The natural eastern border of Canaan followed the Jordan River down to the Salt Sea (or Dead Sea). The Jordan River and the Salt Sea were part of the Great Rift Valley. They were strong barriers, but people could cross them.
This boundary did not include the lands east of the Jordan River. Reuben, Gad, and half of the tribe of Manasseh lived there. These settlements were not part of Canaan. There were outside the first borders of the promised land.