The Integrated Hydrological Units (IHU) of the United Kingdom (UK) define geographical reference units for hydrological purposes including river flow measurement and hydrometric data collection in the UK. The datasets in this series represent the following component polygon layers: Hydrometric Areas with Coastline, Hydrometric Areas without Coastline, Groups, Sections, and Catchments. Each layer represents a different level of spatial detail. The coarsest level, Hydrometric Areas, is provided in two versions to meet differing user needs. Each Hydrometric Area is made up of one or more Groups. 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 in some cases also from local county names. Each Group is made up of smaller units called Sections. A Section is the drainage area of a watercourse between two confluences. Only confluences of named watercourses were considered. Similarly to Groups, 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. Catchments represent the full area upstream from an outlet of every Section. Polygons within each layer do not have gaps and, with the exception of Catchments, polygons within one layer do not overlap. Identifiers and attributes have been calculated so that direct upstream and direct downstream units within the same layer and also related units across layers can be easily selected. The Hydrometric Areas with Coastline layer covers Great Britain and Northern Ireland, but all other layers currently cover Great Britain only as no dataset with river geometries and names with suitable detail is available for Northern Ireland.