The retreating Army of Virginia reaches the Potomac and finds it flooded and impassable

The advance units of General R.E. Lee's retreating Army of Northern Virginia reached the banks of the Potomac and found the river in full flood, trapping him on the northern side.  The remainder of his forces arrived the next day and Lee troops took up strong defensive positions between the washed away fords at Williamsport and Falling Waters against the pursuing Army of the Potomac. General Meade did not immediately attack, however, and Lee was able to slip away across the Potomac on the evening of July 13-14, 1863 when the floods had abated.  (By  John Osborne)