NPS photo

The Potomac River flows at the base of the Blue Ridge Mountains.

While visiting the park's historical district, walk past John Brown's Fort, under the Winchester and Potomac Railroad bridge, and out to the Point. From here one can observe the two rivers that border either side of the town. On the left lies the Potomac River and to the right lies the ShenandoahRiver. About 360 million years ago, the Potomac began cutting its way through the Appalachian Mountains, forming the water gap that lies between Maryland and LoudounHeights. After the Appalachians were worn down, run-off water collected at their base, forming the ShenandoahRiver. When visiting Harpers Ferry today, one can see how the Shenandoah and Potomac gracefully come together, flowing eastward to the Chesapeake Bay as the Potomac River.

According to the United States Geological Survey, Harpers Ferry may experience a flood over 20 feet every 5 to 10 years . The 1936 flood, which reached a record height of 36.5 feet, is estimated to only occur every 125 years. Floods in the park tend to be fairly deep, since there is not much room for water to spread out once it overflows its banks here. During the early months of the year, heavy precipitation that produces rapid runoff is a major flood-causing factor.