The Mohawk Akwesasne council covers parts of New York, Ontario, and Quebec. The international border crosses islands in the St. Lawrence River as well as mainland areas, with tribal members going back and forth to visit family, go to school, get healthcare, go to work, etc. It's very complicated to monitor and regulate, with various tribe, state, provincial, and national authorities involved. There already is at least one COVID19 case being tracked: http://www.akwesasne.ca/mohawk-council-of-akwesasne-and-sain...

Further west, the bridges over the St. Lawrence connecting New York and Ontario get lots of daily cross-border workers, usually Canadians working in New York, as well as the normal flow of trade and tourism going both ways. Shutting it down would have a huge impact on the economies of both countries, but particularly Canada.