The south bank of the Potomac River forms the district's border with Virginia and has two major tributaries, the Anacostia River and Rock Creek. Tiber Creek, a natural watercourse that once passed through the National Mall, was fully enclosed underground during the 1870s. The creek also formed a portion of the now-filled Washington City Canal, which allowed passage through the city to the Anacostia River from 1815 until the 1850s. The Chesapeake and Ohio Canal starts in Georgetown and was used during the 19th century to bypass the Little Falls of the Potomac River, located at the northwest edge of the city at the Atlantic Seaboard fall line.