The borders of Jordan, that are in - Canaan - This verse can never mean that they built the altar on the west side of Jordan, for this was not in their territories; nor could it be a place for the purpose of public worship to their own people, if built on the opposite side of Jordan; besides, the next verse says it was built over against the land of Canaan. It appears that when they came to the river they formed the purpose of building the altar; and when they had crossed it they executed their purpose.
A great altar to see to - A vast mass of earth, stones, etc., elevated to a great height, to serve as a memorial of the transactions that had already taken place. Probably it was intended also to serve as a kind of watchtower, being of a stupendous height, altare infinitae magnitudinis, an altar of an immense size, as the Vulgate terms it.

And the children of Israel heard say,.... Those that dwelt in the land of Canaan, for otherwise, as before observed, the two tribes and a half on the other side Jordan were Israelites also; and this is a further proof that the altar was built on their side, or those in the land of Canaan would have known of the building of it, and have seen them at it, and not come at the knowledge of it by hearsay only, as it seems they did, it being reported to them by some who had been in those parts, and had seen the structure:

behold, the children of Reuben, and the children of Gad, and the half tribe of Manasseh, have built an altar over against the land of Canaan; which clearly shows it was on the other side Jordan, though Noldius, who places it in the land of Canaan, renders the words, "in the neighbourhood of" it (l); which will not much help him, since it might be in the neighbourhood of it, and yet not in it:

in the borders of Jordan; on the banks of it, or in one of the meanders and windings of it, at a place where it ran out and fetched a compass in the land of Canaan:

at the passage of the children of Israel; where they passed over when they first came into Canaan, and where those tribes also passed over at their return; supposed to be the Bethabara beyond Jordan, where John was baptizing, Joh 1:28.

(l) "in vicinia", Noldius, No. 369. p. 80.

THEY BUILD THE ALTAR OF TESTIMONY ON THEIR JOURNEY. (Jos 22:10)

when they came unto the borders of Jordan, that are in the land of Canaan, the children of Reuben . . . built there an altar by Jordan--This altar was probably an immense pile of stones and earth. The generality of our translators supposes that it was reared on the banks of the Jordan, within the limits of Canaan proper. But a little closer examination seems to make the conclusion irresistible that its position was on the eastern side of the river, for these two reasons; first, because it is said (Jos 22:11) to have been built "over against," or in the sight of the land of Canaan--not within it; and secondly, because the declared motive of the trans-jordanic Israelites in erecting it was to prevent their brethren in Canaan ever saying, "in time to come, What have ye to do with the Lord God of Israel? For the Lord hath made Jordan a border between us and you," &c. [Jos 22:24-25]. Such a taunt would be obviously prevented or confuted by the two tribes and a half having on the eastern side of Jordan, within their own land, a facsimile of the altar at Shiloh, as a witness that they acknowledged the same God and practised the same rites of worship as the brethren in Canaan.