ramblings of a catskill fly fisher

Dramatic changes in a six-mile difference

The impact of cold bottom-release water on lower East Branch biodiversity

By TONY BONAVIST
Posted 10/7/20

In previous columns, I’ve written that most mayfly hatches on some Catskill tailwaters, with some exceptions, end by August. But a recent trip to the lower East Branch found excellent hatches …

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ramblings of a catskill fly fisher

Dramatic changes in a six-mile difference

The impact of cold bottom-release water on lower East Branch biodiversity

Posted

In previous columns, I’ve written that most mayfly hatches on some Catskill tailwaters, with some exceptions, end by August. But a recent trip to the lower East Branch found excellent hatches of buff caddis and assorted mayflies. So on that outing, I learned that hatches on the lower East Branch are very different from those on the upper river; they don’t end in August. I had not fished this section in a number of years, so I was not well versed on the hatches there. Instead, most of my fishing was relegated to the upper reaches, downstream of Pepacton Reservoir. It is in this section, between Downsville and the Beaver Kill, that l documented the hatches and made determinations about the river, some of which were inaccurate.

What I did describe in a prior article was the difference in the species composition of mayflies and caddisflies in the East Branch above and below Shinhopple. There are only about five species of mayflies in the upper river and very few caddisflies. More importantly, the prolific Sulphur hatches the upper river was famous for are all but gone. From Shinhopple down-stream, there are at least 11 species of mayflies, including those found in the upper river, plus several species of caddis. Why is there such dramatic changes in species composition between Downsville and Shinhopple, a distance of about six miles? What happened to the Sulphur hatch?

Historically, Sulphur hatches began around June 10 with emergence at dusk on freestone rivers. Those hatches lasted about 10 days. However, once the water releases program began for the East Branch and summer flows increased from 19 cubic feet per second (CFS) to 70 CFS, the Sulphur hatch began about 6 p.m. and lasted several weeks. Those hatches continued for several years. As time past, friends reported fishing huge Sulphur hatches beginning around 1 p.m. on the upper river. No one paid attention to this significant change; they just enjoyed the fishing until the Sulphur hatch all but disappeared in 2016. Then the grumbling began and everyone became interested. There were all kinds of speculation about the cause but no one had an answer. Then in 2019, Sulphurs rebounded to a degree and hope returned to the upper river. Sadly, this year’s Sulphur hatch did not begin until mid-July; it lasted about one week and with fewer flies. My conclusion is something is terribly wrong with upper East Branch’s Sulphur mayfly population.

About a month ago, I began wondering if the constant flow of very cold bottom release water was impacting mayflies on the upper river. I knew that the summer release pattern for the East Branch changed during the years. Since I didn’t know the details, I spoke with Peter Kolesar, who I believed knew the complete history of releases for the East Branch. Kolesar played a major role in the development of the Flexible Flow Management Plan (FFMP) which was implemented in 2007. This is what he told me: summer releases were increased to 95 CFS in 2004, 135 CFS in 2007 and 140 CFS in 2017. That means summer flows doubled since 1977!

Water temperature measured at the United States Geological Survey (USGS) gauge, located half a mile below Pepacton dam, during the period of June 6 to September 28 of this year ranged between 43.0 to 47.5 degrees with a few spikes (see column photo). That is extremely cold water, which means that temperatures in the upper river seldom reach or exceed 55 degrees, even during the summer. What does that all mean for the mayfly populations in that part of the river? When Catskill mayflies evolved, long before the release of cold water, all species were exposed to a variety of flow and temperature extremes. Flows were seasonally low, with temperatures ranging from the low 30s in winter to near 80 in summer. With the release of constantly cold, water from Pepacton, mayfly and caddisflies were no longer exposed to the water temperatures they required to thrive. So, it is my theory, based on what has happened to Sulphur hatches, that the increase in cold water releases has negatively impacted that species and several other mayflies normally found in the upper East Branch.

To determine whether there was any data supporting this theory, I spoke to a biologist with the U.S. Fish and Wildlife Service who was kind enough to forward two studies: one from Norway and one from Australia. Both studies concluded that releases of cold water from deep reservoirs negatively impacted mayflies, severely reducing some populations. So, it appears from my observations and the research provided by these two studies, that the release of very cold water into trout rivers is a mixed blessing. In the upper reaches, the discharge negatively impacts some species of aquatic insects including Sulphur mayflies. At the same time, those releases provide ideal habit for trout and extend cold water habit several miles downriver. As a result, water temperatures remain cool with mayfly and caddis hatches extending well into September. Two rivers in one.

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