It's the same as if you were heading to the part of the reserve just east of what you indicated (notice that the only land access if from the US); you report to CBSA at either the Cornwall or Dundee ports of entry prior to entering the regions indicated. Whether you're arriving from the south or the north the procedure is the same. The border was never "closed down" as you state. The port of entry at Cornwall was just moved to the north side of the river and off the reservation. Makes life much more harder on the Mohawks but that's what they wanted it would seem by intimidating the people who worked at the old port of entry on the island.