Upper course - in the upper course, where the river starts, there is often an upland area. The river's load is comprised of large material in the upper course, as it hasn't been broken down by erosion yet.
Lower course - in the lower course, the land is a lot flatter. The river's load is fine sediment, as erosion has broken down the rocks.
A - as the river flows downhill there is an increase in vertical erosion. The channel is shallow and narrow because there is not a lot of water in it.
B - as the river flows into the middle course, there is some vertical erosion but more lateral erosion. The channel is wider and deeper as a result.
C - in the lower course there is a lot less erosion, with only some lateral erosion. The channel is at its widest and deepest.