1. Graben structure: The structure of Yuncheng basin caused the surrounding mountains to rise due to faults, while the lake area was relatively depressed. This structure leads to water accumulation in low-lying areas, and because of the characteristics of graben structure, the lake water can not leak out, thus forming an internal flow area.
2. Influence of regional water balance and lithosphere structure: Affected by regional water balance, including negative water balance in arid and rainless areas, if there is not enough water flowing into the ocean, an internal flow area will be formed. At the same time, in some closed basins or closed depressions, internal flow basins often appear, which is caused by the control of lithospheric structure.