This being a very old story, there are probably multiple versions of this story. I got this version of the story about New North Bridge from the baike page.
The Story of New North Bridge
Hundreds and hundreds of years ago, Zhu Yuanzhang established the Ming Dynasty and united China under his rule, and the world was at peace. One day, Zhu Yuanzhang had a dream. In this dream, the Dragon King declared that the successive years of war had incurred the wrath of the heavens, so he and his wife were going to take away all the water in Beijing. Zhu Yuanzhang asked his military advisor, Liu Bowen, what he should do. After a few moments of silence, Liu Bowen said, “It seems that the successive years of war really has enraged the Dragon King.” Zhu Yuanzhang asked if there was any way to redress this problem, and Liu Bowen replied that there was, but only a loyal and devoted general could do it. For this purpose, a great general with illustrious accomplishments on the battlefield was found.
Liu Bowen ordered the general, “Today, at noon, you will put on your battle gown, mount your warhorse, and outside the east gate, you will see an old man and woman pushing a watercart, resting by the road. You will go up and break the watercart. Don’t ask for anything and after breaking the watercart, don’t turn your head around, or else it will be hard to keep your life. Once you return to Xizhi Gate, you should be fine.”
The general received the order, and at noon, he went to the east gate, and sure enough, he saw an old couple pushing a watercart resting by the road. He galloped up and broke the watercart, then rode for all his life to Xizhi Gate. Upon reaching the gate, he thought that he was fine and turned his head to look around. What he saw was a massive wave coming down on him, and so he died. It turned out that the elderly couple had been the Dragon King transformed and the great general had been the Ming Dynasty’s General Gao Liang! After Gao Liang punctured the water basket that was the Dragon King’s daughter, the Dragon King’s wife brought their injured daughter away to the Black Dragon Pool in the mountains to the north, where she set up a home. It’s said that the fish in the Black Dragon Pool that like to run into rocks are a sort of dragon, and they are precisely the Dragon Wife’s grandchildren.
By breaking the water basket, Gao Liang provoked the Dragon King into chasing him with a massive wave. After Gao Liang died, the water remained. But the Dragon King’s fury was not yet sated, but he also didn’t dare to provoke Liu Bowen. Thus, he brought his son and his son’s bellyful of sweet water and swam beneath the ground using the spring source of Jade Spring Mountain. This is the reason why the water from Jade Spring Mountain is so sweet. The Dragon King muttered to himself, “Liu Bowen! Liu Bowen! Just because I can’t afford to mess with an old Daoist like you, you think this is finished? There will come a day when you finish repairing this city, and when you do, Liu Bowen, you’ll have to leave! That’s when you’ll have to obey this old dragon!”
Thus, the old dragon and his son waited, one day, two days, one month, two months, one year, two years. Finally, Liu Bowen finished renovating Beijing to be in accordance with the Eight-Armed Nezha Map (The story of which would require a long digression and will perhaps be told at a later date.) Liu Bowen was just preparing to return to Nanjing to report to the Emperor when he suddenly recalled that evil dragon that was making trouble. He thought, this repulsive evil dragon might make trouble once I leave! Ah, if Yao Guangxiao was here keeping watch, it would be fine, but he went and became a monk. What do I do?
Liu Bowen had no other choice than to seek Yao Guangxiao out. On the same day, Liu Bowen found Yao Guangxiao in a temple outside the city. After stating the reason for his visit, he said to Yao Guangxiao, “The Eight-Armed Nezha Map was drawn by the two of us. When I return to report to the Emperor, I intend to say that Beijing was renovated by the two of us. You are still the second military advisor.” Yao Guangxiao was very happy to hear this and agreed to watch over Beijing. Thus, Liu Bowen packed his luggage, gathered his attendants, and left Beijing to go report to the Emperor.
Upon hearing that Liu Bowen had left, the Dragon King brought his son and traveled through the underground waterways to Beijing. Upon reaching Beijing, they spied the mouth of a spring, and charged out it. Not only did they fail to break through it, they got a bump on the head for their troubles. It turned out that some evil-suppressing talisman had been placed above the spring. The Dragon King and his son attempted to charge out of quite a few springs, but they failed each time, the heads eventually swollen from all the bumps they got, and they began to thoroughly loathe Liu Bowen. Finally, in the northeast part of Beijing, the dragon father and son spotted another spring mouth and charged at it. Surprisingly, they smashed through and reached the surface. This place would later be known as New North Bridge.
As soon as they appeared on the surface, the Dragon King transformed into an old man and his son transforming into a young fellow. The two brought the water from the spring up with them, and in the blink of an eye, everywhere around New North Bridge was instantly flooded. The populace wailed and escaped in their panic, leaving only the father and son. They walked on the waves, looking quite pleased with themselves. At this point, someone had already notified Yao Guangxiao of what was going on. Upon learning this news, he thought, Liu Bowen really does know his stuff. He foresaw that the evil dragon would come to make trouble, and the evil dragon really did come!
Yao Guangxiao changed his clothes, took up a sword, and took off with lightning speed to New North Bridge. With a few slashes of his sword, he stopped the flooding, and with a leap, he landed on the waves. He yelled, “Evil creature, you dare attempt to flood Beijing! Take a look at the Second Military Advisor’s power!” The Dragon King was shocked and thought to himself, Liu Bowen is already gone, so where did this Second Military Advisor come from? And he’s no weakling, with a few slashes of his sword, he put a stop to the flooding, so we have to be careful. He shot a glance at his son and the two each revealed their own Azure Dragon Sword. Without the slightest explanation, they fiercely attacked Yao Guangxiao. Yao Guangxiao quickly made to defend himself, and with a flash of cold and threatening sword glows, the three clashed.
Now, if it was just the dragon father, Yao Guangxiao would be able to deal with him, and he would have had it even easier if it was just against the dragon son. However, when father and son worked together, Yao Guangxiao was unable to endure. Each move of his was slower than the last, but just as it seemed like he was about to lose, there was a flash from the clouds. With a yelp of pain, the Dragon King was lying on the waves, bleeding from the thigh. This happened so quickly that both Yao Guangxiao and the dragon’s son were stupefied. Yao Guangxiao was looking for the figure of his mysterious helper when he heard a shout. “Military Advisor Yao, quickly seize the young dragon, I am General Yue Fei of the Song Dynasty!” Yao Guangxiao was elated to hear these words. While he stabbed at the dragon son, he called, “Marshal Yue, please wait!” Marshal Yue did not respond. The dragon son was still stunned, so he was immediately knocked down by Yao Guangxiao’s sword. The father and son were both captured, and the water flooding New North Bridge returned to the ground, never to rise up again.
After the dragon father and son were caught, Yao Guangxiao was in a bind. Just where would he keep this pair of evil dragons? He thought and thought, and finally came up with a good method. He locked up the older dragon in the spring of New North Bridge, building a deep well on top of it, and then tying it up with a long chain. He then erected a temple around the well, and as for what god would be worshiped in the temple? He felt that since it was Marshal Yue that had helped him capture the dragon father, he might as well dedicate the temple to Yue Fei. Before the dragon father was sealed in the spring, he asked Yao Guangxiao, “Military Advisor Yao, do you really have to lock me up for one thousand, ten thousand years? When will I be able to come out?” Yao Guangxiao answered, “Wait for this bridge to turn old, and when it’s repaired, that will be the time when you can come out.” From that moment on, this place was called New North Bridge, and never was there a bridge in that place that needed any sort of repair.
Yao Guangxiao locked the dragon son under the spring beneath the bridge of Chongwen Gate. Before the dragon son was locked away, he asked Yao Guangxiao, “Military Advisor Yao, do you really have to lock me up for one thousand, ten thousand years? When will I be able to come out?” Yao Guangxiao answered, “When you hear the beating of the drum from the city gate, that will be the time when you can come out.” From that moment on, whenever Chongwen Gate was opened or closed, a drum was no longer beat. Instead, a bell was used, which is why old people often say that of the nine gates of Beijing, eight beat drums and one rings a bell.
Now, unfortunately for the dragon son, Chongwen Gate no longer exists, so no longer will the beating of the drum be heard from the city gate. But if you take Line 5 of the Beijing subway to Beixinqiao Station, you might see a bridge there emblazoned with the words ‘北新桥’, apparently built to match with the name of the place. Now if this bridge was to some day be in the need of repair, then perhaps…?