During the retreat from Gettysburg, the Potomac River trapped Gen. Robert E. Lee and his Confederate army when torrential rains on July 4, 1863, flooded the river for more than a week and prevented a crossing into present-day West Virginia. The crisis had deepened on July 3, when Union cavalry behind Lee's army destroyed the pontoon bridge here, cutting a vital link to the Confederacy. On July 10, Confederates finished a new pontoon bridge, but ambulances and hundreds of ordnance and artillery wagons required two days to cross. By early evening on July 13, in another downpour, Gen. James Longstreet's infantry corps began crossing, guided by bonfires on both shores and signal torches on the bridge, followed by Gen. A.P. Hill's corps, and by mid-morning the next day, 30,000 Confederates were across and Lee's army had escaped. Quartermaster John Harman, formerly Stonewall Jackson's chief quartermaster, had built 16 pontoon boats in two days from dismantled sheds and warehouses and wood taken from a Williamsport lumberyard.