Photo of the Day #1604: Lock #20 on the Chesapeake and Ohio (C&O) Canal in Great Falls, Maryland. The C&O Canal (sometimes called the “Grand Old Ditch”) follows the Potomac River from Cumberland to Washington D.C. and was used to travel this river between 1831 to 1924. Locks were used to manage elevation differences, like this lock along a set of falls in the Potomac. July 28, 2008.