The lake surpassed flood stage by almost a full meter Friday, damaging hundreds of homes and threatening hundreds more in upstate New York and Vermont.
Particularly at risk are several island communities, including the Vermont island county of Grand Isle, which faces the loss of its two major east-west road links with the mainland.
Residents haven't been ordered to leave yet, but Vermont Governor Peter Shumlin said Friday he wouldn't rule out a mandatory evacuation.
Shumlin declared the flood area a disaster and called in the Vermont National Guard. Some 67,000 sandbags have been filled, and high-water vehicles are on standby to assist with recovery efforts.
Officials said it has taken weeks for the lake to reach this, its highest level in history, and it will take weeks for it to recede.
The 193-kilometre long lake stretches north into Canada and empties into the Richelieu River, where unprecedented flooding has affected several Quebec communities.
Flooding has been a problem across much of Canada and the U.S. this year. Manitoba is preparing for long months of elevated water levels, while officials in New Brunswick have been keeping a close eye on the flood-prone Saint John River.
Meanwhile, flooding on the Mississippi River this year is threatening to be catastrophic.
With files from the Associated Press