This stonework carried the canal over a tributary stream, the Conococheague Creek (just past my feet), immediately before that stream empties into the Potomac River. Note that the upstream side wall is missing from when a canal boat struck it wrong.
This shot shows the other (downstream) side of the 3-arch aqueduct. This side is obviously in better shape and shows well the magnificent stonework arching over the Conococheague Creek:
NOTE: If you are having trouble following the engineering here, just imagine a deep water-filled ditch (the canal). It's calm and placid and a pair of mules could easily tow a very large canal boat.
OK so far, but now your water-filled ditch comes to the Conococheague Creek, whose average water level is quite a ways below the ditch. Basically, you just build a bridge to carry your ditch over the stream. The waters do not mix or touch.
After the upstream wall of this aqueduct collapsed, it was temporarily replaced by a wooden wall, as canal operations ceased shortly thereafter. The stones of the aqueduct wall that fell into the Conococheague were later rescued and laid out in the canal bed adjacent to the aqueduct. Presumably the National Park Service has a plan to someday--if $$ permits--restore the Conococheague Aqueduct to its original condition.
Here's a couple of views of these large cut stones patiently waiting reuse, with my hat for scale in the 2nd shot: