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 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.