Surface water quality data for the Conwy catchment, North Wales are presented. The data cover stream and estuary survey locations sampled manually from 2007 to 2011 at varying intervals between fortnightly and quarterly. Survey locations were selected as draining the main landscape classes in the catchment, which include upland (>300m) and lowland (<300m) environments under trees, moorland or improved agriculture. Also included are a suite of locations on the major rivers, and in the estuary. The purpose of the data is to provide support for estimating loads and testing hypotheses relating to the relationship between subcatchment properties and water quality, and the relative importance of mixing and in-stream processes in influencing downstream water quality. Streams draining peat, with high dissolved organic carbon concentrations, are well represented. Upland drainage is generally acidic, and lowland circum-neutral. The lowland rivers are not heavily contaminated with nutrients, although they show evidence of some loss of nitrates in particular. There is little industry in the catchment beyond agriculture and tourism, though there is some legacy contamination from metal mines in a small part of the catchment. Measurements are provided for major cations and anions, pH, alkalinity, DOC, Al, Fe and conductivity for most samples, with values for a wide range of trace elements for a limited number of samples. Full details about this dataset can be found at https://doi.org/10.5285/c53a1f93-f64c-4d84-82a7-44038a394c59
