Keil & Delitzsch

But the priests stood with the ark of the covenant "in the midst of Jordan," i.e., in the bed of the river, not merely by the river, "upon dry ground, הכן," lit., firmando, i.e., with a firm foot, whilst all Israel went over upon dry ground, "till all the people were passed over." This could easily have been accomplished in half a day, if the people formed a procession of a mile or upwards in breadth.