From On Desperate Ground: The Marines at The Reservoir, the Korean War’s Greatest Battle, by Hampton Sides (Doubleday, 2018), Kindle pp. 82-83:
When Japan took formal possession of Korea, in 1910, Hamhung was a medieval city steeped in just these sorts of myths and folk traditions. But in the mid-1920s, as the Japanese tightened their grip on the country, modernity began to arrive. A team of Japanese engineers struck upon an ambitious idea: They would build roads into the mountains northwest of Hamhung and harness the might of the Changjin River—Chosin in Japanese—an important tributary that flowed north toward the Yalu. In the highlands, some seventy road miles from Hamhung, the engineers would construct a large dam that would flood the valley floor. The Changjin waters would rise, swallowing the wrinkled country, and the resulting reservoir, with all its scallops and appendages, would extend southward for more than forty miles. It would be a deep lake splayed out in the mountains, practically on the rooftop of Korea.
This scheme alone was considered a nearly impossible feat, but then the engineers envisioned something bolder: They would effectively reverse the course of the river by building a network of pipes near where it entered the lake on its south end. The pipes would snake along, often underground, carrying cold lake water from the mountains to the coast. Thus, a river that had once flowed north would flow south, through man-made conduits. Working with gravity, these tubes of racing water would feed into a series of hydroelectric plants down on the plain that would supply Hamhung and its neighboring port city of Hungnam with enough power to transform the area into a military-industrial center, perhaps the largest in Korea. Some said it was quixotic.
Some said the engineers were tempting fate, manipulating sacrosanct forces of nature. But the immense project worked as planned. The Chosin Reservoir was completed in 1929, the year Lee was born, and, with dizzying speed, Hamhung-Hungnam underwent a metamorphosis, much of it under the direction of the Noguchi Corporation, a Japanese conglomerate founded by a chemical engineering mogul named Jun Noguchi, who was said to be the “entrepreneurial king of the peninsula.” A nitrogen fertilizer plant, the largest in the Far East, was quickly constructed, and the area became one of the world’s largest producers of ammonium sulfate. Then came oil refineries, chemical concerns, textile mills, metal foundries, munitions works. They produced dynamite and mercury oxide powder and high-octane aviation fuel. It was a grinding, stinking, spewing complex of industries designed to fuel Japan’s expansionist aims across Asia.
Thousands of peasants, many of them displaced by the new lake, moved down from the mountains to work in the factories. Schools sprang up, a train station, a city hall, suburbs, all of it stitched together with streetcars and underground sewer systems and electricity and telegraph wire. It was a modern marvel of civil planning and central design—at least that was how the authorities portrayed the region’s transformation. Through Japanese ingenuity and Korean sweat, men had built a lake that built a city.