The velocity of the river is dependent upon several factors. These factors include the amount of discharge, gradient of the land, shape of channel cross-section and the roughness.
River discharge is usually measured using cubic metres per second and it can be presented in a hydrograph. A hydrograph is graph that plots the amount of discharge against time, so it shows the changes in the river discharge over a certain period of time. The larger the discharge, the faster the river will flow. This is due to the fact that when there is more material in the river, more amount of gravity would be acting on it. Therefore, they would flow faster down the river.
The steeper the river channel, the faster is the river flow, all other factors being equal.
The rougher the river the slower the speed of the river. The amount of friction between a rough surface and the water in the river would be huge. Hence, the river would be slowed down.
The shape of the channel cross-section affects the amount of water that would come in contact with the riverbed. This in turn affect the friction between the two bodies and thus also the speed of the river. The more the water is in contact with the riverbed, the more friction there is and the speed of the river would be slower. The total length of the riversides and bed is called the wetted perimeter. The speed of the river is not constant in a rive channel. The speed of the river is much slower near the riverbed and sides than the speed of the water in the mid-stream. The water over the deeper side of the channel flows faster than the shallow sides.