What are the conotaions of the word "ve'Alav Mateh Menasheh"?
Rashi (citing Targum Onkelos): It means 'next to'. 1
See Torah Temimah, note 9. Sifsei Chachamim: Rashi did not need to explain the same word in Pesukim 5 &12, since there the Torah writes "ve'ha'Chonim Alav" (where it is perhaps more obvious). Refer to 2:20:151:1. Alternatively, because the Gemara in Menachos, 96b learns it from here. Refer also to 2:20:151:3.
Why does the Torah not write "ve'ha'Chonim alav" here, like it does in Pesukim 5, 12 & 27?
Moshav Zekenim (citing R. Yehudah ha'Chasid): Because Efrayim and Menasheh are like one tribe.
Moshav Zekenim: Because every tribe had a river coming out of Miriam's well [that supplied it with water] - and Efrayim and Menasheh had only one river.