismatches between invertebrates and their environment, desynchronizing life history events, like hatching.” (1) 

When we discuss the life history of Catskill mayflies, we must keep in mind that before reservoirs and cold bottom releases, all species of this order were exposed to extreme changes in water temperature. While winter temperatures registered in the mid-30s, summer temperatures could rise well into the 80s. As a result, Catskill mayflies evolved over many thousands of years as organisms subject to extreme water temperature change. 

When those same species that evolved under those conditions were exposed to very cold homogenous water temperatures of 50 degrees Fahrenheit, many could not survive. 

Based on this background information and my observations along the upper East Branch, I believe that the increase in cold bottom water from Pepacton Reservoir to 140 CFS during the summer is just too cold for most species of our indigenous mayflies.

Those that survive in that environment, I consider “cold water mayflies,” in that their life cycles comply with early-season cold water conditions. All the other mayflies that I described, found from Shinhopple downriver, do not appear to survive in the cold- water environment found in the upper East Branch. Those mayflies need a progression in water temperature change to complete their life cycles and thrive. 

Folks who read this article will likely think that I’m a heretic in blaming the current summer releases of cold water as the reason certain mayflies are not present in the upper East Branch of the Delaware River. But when you take into account that most of our “summer mayflies” evolved when water temperatures fluctuated between 33 degrees and 80 degrees, then were rather quickly exposed to temperatures that never exceed 50 degrees, I believe anglers will understand the logic behind my theory. While I would never recommend that the summer flow regime be changed, after all the work that was done to get where we are today, just keep in mind that sometimes there can be “too much of a good thing.”

(1) Lyles, Chloe. “Impacts of reservoir management strategies on aquatic invertebrates in the Colorado River basin.” Utah State University, June 2023.