The upper course of a stream/river is characterized by the presence of steepest slope. In this part the vertical erosion, particularly corrasion, is most strong. Erosion in this part creates a V- shaped gorge or ravine. The stream flows along the bottom still eroding vertically the stream-bed.
Valley sides are very steep, almost like a will from which through mass movement (a form of weathering) materials fall into the stream channel. Such valleys in which down-cutting is predominant are called youthful valleys.
The upper courses of the rivers are not straight, rather they are zig-zag with streams winding between interlocking spurs (ridges) projecting from the valley walls.
Because of the preponderance of erosion in this part, the effects of differential erosion are clearly visible. Differential erosion results where a stream cuts through rocks of different resistance. Juxtaposition of hard and soft rocks undergoing down-cutting produces steeper gradient.
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Steeper gradient in turn gives the stream more energy, or more erosive capacity. Presence of rapids, cascades and water-falls clearly indicate the presence of more resistant rocks in the channels and very steep gradient. In case the rocks are more resistant they are not easily weathered and eroded.
Naturally, under such conditions the stream will be able to carve out only valleys which are narrow and steep-sided. If the stream has to cut through less resistant rocks, the valleys will be more open with gentle slopes.
As the erosional work of a youthful stream precedes further, the suspended load increases as a consequence of the reduction in gradient. Now, valley sides are subject to lateral erosion so that the valley becomes wider.
Remember that in the act of valley widening, mass movement of material from the steeper walls of the channel plays very important role. Thus, the lateral erosion expands the valley floor, interlocking spurs are eroded significantly, and ultimately an open flood plain is produced.