Antietam Creek is a tributary of the Potomac River located in south central Pennsylvania and western Maryland in the United States, a region known as Hagerstown Valley. The creek became famous as a focal point of the Battle of Antietam during the American Civil War. The term Antietam is thought to be a derivative of a Native American phrase meaning "swift flowing stream."
The creek is formed in Franklin County, Pennsylvania at the confluence of the West and East Branches of Antietam Creek about 2.3 miles (3.7 km) south of Waynesboro, Pennsylvania. The stream runs for about 0.5 mile (0.8 km) before entering Washington County, Maryland. The total length of the creek is 41 miles (65 km).
The watershed area is 93 square miles (241 km^(2)) and includes parts of Franklin County, Pennsylvania and Washington County, Maryland. Major tributaries in Pennsylvania include the East Branch of Antietam Creek, the West Branch of Antietam Creek, Red Run and Falls Creek. Major tributaries in Maryland include Little Antietam Creek, Beaver Creek, and Marsh Run.
The creek is noted for numerous well preserved stone arch bridges dating to the 19th Century that still traverse the creek, the most famous of which is the 125 foot (38 meter) long Burnside Bridge in the Antietam National Battlefield.
Most of the watershed area is relatively rural in nature but the area surrounding Hagerstown, Maryland, is threatened by urban sprawl. The area is also heavily cultivated and waste runoff from farms is a growing ecological concern.
The creek was a major topographic feature during the Battle of Antietam (called the Battle of Sharpsburg in the American South) fought on September 17, 1862, near Sharpsburg, Maryland. Burnside Bridge, which traverses the creek, became a major focus of combat as Union forces under General Ambrose Burnside repeatedly tried to capture the bridge from Confederate forces guarding the crossing from a high bluff overlooking the creek. The day of the battle is known as "the day Antietam Creek ran red" due to the blood of thousands of Union casualties mixing with the creek waters.
Source: Answers.com, Wikipedia, and Maryland.gov