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
you can check with the website for Shipping ministry for the data for the river meandering data, cross sectional data, water depth, velocity data etc. you can get from ministry of shipping, Eroupe