For those of you that haven't made it here, Great Falls stopped commercial
travel up the Potomac River, hence the settlement in Washington. To enable
commerce further inland (primarily to get more tobacco fields) A series of canals
were built to get above the falls. The C&O Canal is on the MD side and starts at
Great Falls and comes out near the now infamous Watergate Hotel.

The National Park is actually situated on both sides of the river, MD and VA.