Simple mathematical approaches are used to investigate the relative importance of salinity, temperature, dissolved load, stratification of suspended load, and estuarine mixing on the direct formation of particulate hyperpycnal currents from river outflow. Specifically, the critical concentration of suspended sediment for hyperpycnal flow formation is related to a reference concentration that does not take any of these processes into account. The results show that the most important process in reducing this critical concentration is estuarine mixing, which can reduce it by several orders of magnitude. The other processes are relatively secondary but can all reduce the critical concentration by about one order of magnitude. Examples from rivers where hyperpycnal flows are known to form illustrate the influence of the described processes.