Imboden, learning that a 3,000 strong Union cavalry was approaching, formed a semicircle to defend the town. He managed to fend them off until nightfall when a brigade of Confederate cavalry arrived to drive them off. By July 11, most of Lee's infantry had reached Williamsport. He strengthened the defenses and waited for the Potomac river to go down so they could cross. Meade’s army reached Williamsport on the 12th and began probing the Confederate defenses. The Potomac finally was low enough to cross on July 13. A pontoon bridge had been constructed by Confederate engineers about downstream at Falling Waters. At both locations the Confederate army began crossing the river that evening.