This dataset provides 100 model realisations of daily river flow in cubic metres per second (m3/s) for 1,366 catchments, for the period 1962 to 2015. The dataset is model output from the DECIPHeR hydrological model driven by observed climate data (CEH-GEAR rainfall and CHESS-PE potential evapotranspiration). The modelled catchments correspond to locations of National River Flow Archive (NRFA) gauging stations and provide good spatial coverage across the UK. The dataset was produced as part of MaRIUS (Managing the Risks, Impacts and Uncertainties of drought and water Scarcity) to provide national scale probabilistic flow simulations and predictions for UK drought risk analysis. MaRIUS was a UK NERC-funded research project (2014-2017) that developed a risk-based approach to drought and water scarcity.