The long-term measured data of Sanmenxia Reservoir are analysed to enable consideration of the causes of water levels at Tongguan station. Measures to reduce flood heights are also analyzed. The main causes of rising water levels at Tongguan station are limited flow in the reservoir in flood season and high water levels in front of the dam in the dry season.To change the operational mode of Sanmenxia Reservoir is the most effective measure to reducing flood heights at Tongguan.