Joshua 4:9 states:
וּשְׁתֵּ֧ים עֶשְׂרֵ֣ה אֲבָנִ֗ים הֵקִ֣ים יְהוֹשֻׁעַ֮ בְּת֣וֹךְ הַיַּרְדֵּן֒ תַּ֗חַת מַצַּב֙ רַגְלֵ֣י הַכֹּהֲנִ֔ים נֹשְׂאֵ֖י אֲר֣וֹן הַבְּרִ֑ית וַיִּ֣הְיוּ שָׁ֔ם עַ֖ד הַיּ֥וֹם הַזֶּֽה׃
Joshua also set up twelve stones in the middle of the Jordan, at the spot where the feet of the priests bearing the Ark of the Covenant had stood; and they have remained there to this day.
The passuk itself does not give a reason for placing the stones in the middle of the river.
Conversely, 12 stones were taken from the Jordan river (ibid. 4:20-22) and brought to their encampment as a symbol/testament, for future generations, that the Jews crossed the Jordan river. In this case, the reason is stated (ibid. 4:21-22):
וַיֹּ֛אמֶר אֶל־בְּנֵ֥י יִשְׂרָאֵ֖ל לֵאמֹ֑ר אֲשֶׁר֩ יִשְׁאָל֨וּן בְּנֵיכֶ֤ם מָחָר֙ אֶת־אֲבוֹתָ֣ם לֵאמֹ֔ר מָ֖ה הָאֲבָנִ֥ים הָאֵֽלֶּה׃ וְהוֹדַעְתֶּ֖ם אֶת־בְּנֵיכֶ֣ם לֵאמֹ֑ר בַּיַּבָּשָׁה֙ עָבַ֣ר יִשְׂרָאֵ֔ל אֶת־הַיַּרְדֵּ֖ן הַזֶּֽה׃
“In time to come, when your children ask their fathers, ‘What is the meaning of those stones?’ tell your children: ‘Here the Israelites crossed the Jordan on dry land.’
What is the purpose of the 12 stones that remained in the Jordan river, if they could (presumably) not be seen?