The Hunger Stones
The Torah writes (Devarim 27:2-4):
And it shall be on the day when you shall pass over the Yarden to the land which the Lord Your G-d gives thee, that you shalt set up great stones, and cover them with plaster. And you shalt write upon them all the words of this Torah, when you passed over, that you mayst go in to the land which the Lord Your G-d gives you, a land flowing with milk and honey; as the Lord G-d of your fathers has promised your. And it shall be when you have gone over the Jordan, that you shall set up these stones, which I command you this day in mount Eval, and you shalt cover them with plaster.
However, later on we find a separate set of stones, as it is written in Sefer Yehoshua (4:9):
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.
As Rashi (ibid) says:
These were different stones that Yehoshua set up in the midst of the Jordan
What was the purpose of these stones? Abarbanel, Radak and Malbim state that the purpose of these stones was to remember the miracle of the water stopping so people in the future that pass over this area can see them and remember.
A similar phenomena exists in Europe (Wikipedia):
A hunger stone (German: Hungerstein) is a type of hydrological landmark common in Central Europe. Hunger stones serve as famine memorials and warnings and were erected in Germany and in ethnic German settlements throughout Europe in the 15th through 19th centuries. These stones were embedded into a river during droughts to mark the water level as a warning to future generations that they will have to endure famine-related hardships if the water sinks to this level again. One famous example in the Elbe river in Děčín, Czech Republic, has "Wenn du mich siehst, dann weine" ("If you see me, then weep") carved into it as a warning.
