This dataset consists of headwater quality data for sites across Wales between 2021 and 2023), collected as part of the Environment and Rural Affairs Monitoring and Modelling Programme (ERAMMP) National Field Survey. Data include water chemistry and ecological quality metrics, River Habitat Survey data (including information on substrates, flow types, aquatic vegetation, channel and bank materials, modifications and natural features, riparian land use, artificial features, spot check records, tree cover and associated features and Habitat Modification and Habitat Quality indices), macro-invertebrate taxonomic and abundance data, diatom taxonomic and abundance data, headwater quality indices derived from both the invertebrate and diatom communities and associated environmental data. Headwater streams were surveyed within a set of 101 x 1km squares across Wales, collected as part of the Environment and Rural Affairs Monitoring and Modelling Programme (ERAMMP) National Field Survey. ERAMMP follows an earlier monitoring programme, the Glastir Monitoring and Evaluation Programme (GMEP), which was set up by the Welsh Government in 2013 to monitor the effects of the Glastir agri-environment scheme on the environment and ran from 2013 to 2016. The field survey element for both GMEP and ERAMMP was based on a stratified random sampling design of 300 x 1km square sites across Wales (225 of which were resurveyed for ERAMMP in 2021-23, though not all squares contained headwater streams). It was managed by the UK Centre for Ecology & Hydrology. Full details about this dataset can be found at https://doi.org/10.5285/776f99f4-1506-4f7f-920f-8318561d4085