The graded river and base level

The graded river and base level

Base level: is the lowest point to which erosion by running water can occur. In the case of rivers the theoretical limit is the sea, although there are exceptions where a local base level may exist, such as with the great Lakes of the USA. Changes in base level result from:

  1. Climatic change: Glaciations and changes in rainfall.
  2. Tectonic change: Where land is uplifted after plate movement or volcanic activity. Changes can be positive sea level rises in relation to the land, or negative sea level fall in relation to the land.

The graded river and base level

If land emerges from the sea following a negative change in base level, the potential energy of a river for erosion is revived and a re-grading of the river can occur.

The graded river and base level

As shown in the diagram above river terraces offer superb sites for the location of settlement. They are the remains of former flood plains that lie far above the extent of present day flooding as a result of vertical erosion caused by rejuvenation.

If land is uplifted for a considerable period of time a river may cut downwards and form incised meanders. There are two types:

Entrenched meanders Symmetrical cross section, and occur if valley sides are resistant to erosion, or rapid incision by the river
Ingrown meanders Less rapid uplift of land, allowing river to shift laterally across its floodplain

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