Canals are waterway channels, or artificial waterways, for water conveyance, or for servicing water transport vehicles. They carry free surface flow under atmospheric pressure, and can be thought of as artificial rivers.
In most cases, a canal has a series of dams and locks that create reservoirs of low speed current flow. These reservoirs are referred to as slack water levels, often just called levels.
A canal can be called a navigation canal when it parallels a river and shares part of its waters and drainage basin, and leverages its resources by building dams and locks to increase and lengthen its stretches of slack water levels while staying in its valley.
A canal can cut across a drainage divide atop a ridge, generally requiring an external water source above the highest elevation. The best-known example of such a canal is the Panama Canal.
Many canals have been built at elevations, above valleys and other waterways.
Canals with sources of water at a higher level can deliver water to a destination such as a city where water is needed. The Roman Empire's aqueducts were such water supply canals.