Floodgates are being opened at a New York Power Authority dam in Massena to spill excess water from Lake Ontario into the St. Lawrence River.
The gates at Long Sault Dam were scheduled to open at 7 a.m. Monday. The power authority says there will be rapidly changing river flows downstream.
Outflows from Lake Ontario are established weekly by the International Lake Ontario and St. Lawrence River Board.
The decision to open the floodgates Monday is in response to higher than normal water levels and ice conditions on Lake Ontario.
The spilling may continue through March 23.
Heavy spring rains last year filled Lake Ontario to a record high level, resulting in extensive shoreline flooding.
Officials said opening dam floodgates would have worsened floods that were also occurring downstream, particularly in Montreal.