Many small estuaries and coastal lagoons in different parts of the world may be classified as temporarily closed/open ecosystems. They are blocked off from the sea for varying lengths of time by a sand bar, which forms at the estuarine mouth. The lengths of the closed and open phases, which are determined primarily by the interaction of river inflow and the sea in the mouth region, affect the structure and functioning of the estuarine biotic community. Freshwater inflow to such estuaries is normally not measured, and observations on the duration of estuarine mouth openings/closures are very scarce. As a result, relevant management decisions are often made on the basis of general experience and intuitive judgment. This paper describes an innovative approach for linking hydrologic data to mouth state in ungauged estuaries. A key characteristic in the method is the stream/river flow duration curve. It is first established for a daily index, which reflects the upstream catchment wetness and is calculated using rainfall information from the nearest rain gauge(s). This duration curve is then used to convert the current precipitation index time series into a continuous daily inflow time series at the ungauged estuarine mouth location. The conversion is based on the assumption that precipitation index values in a small catchment, and daily inflows to the estuarine mouth correspond to similar probabilities on their respective duration curves. The paper further illustrates how the generated inflow data could be used for the simulation of a continuous time series of estuary mouth openings/closures. Inflows are routed through a reservoir model, and the estuary mouth is considered open on days when the spillage from an estuarine “reservoir” occurs. The approach is illustrated using limited observed data on estuary mouth conditions from the South African coastline.