Canals are man made waterways, built in the UK (and the rest of Europe) long before roads as we know them and before the internal combustion engine. They are not used for irrigation.
They are not rivers and therefore have no current. The water level is strictly controlled and systems of lock gates enable the canal to gain or lose elevation where necessary.
We have one canal "The Leeds to Liverpool" which passes over the top of our mountain range called the Pennines.
We in the UK have many thousands of miles of canals, that link all major cities. In fact just one city "Birmingham" has more miles of canal than Venice. They were all dug out by hand and were called "Navigation canals" as they navigated from A to B.
This accounts for the English slang word for a labourer who is referred to as a "Navvy".
They were originally used to transport heavy loads to and from factories as a single horse could pull 40 tons of laden "Barge". To move such weights on land would have required large teams of horses and hence was too expensive.
Transport in this way is at a slow walking speed. Therefore the people operating these Barges would live on the boat. Possibly the first "Houseboats" as we now know them.
Today (with the advent of the internal combustion engine) often these boats are used for leisure craft which are shown in this picture.
They are canal boats. Often referred to as a narrow boat or long boat. The long thin shape is fixed as the canal width is fixed and two need to be able to cross in opposite directions.
Not a lot of people knows that.... as Michael Caine would say