This data set archives all inputs, outputs and scripts needed to reproduce the findings of W.H. Farmer and S. Levin in the 2017 Journal of the American Water Resources Association article entitled “Characterizing Uncertainty in Daily Streamflow Estimates at Ungauged Locations in Support of the Massachusetts Sustainable Yield Estimator”. Input data includes observed streamflow values, in cubic feet per second, for 66 streamgages in and around Massachusetts from 01 October 1960 through 30 September 2004. Cross-validated streamflows, in cubic feet per second, and estimated correlations are included for all basin pairs as archived by Archfield et al. (2010; USGS SIR 2009–5227). Comma-separated-values files contain output data, including all estimated daily confidence intervals, performance thereof (coverage ratio, average width indices and interval skill scores), and multi-day aggregated performance metrics. ESRI ArcGIS shapefiles are available for all maps produced in the original publication. This archive also includes an R script capable of reading the input files and producing output files and figures. See the README.txt file for a full description of model application. The larger publication can be found at https://doi.org/10.1111/1752-1688.12603.