River catchments in Somerset

The river catchment, or drainage basin, is all the land from the highest land (headwaters) to the sea, drained by a single river and its tributaries.

Catchment areas vary greatly in size - a major river may have a catchment area in excess of a thousand square kilometres, whereas a small tributary will have a catchment area of only a few hectares.

River catchments are separated from each other by watersheds. The characteristics of any river (physical, chemical, biological) are determined by the nature of the entire catchment and the activities, both human and natural, that take place within it.