Is there any possibility to download for free the UK river and catchment files (e.g. vector or raster) readable for GIS?
I need them for research purposes.
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FYI a colleague of mine suggested me the following website, where you can find the UK (detailed) river network: https://www.ordnancesurvey.co.uk/business-and-government/products/os-open-rivers.html
On this website there is also a lot more..
Have you considered Natural Earth Data: http://www.naturalearthdata.com/downloads/10m-physical-vectors/
HydroSheds is a global hydrology dataset based on SRTM topography data set (DEM). It includes glboal stream network and basins (vectors) and other raster outputs (flow dir., flow accu. etc.) derived from either 15 and 30 sec resolution. Rasters are also available at 3 sec resolutions. For the UK make sure you download European region.
You ask where you can get a river network for the UK for research purposes. This suggests you are in academia? If so you should have access to the definitive river network of the UK from the Ordnance Survey via the edina digimap website. You can download the WaterLayer. Be aware this is a very large dataset and can only be downloaded in tiles.
Depending upon the quality of research you want to carry out you could use the Open Rivers dataset which is easier to access and less restrictive in licensing. The issue with this dataset is that it is a generalised version of the WaterLayer and therefore missing many first order streams. So if that is where you are doing your research, head water streams, then this is not a good network to work with. Last time I looked at it it also had various topologically issues but they may have fixed those.
As for catchment boundaries, you would typically generate those yourself. OS provide a DEM, OS Terrain 50, which would need processing before it could be used to generate hydrological catchments. Again through the Edina digimap website you can obtain 5m resolution DEM data.
You could explore the spatial data catalogue which has a wealth of data, including rivers and catchments specific to the UK.
Finally I would not use the hydrosheds data as it is built from 90m SRTM which is fairly crude and will deviate considerably to what is actually on the ground, unless you intend to use it as some sort of overview/backdrop to other datasets?