At the Horseshoe Falls, just west of Llangollen in North Wales, water is channelled from the River Dee into the Llangollen Canal.
The falls are at nearly 100 metres above sea level and the river rapidly loses height as it continues down the valley.
The canal appears to have a flat path, but the water can be seen to be travelling along the canal channel, so there must in fact be a very slight down-valley slope.
By the time you get to Pontcysyllte Aqueduct, where the canal crosses from the north side of the valley to the south side, the river has dropped by about 30 metres. The canal has hardly dropped at all – just enough to keep the water flowing, so to bring the canal across the valley a spectacular aqueduct was built.
You can walk over on the tow path, if you have a head for heights. You need a head for heights on a boat too – one side is the towpath, but the other side is the thin edge of the water channel and then a very long drop!
The canal needed to head south to bring the water into the canal system, but the steady gentle slope needed to be kept all the way so that the water could be transported over a long distance.
After the Pontcysyllte Aqueduct the canal leaves the Dee Valley…but it can’t maintain the water flow if it goes over the hill, so there is a tunnel through it…
…and then a second one…
…and then a second aqueduct.
The canal is then in Shropshire, where the landscape is flatter, and it is easier to maintain the steady gentle slope. Eventually it links into the rest of the canal system, providing water for the locks that enable the boats to descend to the coast.