What did God tell Joshua to do after the Israelites crossed the Jordan River in Joshua 4?

Put twelve stones in the middle of the Jordan river.

(To remember how God stopped the Jordan river to allow the Israelites to cross without getting their feet wet.)
The LORD spake unto Joshua, saying,

Take you twelve men out of the people, out of every tribe a man,

And command ye them, saying, Take you hence out of the midst of Jordan, out of the place where the priests' feet stood firm, twelve stones. ...

And Joshua set up twelve stones in the midst of Jordan, in the place where the feet of the priests which bare the ark of the covenant stood: and they are there unto this day. Joshua 4:1-9