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Abstract: Lake Chien-Hu, or the Lake of Mirror, was one of the ancient reservoirs to the southeast of Hangchow, Chekiang. Its dikes totally 63.5 km long plus a series of locks dammed up the several rivers flowing northward from the Kwei-Chi Hills and formed a large lake with a surface of about 206 square kilometers. For about 800 years since its completion in the East Han Dynasty, the reservoir checked the monsoonal floods and facilitated the irrigation works over the Shan-Hwei plain to the south shore of the Bay of Hangchow. Then, fluvial deposit raised the reservoir bottom and, during the early Sung Dynasty, it had become so shallow in some parts, that people began to reclaim paddy Melds by embankments. The continuance of these processes, natural and 'human, further reduced the water surface, uatil the whole reservoir was finally filled up in the South Sung Dynasty.