why does river discharge increase downstream a level
1 Tributaries-downstream of a confluence, the gradient may increase because discharge (and therefore the rate of downward erosion on the river's bed) is higher than upstream of the confluence. 2  Pools and riffles-on a small scale, particularly in the upper course, the long profile is stepped.ischarge is the volume of flow of water per unit time, often measured in cubic metres per second (or cumecs). In general, discharge increases with distance from the source to the mouth of a river. The extra water enters the river from tributary streams, surface runoff, throughflow and baseflow.