As noted in the first two installments of this series, in the June 25 and July 9 issues, trout are vulnerable to high water temperatures, and temperatures in the Upper Delaware as far upstream as Lordville reach “severely stressful” levels, defined as 75°F or above, with significant frequency.

At its Water Water Everywhere Conference in 2011, Friends of the Upper Delaware adopted a mission of finding a way to relieve such summertime thermal stress at Lordville. Jim Serio of Hancock, my colleagues at the Columbia Water Center and I set out to find a way to achieve this goal. We estimated just how much additional water is necessary to alleviate thermal stress at Lordville, and showed that in most summers there is sufficient water in the reservoirs to do so without imperiling New York City’s (NYC’s) water supply.

First, to understand the magnitude of the stress problems at Lordville, we needed to count the stress days, find out just how high temperatures can get, and determine how long stress events last. Online data from the United States Geological Survey (USGS) Lordville gage show that there are nine stress days in an average summer, with five of them in July. We also measured the intensity of stress, with a “stress-degree-day” concept similar to that commonly used by homeowners to assess heating burdens. Luckily, river temperatures rarely get very far above the stress benchmark of 75°F, so that in an average summer there are about 7.3 stress-degree-days. But, as in any insurance situation, we need to take into account how bad things might get. The worst year since the current flow regime plan went to into effect was 2010, when a heat wave caused 15 stress days, with a run of eight consecutive stress days during which the river temperature got to a high of 78°F.

So how much additional reservoir water would be required to keep maximum daily temperatures below the 75°F stress threshold? River water temperatures depend on many factors including reservoir releases, natural stream flows and, of course, the weather. The statistical method called “regression analysis” permitted us to take all these factors into account simultaneously, and led to an estimate that about 118 cubic feet per second (CFS) of additional Cannonsville water would reduce the Lordville daily maximum temperature by 1°F. From this estimate and our tally of 7.3 stress degree days in an average summer, we conclude that, on average, about 556 million gallons (MG) of water would be required. There are 271 billion gallons of storage capacity in NYC’s Delaware dams, and the city’s own daily diversion from those dams is 586 MG, meaning that adequate protection for an entire typical summer could be provided by an amount of water equaling only one day of NYC consumption.

Can NYC spare that much water? The answer is a clear “yes,” at least during typical summers. We come to this conclusion from tracking the amount of water that has actually been in reservoir storage since the Flexible Flow Management Plan (FFMP) first went into effect in 2007, from tracking the city’s own estimates of excess water availability since 2011, and from tracking the amount of unused water in a Special Needs Bank that is already incorporated in the FFMP. Our research also showed that stress events are highly correlated with heat waves, which themselves are quite well forecasted by the U.S. Weather Bureau. Thus, stress events can be anticipated and relief can be planned.

In conclusion: (1) There is a real need for thermal stress relief for the trout of the Delaware; (2) The outlines of how to achieve thermal stress relief are well understood; and (3) In all but the most exceptional circumstances, there is sufficient water in the reservoirs to implement such a system.