The surprise attack is successful. Although there are casualties on both sides, they appear to be greater among the Lamanites. Thus, the Lamanites saw a difficult task in attempting to fight Lehi’s army; they also saw that they might cross the river and perhaps escape, so they fled into the river to presumed safety on the other side.
Lehi, the Nephite army’s leader, of course, knew that Moroni’s second army was on that side of the river, so he halted his army to prevent the Lamanites from coming back that way in order to escape their new battlefront.