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Drainage is the movement of water derived from the rain, snowfall and melting
ice and snow, over the land (and through it in subterranean waterways) which
results eventually in its discharge into the sea. The flow of streams and
rivers is influenced by the underlying rocks, how they are arranged and whether
there are any structural features that the water may follow. Further factors
affecting drainage include soil type, climate and the influence of man.
There are a number of recognizable patterns which can be related to geology: |
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| 1. Dendritic | A random branching unaffected by surface rocks. |
| 2. Trellis | Streams aligned with the trend of underlying rocks. |
| 3. Parallel | Streams running parallel to each other due to folded rocks, or steep slopes with little or no vegetation. |
| 4. Rectangular | Controlled faults and joints, the latter often in igneous rocks. |
| 5. Annular | Formation of streams in circular patterns around a structure of the same shape (e.g. an igneous intrusion). |
| 6. Barbed | A drainage pattern where the tributaries imply a direction of flow contrary to what actually happens. |
| 7. Radial | Streams flowing outwards from a higher area. |
| 8. Centripetal | The flow of streams into a central depression where there may be a lake or river. |
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When a drainage pattern is a direct result of underlying geology, it is said to be ' accordant ' (the opposite case being ' discordant '). |
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