Because the Yellow River is turbid, ordinary fish can't grow. Only the pollution-resistant carp grows very well, and because its environment is yellow muddy water, the Yellow River carp is golden scales and very beautiful. The ancients found that golden carp often rose against the current, forming a jumping group at Longmen Waterfall. But above the waterfall, no fish can survive because of the fast current. So the ancients imagined that these golden carp might have jumped over the Longmen and turned into dragons. Later, there was a saying that "a dragon gate is worth ten times".
At present, due to the erosion of rivers for hundreds of years, the riverbed collapses and the waterfalls gradually recede. Now the waterfall has retreated dozens of kilometers to the hukou. There is no waterfall in Longmen. However, the legend of yue longmen, a carp, has been handed down and influenced Japanese and other places. Japanese custom hangs the carp flag on Children's Day, hoping that children can jump over the dragon gate and become dragons like carp. Although there is a saying in China that "one hopes for success", in feudal times, "dragon" was the patent of the emperor. If the children of ordinary people really turned into dragons, they might be beheaded, so no one dared to take the risk of claiming that their children would become dragons.