I’m guessing that some of you who read this column fish the East and West Branches of the Delaware and the Neversink Rivers below the reservoirs. The tailwaters. 

At the outset of trout season, once the early fly hatches begin, the tailwaters and freestone rivers all have the same species of mayfly emergences. The species that hatch then include quill Gordons, blue quills, Hendricksons and pale evening duns. There might be some early olives in the mix too. I call these early species of flies “cold-water hatches,” because these insects begin to hatch when the water temperatures approach 50 degrees Fahrenheit. 

As the season progresses and the freestone rivers begin to warm with the increase in air temperature, the tail waters do not.

Instead, the water temperatures remain constant due to the release of very cold bottom water. That means they stay in the very low 50s throughout the summer in the upper reaches of the East and West Branches of the Delaware and the Neversink River, below their respective dams. 

While I do not have a great deal of first-hand knowledge about the hatches that appear along the upper sections of the West Branch and Neversink, I am very familiar with the upper East Branch. What I have observed on that river is that the species diversity of the mayfly population has been significantly altered by the bottom release of very cold water. The only species found in the upper river are quill Gordons, Hendrickson blue quills, pale evening duns, sulphurs and olives. There are no March browns, grey foxes, green and brown drakes, light Cahills, Isonychia or tiny tricos.

Once the water temperature warms to more favorable levels several miles downriver, around Shinhopple, all those other flies are present. 

In addition to having a significant impact on the species diversity in the upper reaches of the tailwaters, the cold water has altered the dates that some mayflies hatch. That cold water has also had a profound impact on the length of some fly hatches too. For example, the little sulphur mayfly, a species that hatches from freestone rivers in early June and last a week or so, begins at about the same time on the tailwaters. But instead of emerging for a week or two, sulphurs have been known to last well into August. In addition to and because of the cold water, sulphurs might hatch around noon, not at dusk.

While I know that some of what I have written here is a bit repetitive, it is important to emphasize how much the releases of cold bottom water from Catskill reservoirs has changed the hatching behavior of some species of aquatic insects. What that means for us as anglers is that it is necessary to familiarize ourselves with the time of day and season that the important hatches occur. 

The anglers with whom I am friends are on Catskill tailwaters sometimes several times a week, so they have first-hand knowledge of the hatching dates and times of the different flies. 

It is also important to keep in mind that the same species can hatch at different times on different tailwaters.

In addition to sulphur mayflies, I’ve observed that pale evening duns are also affected by the cold releases from Pepacton Reservoir. On freestone rivers that fly normally emerges right at dusk, generally in mid-to-late May. On the upper East Branch, pale evening duns have been observed as early as 5 pm. And while this hatch lasts a week or so on freestone rivers, I’ve seen duns on the water well into late June on that section of river.

What this all means to us as anglers is that there can be two distinct seasons for the same fly hatches, depending on whether we fish the freestone, the tailwaters or both. Fly fishers not familiar with these anomalies need to check with fellow anglers and the local fly shops to find out what insects are hatching on the freestone rivers when compared to the tailwaters. In that way they can take advantage of both the freestone and tailwater hatches and likely extend their fishing during the same fly hatches, but for a much longer period of time. 

The implementation of the Water Releases Legislation in 1976 along with the increased releases resulting from the Flexible Flow Management Plan have made the rivers in the Delaware system world-class trout fisheries. The fact that they provide season-long dry fly fishing to species of mayfly hatches that have long been over on the freestone rivers is a huge bonus for us as anglers. Especially since there is little or no fishing in the famous freestones, including the Beaver Kill, where flows can be extremely low and water temperatures extremely high. Conditions lethal to trout, with no fishing.