What is the meaning of "mi'Nachal ba'Derech Yishteh"?
Rashi: [Pharaoh's] land drank from the Nile river, along its course. It did not need rain. Therefore, he lifted his head and boasted "Li Ye'ori va'Ani Asisini" (Yechezkel 29:3).
Rashi (6): Sancheriv's army drank from the Yarden. He boasted "Ani Karti v'Shasisi Mayim v'Achriv..." (Yeshayah 37:25).
Radak: [David] killed so many until a river of their blood flowed. "Yishteh" is a metaphor, like "v'Dam Chalalim Yishteh" (Bamidbar 23:24).
Malbim: This reveals that the Rosh is Ir ha'Mayim, the head of Rabah. There was the river from which they drank water.
Why does it say "Al Ken Yarim Rosh"?
Rashi: Refer to 110:7:1:1-2.
Radak: After [David] won all his wars, he raised his head over all of them - "va'Ya'as David Shem b'Shuvu me'Hakoso Es Aram b'Gei Melach..." (Shmuel II, 8:13)
Malbim: [Because they drank water from the river,] this part (Ir ha'Mayim) was the head and elevated over the body, i.e. the rest of the country.
