A natural or artificial conveyance through which liquid (typically water) having a free surface moves. The free surface is the interface with a gas (usually the atmosphere), along which the pressure is constant. The liquid is accelerated or decelerated in the flow direction due to an imbalance between the driving gravity force and the viscous boundary resistance force (friction). Such flows occur naturally in rivers, streams, and estuaries as a part of the hydrologic process of surface runoff and artificially in free-surface conduits for the transport of water for irrigation, water supply, drainage, flood control, and other useful purposes. In contrast to full pipe flow, the free surface introduces an additional freedom into the description of open-channel flow, which is the position of the free surface itself as it adjusts to the imposed flow conditions.