Unit 7:  External dynamics of the Earth                                      1   2   3

3.3. Rivers

Rivers are permanent watercourse that flow into an ocean, a lake or other river.

Rivers carry out erosion, transportation and sedimentation. The dominant process is different in different parts of the river. From source to mouth we can distinguish:

     - The upper course.

       River flows very fast because the slope of the terrain is very accused.

       The dominant process is erosion, caused by water and the materials

  which are dragged by it.

  As a result the river excavates its valley and makes it deep and narrow

  (V-shaped valleys).

  Gorges, ravines, waterfalls and rapids are found in this part of the river.

- The middle course.

  The slope is less strong and the speed of water decreases,

  so that the dominant process is transportation.

  The river flows describing large curves called meanders and the valley widens.

- The lower course.

  The slope and the speed of water are almost inexistent,

  so that the dominant process is sedimentation and the valley becomes flat.

  The sediments of the rivers are called alluvium and form alluvial plains

  and alluvial terraces. In the mouth we can find deltas.

Animation: Floodplain (aluvial plain) and fluvial terraces (McGraw Hill)