The eastern border was the Salt Sea as far as the mouth of the Jordan. The northern border started from the bay of the sea at the mouth of the Jordan,
Judah and Joseph were the two sons of Jacob on whom Reuben's forfeited birth-right devolved. Judah had the dominion entailed on him, and Joseph the double portion, and therefore these two tribes were first seated, Judah in the southern part of the land of Canaan and Joseph in the northern part, and on them the other seven did attend, and had their respective lots as...
Commenting on Joshua 15:1-12
And the east border was the salt sea, even unto the end of Jordan,.... To the place where Jordan fell into it; so that this border was the whole length of the salt sea, which Josephus says (y) was five hundred eighty furlongs; and, according to Pliny (z), an hundred miles: and their border in the north quarter was from the bay of the sea...
the end--that is, the mouth of the Jordan.