http://www.theheritagetrail.co.uk/industrial/pontcysyllte%20aquaduct.htm

An interesting feature here is that the water in the canal above is fed from the River Dee below about four miles behind me, so 4 miles back the river was above the canal !
The canal drops one inch per mile and is continually fed from the river, in this pic the water flows from left to right.
About three miles further on at Chirk is an equally impressive aqueduct crossing the River Ceriog and the beautiful Ceriog Valley.