This dataset is part of Integrated Hydrological Units (IHU) of the UK, a set of geographical reference units for hydrological purposes including river flow measurement and hydrometric data collection. A Section is the drainage area of a watercourse between two confluences. Only confluences of named watercourses were considered. Each Section carries a name constructed from names of the major river flowing through the Section, the major river flowing into the Section, and the major river into which the Section flows. Sections are spatially consistent with Groups: each Group is made up of one or more Section. Each Section is associated with one Catchment representing the full area upstream from the Section outlet. Identifiers and attributes have been calculated so that direct upstream and direct downstream IHU units can be selected. This layer currently covers Great Britain only as no dataset with river geometries and names with suitable detail is available for Northern Ireland.