Rock-fill dykes are often damaged caused by rapid flow currents in a mountain river. Based on the relationship between the rock size on cover layer and its incipient velocity, it is found that rock weight is directly proportional to the 6th-9th power of incipient velocity, and 50% increase of the velocity may result in about 40 times increase of the rock weight. Therefore, it is inappropriate to improve the stability of rock-fill dykes by simply increasing the rock weight. Some new measures should be used to reach this purpose.