On Saturday morning, a tipster called in a complaint to the DEC about dead fish in the Plattekill Creek just below the vllage of Saugerties' Blue Mountain Reservoir.
The timing seems to point to the Saugerties water department: On Friday afternoon, the reservoir was treated with copper sulfate, a chemical commonly used to kill algae in municipal water systems. (Hat tip to Occam's Razor.) But the village says they're not related, the Freeman reports:
On Saturday, about 20 dead fish were found in the Plattekill Creek downstream from the reservoir, [village Water Department Superintendent Joe] Bisignano said. He said there were no dead fish in the reservoir itself and there is no way to know if the deaths were related to the copper sulfate.
Bisignano said every reservoir in the nation uses copper sulfate and he does not believe there is cause for concern. He added that water was running over the dam in the reservoir, but the water level in the stream below it looked a little low because there had not been a recent rainfall.
From Cornell Cooperative Extension, here's a fact sheet on copper sulfate. It's highly toxic to fish:
Copper sulfate is very toxic to fish. Its toxicity to fish varies with the species and the physical and chemical characteristics of the water (12). Even at recommended rates of application, this material may be poisonous to trout and other fish, especially in soft or acid waters. Its toxicity to fish generally decreases as water hardness increases. Fish eggs are more resistant than young fish fry to the toxic effects of copper sulfate (3). Very small amounts of this material can have damaging effects on fish. Permits are being required in some situations for application of copper sulfate to water bodies. Further field studies have been required by the EPA (16). Direct application of copper sulfate to water may cause a significant decrease in populations of aquatic invertebrates, plants and fish (17).
Copper sulfate is indeed a very common approach to the problem of algae in reservoirs -- and far cheaper than the watershed-wide approach of reducing the levels of algae-feeding nutrients in the water. But in some reservoirs, the use of copper sulfate has become controversial, and planners are looking for alternatives. Here's a story from 2006 about the fight over the pesticide in the reservoir for Napa, CA.