Antietam Creek is a 41.7-mile-long tributary of the Potomac River that runs through southern Pennsylvania and western Maryland. Its name means "swift-flowing stream" in Algonquian, and the creek was the site of the 17 September 1862 Battle of Antietam during the American Civil War; the creek ran red with blood as the Union Army lost a quarter of its number, many of whom died attempting to cross the creek at Burnside's Bridge.