A BIG blackout. A single $2 electrical switch at Niagara Falls failed in 1965, and left 25 million people in the dark - imagine what happens if you destroy or seriously damage it.
On 9/11/01, the New York State Police and the FBI shut down the Robert Moses Parkway leading to and from the Falls, and the Falls themselves, because they were afraid that the power generating facilities were also a target - shut down Niagara Falls, and you'll have just about everyone from Ontario to Virginia in the dark. By itself it might be "lame", but as part of a multi-pronged attack, that's a different story. First you turn out the lights, then you start hitting targets while everyone's running around trying to figure out what the hell's going on...