Dear Christos, some of the ...

Dear Christos, some of the organisation sharing data on rivers are - 

• European river catchments  - The purpose of the European river catchments (ERC) is to provide a homogeneous European catchments dataset at scale 1:1 million that can be used together with the digital topographic data of EuroGlobalMap http://www.eea.europa.eu/data-and-maps/data/european-river-catchments-1

• ESRI - Europe Major Rivers - Europe Major Rivers is a polygon theme representing the major rivers of Europe. Attribute information includes the name, type, country, rank and area of the river. This dataset is a part of the 2008 ESRI Data & Maps collection https://earthworks.stanford.edu/catalog/columbia-columbia-esri-2008-europe-mjrrivers

• Natural Earth - Rivers + lake centerlines -  Single-line drainages including optional lake centerlines and supplementary data for North America and Europe. http://www.naturalearthdata.com/downloads/10m-physical-vectors/10m-rivers-lake-centerlines/

Hope this is useful for your work. Best luck!