The remains of the Potomac Aqueduct Bridge sit near the Georgetown waterfront, where the old bridge once carried canal boats between the C&O Canal and the Alexandria Canal. The bridge opened in 1889 and was demolished in 1933 after the canals closed and the Key Bridge took over the crossing.
What is left is modest, but the spot is peaceful and gives a good view toward the Key Bridge. It is a worthwhile short detour if you are already walking the canal or the waterfront.