This paper describes the basic concepts of open channel flow and important conditions effecting the flow behaviour of river channels. The study of open channels traditionally includes the discussion of flow in river. In this paper the phenomenon of flow in river channels can be explained by diagrams. Rivers are large natural streams of water flowing in channels and emptying into larger bodies of water. The river source, also called the headwaters, is the beginning of a river. Often located in mountains, the source may be fed by an underground spring, or by runoff from rain, or snowmelt,. Discharge is the volume of water moving down a river per unit of time .In general the river discharge can be computed by multiplying the area of water in channel cross section by the average velocity in the cross section. In order to determine the discharge flowing through the channel the mean velocity of flow is required. For measuring discharge the water velocity can be determined by current-meter .With an increase in depth of flow the velocity will be increased. Particularly in large channels during rainy season the discharge ranges from a minimum value to very large value. Hence the description of channel sections with variation of velocity is very important in Flow dynamics of open channels.
