Joshua also set up twelve stones in the middle of the Jordan, in the place where the priests who carried the ark of the covenant stood. And the stones are there to this day.
9. And Joshua set up twelve stones, etc Apparently there was no use of stones under the water, and it may therefore seem to have been absurd to bury stones at a depth. The others which were placed in Gilgal being publicly visible, furnished occasion for inquiry; but stones hidden from the eyes of men at the bottom of the water could have no effect in inciting their minds.
We may well imagine how busy Joshua and all the men of war were while they were passing over Jordan, when besides their own marching into an enemy's country, and in the face of the enemy, which could not but occasion them many thoughts of hear, they had their wives, and children, and families, their cattle, and tents, and all their effects, bag and baggage...
Commenting on Joshua 4:1-9
And Joshua set up twelve stones in the midst of Jordan,.... Twelve other stones, as the Septuagint and Vulgate Latin versions, and so Jarchi; which he set one upon another, so that they might be seen above the water; or however the water of Jordan being clear, they might easily be discerned by those who either passed over the river, or walked by the side...