The well-known and enormously controversial Three Gorges Reservoir (TGR) on the Yangtze River, China, impounded about 9.936 × 10⁹ m³ of water (between Qingxichang and Huanglingmiaodou) for the first time from May 25 to June 10, 2003, finally elevating the water level to about 135 m above mean sea level at the dam. Mainly on the basis of daily, monthly, and annual water and sediment data sets of the Yangtze River at mainstream hydrological stations above and below the TGR Dam in 2003, we preliminarily examined the effects made by the TGR water impoundment in June 2003 on the Yangtze River sediment. According to sediment transport balance method, the estimated amount of suspended sediment impounded in the TGR is (1) about 3.1816 × 10⁶ t between Qingxichang and Huanglingmiaodou during June 1 to June 10; (2) about 1.247 × 10⁸ t between Cuntan and Yichang in 2003, which in June accounted for 25.9% and from June to December accounted for 99.2%; and (3) about 1.359 × 10⁸ t between Cuntan and Yichang in 2003, if considering eroded sediment from the channel between the TGR Dam and Yichang. The TGR sedimentation resulting from the TGR water impoundment leads to unnaturally clear water discharged from the TGR Dam, which results in the Yangtze River suspended sediment concentration and sediment load downstream of the TGR Dam in 2003 being reduced more than those above the TGR Dam, and the channel just below the TGR Dam significantly eroded (about 4.759 × 10⁵ t between Yichang and Shashi from June 1 to June 10).