After Dayu locked the yellow dragon in the stone trough of Hukou, he rushed to develop other rivers. A few years later, he came to the Yellow River again to observe the river conditions. One day, when Dayu was walking along the Yellow River Gorge, he suddenly encountered an old woman by the river, crying with snot and tears, which made Dayu very suspicious. 

He rushed forward to help the old woman and said to her softly: "Madam, tell me what's the problem. I'll help you solve it." The old woman looked at Dayu and thought he was very kind. She didn't think Dayu was a bad person, so she cried and said, "My son, the yellow dragon living in this river was locked under the stone trough of Hukou by Lord Yu that year. It dares not make trouble again. But every year it must eat virgin boys and girls on April 18th to celebrate his birthday. My little daughter was dedicated to the yellow dragon when she was 12 years old. This year my little grandson is 12 years old again. I'm afraid. I'm afraid that he will be elected by yellow dragon again. "The old woman cried again. Dayu comforted the old woman, stood by the river, and shouted: "Yellow dragon, you evil demon, how you dare not come to see me. I will have your body smashed to pieces and bones ground to powder today."

The yellow dragon came out and knelt down on Dayu tremblingly. In front of him, he vowed: "I dare not do harm to people anymore, please spare my life, Lord Yu." After listening, Dayu watched for a while, and felt that the yellow dragon was a little sincere, and said: "Since you have the determination to change your past, I will leave you alive and do more good deeds in the future to redeem your sins." The yellow dragon  nodded and entered the Dragon Palace with a breeze. Dayu blocked the front door and the back door. He still felt uneasy. He placed a jewel of locking dragon on the middle door, so that he could never get out of water to do harm to people forever. This day is the day of early spring, then people call this place Mengmen Dingbao.

In ancient times, there was a Dragon King Temple in front of Mengmen Mountain. The people on both sides of the Yellow River gather here twice a year to hold a memorial service for the Dragon King. They would kill pigs and sheep and sing for three days. It is called the "Temple Fair." Only in this way, the Dragon King would fall asleep in intoxication. Otherwise, the Dragon King would make trouble. There would be dust and stone flying as in storm. The flood would sweep across and drown the crops on both banks. The people would not be able to live in peace. Over time, the "Temple Fair" has become a custom of the people in the Mengmen Mountain. The Dragon King is a greedy and lustful king. Even though there were hundreds of maids of honour in the Dragon Palace, the Dragon King was always greedy for the ordinary people. During the "Temple Fair", as long as there was a lady who walked into the temple gate, she would disappear in the blink of an eye. It turned out that the Dragon King robbed her into the palace for him to have fun. Therefore, during the temple fair in this place, although it was very lively, there would not be a girl.

Later, when Dayu ruled the water, he passed through here. Knowing about this, he came up with a way to conquer the shameless Dragon King. Whenever "Temple Fair", he sent a brave strongman to stand on Mengmen Mountain and guard the dragon gate. Strangely speaking, the Dragon King was afraid of Yu's actions and never dared to do anything wrong. Since then, at the "Temple Fair", the women never disappeared. Over the years, the Mengmen has grown with the water, and even the largest floods haven’t submerged the Mengmen. If you visit here now, you can still see the scenery of Mengmen.

"Mengmen" is located in the center of the Yellow River. It looks like a boat from a distance and a mountain nearby. It is connected to the dragon gate in the south and Hukou in the north. It is a gem put by Dayu to control the dragon in Mengmen when he controlled the flood. Because it is a gem, it attracts the "night moon".

In the oldest days, there were ten suns in the sky, and they were on duty in the sky every day. One day, ten suns came to the sky together, and people in the world suffered a catastrophe. The ten suns were burning like fireballs, and the earth was about to catch fire. People all went into the caves to hide. The Jade Emperor quickly learned about this and sent Hou Yi to rescue the people on the ground. Hou Yi came to the world and shot down nine suns in a row. These nine suns cried in the sea all year round, regretting it, and feeling that they had no future. Suddenly one day, they heard that Mengmen was a gem. Through the gem, you could see everything in the sky. This was a great wonder. They said happily: "Now it's going to be a bright day." As soon as the brothers discussed it, they all came to Mengmen.

When the nine suns came to Mengmen, they saw the gems glittering in the water, floating with the water, giving off a strange light. When the water was high, the door would be high. When the water was low, the door would be low, which was very fun. Especially at night, the Mengmen was even more peculiar. The gods from all walks of life in the sky came to meet. They talked with ease and fluency. Chang’e in the moon was also dressed very coquettishly, dancing lightly, flashing in Mengmen. It was really lively and extraordinary. The nine brothers looked at it and felt very unhappy. They thought: How dare the moon compared with them back then! So the nine brothers were determined to compete with the moon, and they all jumped around the gem. Therefore, whenever the bright moon is in the sky at night, the beautiful girls will dance, and the nine brothers also jump in the Mengmen, trying to get out of the dragon gate and compete with the moon.

Five kilometers downstream of Hukou Waterfall, below the "Shili Dragon Trough", in the riverbed at the bottom of the Yellow River, there are two boulders fusiformis standing majestically in the giant current. They form two river islands. This is the Mengmen Mountain known as the "Stage of the Nine Rivers" in ancient times. It is said that these two small islands were originally a mountain. It blocked the river and caused floods. When Dayu controlled the water, he split the mountain into two and led the water to flow smoothly. These two islands look like a boat from a distance, like a mountain up close, and look down like a door. It is also said that in ancient times, the descendants of the Meng family brothers were washed away by the river and were rescued here, so the two islands are called Mengmen Mountain.

Big Mengmen Island is about 300 meters long, 50 meters wide, and about 10 meters above the water. On the stone cliff to the south, there are four characters "Lying to Control the Flood" inscribed by Xu Huanying, the guardian of Jinming County during the the early years of Yongzheng in Qing dynastry. The plaque is 2.5 meters long and 1 meter wide, which is a true portrayal of the wonders of the landscape here. Little Mengmen Island is more than 10 meters upstream of Big Mengmen Island. It is only fifty to sixty meters long. These two islands are all composed of massive gray-green sandstone produced horizontally. The rock is hard and has strong resistance to weathering.

Facing the turbulent mud flow, Mengmen stood upright with its head high. Even though the water is overwhelming, it never drowns the Mengmen. Mengmen "connects the dragon gate to the south with a trace of the sky in the north". Its majestic figure, dragon gate and Hukou form the Three Wonders of the Yellow River, and it is also known for its unique style. There are beautiful sentences in poems as follows “Fog and rain in four seasons sweep Hukou. The waves on both sides shock Mengmen."   
According to "Geographical Knowledge", Mengmen was originally a crack in the Yellow River bed. Hukou Waterfall appeared here at that time. As the earth's crust had risen for a long time, the downward cutting effect of the river was enhanced. There was headward erosion. The crack point moved up. The waterfall moved from Mengmen to its current location. The deep pool under the waterfall has developed into today's "Shili Dragon Trough". Mengmen Mountain is the rock mass left over from the deep pool of the waterfall.

Yellow River Mengmen is not only charming, but also has the beauty of "Night of Mengmen". In the study of the development history of the Yellow River, it also has important scientific value.