Abstract: The water-sediment regulation provides a good opportunity for the observation of the hyperpycnal flows at the Huanghe (Yellow River) estuary. The concentration (SSC) and median grain size of the suspended sediment are important factors to the formation of hyperpycnal flows. During the 10th water-sediment regulation on June 19^th, 2010, in order to study the variations in water and sediment discharge off the Huanghe River mouth and to predict the formation of hyperypycnal flow, synchronous survey were carried out at the Lijin station, which includes the sampling of the suspended sediment and measurement of the bathymetry and current system. The results suggest that when the reservoirs on the upper reaches of the river started to release water with low suspended sediment concentration (SSC) on schedule, the water discharge recorded at the Lijin station increased sharply, together with the intensification of riverbed erosion. A great amount of the coarser particles were resuspended from the riverbed, resulting in the increase in suspended sediment concentration (SSC) and sediment load (SL) of the water, as the major contributor to the rise of suspended concentration of coarser sediment. In the later stage, under the influence of hyperpycnal discharge off the Xiaolangdi Reservoir, the water discharge recorded at the Lijin station decreased quickly, while the SSC and SL remarkably increased off the Lijin station up to the river mouth. The remarkable increase in fine sediment concentration led to the heavy siltation on the riverbed. It was possible to predict the hyperpycnal process at the river mouth, since both high SSC and fine particles concentration are the necessary requirements for the formation of hyperpycnal flow. The hyperpycnal flow would not only deliver a large amount of sediments from the river to the coastal sea in a short period, but also bring impact to the geomorphological and biogeochemical evolutions of the estuary.