Official rules are yes by air and no by land.
Is it possible for a canadian citizen to visit america by land or by air?
Land depends in the port entry.
Really? Some allow it??
At the end of the day it is up to the individual agent.
Vermont border!!
Are U.S. citizens able to enter the U.S through the land borders in Niagara?
Definitely not officially.
Plenty of "spouses" have been able to get through the NY borders; it depends on the agent.
A lot of success by the Buffalo/Niagara Falls border points to U.S.