Canals

Canals and navigations are human-made channels for water conveyance (supply), or to service water transport vehicles. In the vernacular, both are referred to as 'canals', and in most cases, the engineered works will have a series of dams and locks that create areas of low speed current flow. These areas are referred to as 'slack water levels', often just called 'levels'.

The main difference between them is that a navigation parallels a river and shares part of its waters, its drainage basin and leverages its resources by building dams and locks to increase and lengthen its long stretches of deep enough slack water levels while staying in its valley. In contrast, a canal cuts across a drainage divide atop a ridge (however humble), generally requiring an external water source above the highest elevation. Many canals have been built at elevations towering over valleys and others water ways crossing far below. Cities need a lot of fresh water and many canals with sources of water at a higher level can deliver water to a destination where there is a lack of water. The Roman Empire's Aqueducts were such water supply canals.

A navigation is a series of channels that run roughly parallel to the valley and stream bed of an unimproved river. A navigation always shares the drainage basin of the river. A vessel uses the calm parts of the river itself as well as improvements, traversing the same changes in height.

A true canal is a channel that cuts across a drainage divide, making a navigable channel connecting two different drainage basins.

Most commercially important canals of the first half of the 19th century were a little of each, using rivers in long stretches, and divide crossing canals in others. This is true for many canals still in use.

Both navigations and canals use engineered structures to improve navigation:

...
Wikipedia