Why did Huayin immigrants move in 1956?

In order to support the construction of Sanmenxia Reservoir. According to the official information of huayin city government in Shaanxi Province, 1956 to 1960, in order to support the construction of Sanmenxia Reservoir, huayin city immigrants left the land and began to move under the call of the government to "move one family and protect thousands of households". Huayin city belongs to Weinan City, Shaanxi Province.