Why do discharge and velocity increase downstream?
The volume of water passing a given point on the river is known as its discharge. Discharge is measured in cubic metres per second (cumecs). To calculate discharge use the formula:
Discharge = velocity x cross-sectional area
The discharge of a river increases along its course as tributaries join it adding more water.
Velocity also increases along the course of a river. Even though the descent in the upper courses is steeper the lower course has the greatest velocity. The reason for this is because velocity is affected by how much water is in contact with the bed and banks. The small channel in the upper course of the river means there is more friction which means the velocity slows.
In the lower course of the river, a lower proportion of water is in contact with the bed and banks so there is less friction meaning velocity is much higher. Again, the added water from the tributaries flowing into the river increases speed due to the additional discharge.