In this paper, a method for simulating river headwater flows in Australia's southern Murray-Darling Basin is described. This work was conducted for the purpose of analysing competing demands for water in the Basin. The methodology described is largely driven by two issues. Firstly, the spatial relationships between river headwaters need to be captured in conjunction with any temporal correlations that may exist (adding to the complexity is the fact that some river headwaters have no flows for consecutive months). The second issue requires selecting a robust methodology which can be used within a large-scale simulation framework.