I'm looking for a set of outlines of the river basins in the UK - I don't need a particularly detailed or highly accurate map - just a map saying point X will drain into the Thames or point Y will just go into the sea! Just the main river names would be fine. I have the full Environment Agency rivers set and the OS DTMs if I want... but how do I convert this into a catchment boundary?
The trick is to Google "UK river catchment map". (Other combinations that include "watershed" don't work.) You will find
- River basin management plan documents.
- River basin district maps (downloadable files).
- An interactive map.
- Maps by the UK Environment Agency.
- Scottish river basin maps and data.
A search involving "watershed" did turn up a historical (1883) map in the Rumsey Map Collection.
If you don't find a map, SAGA GIS has a module called watershed basins that uses a dem and the channel network (your rivers). If you use a coarse dem this goes fast. http://www.saga-gis.org/saga_modules_doc/ta_channels/index.html
https://data.gov.uk/dataset/integrated-hydrological-units-of-the-united-kingdom-catchments has data for catchments in GB, but not Northern Ireland. Although derived from a lower resolution grid than EA Lidar, these should be sufficient for most purposes