Yes, this will be OK
U.S. Governement's website states this for US Citizens traveling to Canada.
Valid at time of entry.
And the Canadian Government's website also does not list any validity period requirements for US citizens.
If you are a citizen of the United States, you need a passport to fly to or transit through a Canadian airport, but not to enter Canada by land or by boat. You must carry proof of your citizenship, such as a birth certificate, certificate of citizenship or naturalization, or a Certificate of Indian Status, as well as photo identification.