Why are canals slower than natural rivers?

[Vladki]

E.g Large river allows 50 km/h, while large canal only 11 km/h. Similar for other river/canal pairs.  It just feels not right. If the river is canalised it should be safer to sail faster - straight banks, constant depth, etc...

[jonbridg]

The 11km/h (about 6 knots) limit for the large ship canal would be based on data from the Manchester Ship Canal - the UK's largest - and probably applies to the largest permitted vessels:
https://en.wikipedia.org/wiki/Manchester_Ship_Canal#Present_day

Smaller vessels, which create less wash, could conceivably be allowed to sail faster. This blog claims 10-12knots (about 18-22km/h) for MV Royal Iris Of The Mersey:
http://ianhelencanals.blogspot.com/2014/11/cruising-length-of-manchester-ship-canal.html - 18th paragraph