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The results are discussed of the long-term observations of the system of mega-cusps (large-scale shoreline cusps) revealed on the coast of the Gulf of Terpenya of Sakhalin Island. Typical spatial step of the rhythmic forms is about 800 m and the amplitude of shoreline undulations is of tens of meters. Despite of significant changes in temporal scales of the seasons, years and decades the main morphological features of mega-cusps are kept over the period of more than 60 years. The model is suggested explaining creation of rhythmic shoreline patterns by self-organization of bottom relief under the wave impact. Development of mega-cusps is shown to be connected with generation of nearshore circulation cells. Due to flows within the cells a small initial perturbation of shoreline contour can increase in time. A positive feedback between the coastal morphology and the hydrodynamics arises when a certain relation is established between the length of perturbation, the width of surf zone and the velocities of nearshore currents. It is concluded that the system of mega-cusps in the Gulf of Terpenya is mainly supported by the moderate waves of significant wave height about 1.6 m.

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