Most canals are fairly static when it comes to water. Water is poured into the summit pound (and, sometimes elsewhere) to replace that “lost” when passing through a lock. As water is frequently scarce, this is the reason that emptying a full lock or filling a empty lock unnecessarily (e.g. when you’re above an empty lock, and someone is approaching from below but still a few minutes off) is considered very poor form. If you’re moored up on most canals, there is a modest movement of water in the general direction of “downhill”, but not enough to be of any real concern.
However, the Llangollen canal is used to as a feeder to transport water from the Welsh mountains in the Cerriog Valley above Llangollen at Horseshoe Falls, down to a large reservoir at Hurlestone Junction, from where it goes on to Manchester (or somewhere!).
(This picture of Horseshoe Falls, technically the start of the canal, was taken on an earlier visit in 2007).
This transporting of water means there is a considerable flow down the Llangollen Canal at all times, making it more like a river. There are two significant effects resulting from this. Firstly, each lock has a large by-wash channel which carries water down around the lock when boats are not moving through – a bit a weir on a river. For some reason here, these inevitably enter and exit the main channel just above and below the lock gates (unlike most river locks, which give you much more room), and cause major cross currents just by the lock itself. Just what you need when you’re trying to slot a 58 ft by 6ft 8in boat into a 72 ft by 7 ft brick slot without crashing into anything and destroying all the crockery.
The second effect is that while the canal is relatively wide and deep, the flow is fairly negligible, but when the canal narrows, Bernoulli’s principle means that the flow speeds up considerably. The canal narrows at every bridge hole, sometimes to little more than the width of the boat, and heading up stream you suddenly find yourself going nowhere, halfway under a bridge. And when you finally pop out the other side, the boat suddenly surges forward again.
It’s even worse in long narrow tunnels like that at Chirk, or in long narrow aqueducts. To keep going upstream, you need a significant amount of power to overcome the downhill flow and keep moving forward against the fast flow, but there’s something called prop walk which tries its hardest to screw the boat around, and in a cramped tunnel it can make it nearly impossible to track straight down the middle without scrubbing the front on one wall and the stern on the other. Deep joy. And scraped paintwork. Guaranteed.
It’s a lot easier going downhill! Or if you must go uphill, use a horse.