“And Joshua also set up twelve stones in the middle of the Jordan.” Not only was there a heap of stones on the bank of the Jordan to mark the crossing spot where Israel crossed the Jordan, but Joshua also had a memorial of stones placed in the Jordan where the priests stood. The REV adds the word “also” to make it clear there was a second set of stones, as do some other versions (cp. CEB; CJB; CSB; GWN; JPS; NET; NLT). While the stones for the monument set up where the people lodged were taken from the place where the priest's feet stood, this second monument was set up where the priest's feet stood. This could have been a monument that in some way commemorated the role that Yahweh, via his ark and priests, played in drying up the Jordan. Also, since the pile was placed where the priest’s feet stood at the edge of the Jordan, since the Jordan was at flood stage, it almost certainly meant that this second pile of stones was on the east bank of the Jordan most of the year (Josh. 3:12-13, 15.) Predictably, however, since the Jordan flooded every year, it would not have lasted too long before it started to break down, get covered in mud, and/or wash away. Perhaps a few decades.
“they are there to this day.” So when the account that is recorded in Joshua 4 was written, the stones in the Jordan were still there. They would have been very large stones, and the top of the pile was almost certainly visible late in the dry season when the Jordan River ran very low. Of course, over time they would have washed away.