Defining the exact borders of the tribal lands on the eastern side of the Jordan River requires a careful look at the area's unique landscape and water sources. The western edge of the territory given to the tribes of Reuben and Gad creates a complex geographical picture centered around the Jordan River.
How the Jordan River relates to this territory is a subject of discussion. One approach understands the river simply as the western boundary line for these tribes [ביאור יש״ר, ביאור שטיינזלץ]. However, another major perspective argues that the territory of Gad did not just stop at the eastern bank. Instead, it reached further west to include the entire width of the river itself, taking in a small strip of land on the western bank as well [רש״י, מזרחי, שפתי חכמים].
The commentators agree that the river does not act as a boundary for its entire length. The border for Reuben and Gad only begins in the north near the Kinneret and stretches south down to the Dead Sea. The northern section of the Jordan River, from its early sources down to the Kinneret, belonged to the half-tribe of Manasseh and did not border Reuben and Gad at all [העמק דבר, ביאור יש״ר].
Identifying the exact northern starting point of this boundary brings up an interesting geographical issue. While it might seem to refer to the Sea of Galilee itself, this creates a problem because the Jordan River flows directly through the middle of the lake. To resolve this, some explain that the boundary marker is not the body of water, but the ancient city of Kinneret, which early scholars identify as the settlement of Ginosar [ברכת אשר על התורה].
Further south, the border comes to an end in the area of the Dead Sea, at the base of the mountain peaks where water forcefully pours down. This location refers to a place where streams burst from the rock and fall with great force [בכור שור]. These are essentially waterfalls dropping from the high mountains toward the east, or streams flowing down into the Jordan River from the eastern side [ביאור שטיינזלץ].