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. Groups represent the intermediate units of IHU in terms of spatial resolution. The most common size of a group is around 400 square kilometres. Each Group carries a name constructed from names of the major river flowing through the Group, the major river flowing into the Group, the major river into which the Group flows, and sometimes also from local county names. Each Group consists of one or more Section and Groups can be combined to form Hydrometric Areas without Coastline. 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. Full details about this dataset can be found at https://doi.org/10.5285/f1cd5e33-2633-4304-bbc2-b8d34711d902