GUILDERLAND — Joseph Hein was crossing a pedestrian bridge over the Normanskill near French’s Hollow on Monday morning when he noticed a sheen of oil on some water that had pooled there.

Hein, who said he knew a hydraulic plant for the Watervliet Reservoir was nearby, called the state’s Department of Environmental Conservation to report what he had seen.

“It didn’t look like a big spill, maybe just a small burp,” said Hein. “My guess was that, when the hydraulic plant was turned off, it released a little bit of oil.”

Rick Georgeson, a representative for DEC Region 4, said, on Wednesday, that the spill had been caused by “a problem with the electric generation equipment at the hydroelectric dam” and that an “unknown amount of petroleum product was released.”

A spill cleanup contractor, Op-Tech Environmental Services, was sent to the scene shortly after noon on Monday, and placed booms — which would absorb and contain the spilled material —downstream.

The cleanup process, according to Georgeson, is ongoing.

Hein said on Wednesday that he had received an e-mail from the DEC thanking him for the notification, as it narrowed the window of time between the spill and the cleanup, minimizing potential damage.

He said he had gone back down to the bridge after the booms had been placed and the sheen on top of the water was gone.