These data are cellulolytic decomposition in eight Welsh upland rivers with contrasting land-use, moorland and exotic conifer, in response to riparian deciduous leaf addition. Eight sampling reaches were chosen at two sites, Llyn Brianne (4 reaches) and Plynlimon (4 reaches). The experiment consisted of adding deciduous leaves to half of the reaches whilst the other half were maintained as a control (no addition of deciduous leaves). To characterise the cellulolytic decomposition of the studied streams, cotton strips were placed and then collected during January 2013 (before deciduous leaf addition) and March 2013 (after deciduous leaf addition) in each sampling reach. The main goal of this survey was to examine how aquatic biodiversity and litter decomposition respond to leaf addition in moorland and conifer forested rivers. Dr Isabelle Durance was responsible for organising the surveys, Dr Dan Perkins was in charge of collecting, processing and sorting the samples. The work was carried out under Diversity in Upland Rivers for Ecosystem Service Sustainability (DURESS) project (Grant reference NERC NE/J014818/1). DURESS was a project funded by the Natural Environment Research Council (NERC) Biodiversity and Ecosystem Service Sustainability (BESS) programme. Full details about this dataset can be found at https://doi.org/10.5285/53b4ec90-cb8a-49c6-a6f9-18a77f980b0f