In the column printed in the June 25 issue of TRR, I discussed how trout are thermally stressed at temperatures above 68°F, and are so seriously stressed at temperatures above 75°F that they will actually die. Today’s column shows that those conditions can be all too prevalent during the summertime on the Upper Delaware River. Therefore, one must consider strategies like increasing cold water releases from the New York City dams to mitigate high temperatures, and limiting fishing activities in midsummer.

I obtained information on water temperatures online from the USGS gages on the Delaware at Stilesville, essentially at the discharge point from the Cannonsville reservoir into the West Branch, all the way down to Callicoon, 43 miles downstream on the Main Stem.

I’ll focus on July, since that’s the most perilous month for trout. Much depends on the cold water discharged from the bottom of the Cannonsville dam, which over the last decade has averaged 47°F. As it proceeds downriver, this water is warmed by the atmosphere and by mixing with warmer water from tributaries. Eight miles downriver at Hale Eddy, July temperatures average 54°F, while 16 miles downriver on the West Branch at Hancock, July temperatures average 61°F. And since even the daily maximum temperature at Hancock averages 65°F, by and large the trout in the West Branch of the Delaware suffer little during the summer.

But on the Main Stem from Hancock to Callicoon, temperatures rise to dangerous levels. At Callicoon, 43 miles below the dam, generally taken to be the lower end of the springtime trout fishery, the average July temperature is 72°F, while the average daily maximum is a lethal 75°F. Conclusion: While trout fishing in the spring at Callicoon can be excellent, no one should fish there for trout in midsummer.

Motivated by the marvelous population of wild rainbows in this section of river, the fishing conservation community has focused on protecting the trout in the Main Stem down to Lordville, which is 24 miles below the dam. How bad can things get at Lordville? Here the average July temperature is 68°F, while the average daily maximum is 70°F. So, it looks as if conditions at Lordville are generally favorable for the trout.

But, we need to take a closer look at Lordville, because temperatures really spike there when the Upper Delaware is experiencing a heat wave—sequences of days with air temperatures in the 90s. Fish biology experts like Mark Hartle of the Pennsylvania Fish and Boat Commission judge that any day in which the maximum water temperature exceeds 75° is an “extreme stress day” for trout. With this in mind, we found that at Lordville over the last decade, 16% of July days were extremely stressful. Deeply troubling is the fact that in some years, like 2012, the worst year in the decade, fully 48% of July days were extremely stressful.

This analysis, and many other details that space doesn’t permit going into, leaves me with a couple of conclusions. First, is that summertime conditions as far downriver as Callicoon get so bad in midsummer that it is highly doubtful that any relief can be afforded there by additional coldwater releases from the dams. Second is that, although the stress on trout at Lordville is substantial, it is possible to take remedial action there as long as: (1) the stress conditions can be forecast in advance, (2) one can figure out how much additional water is necessary to drive temperatures below the stress level, and (3) enough water is available in the reservoirs so that additional releases will not create substantial risk to New York City’s water supply, or the water rights of down-basin users. Our next column will explore these issues and show that a solution is indeed available.