1. Introduction
The initial design of the Three Gorges Project （TGP） planned for three phases in the impoundment of its reservoir： the power generation period, with the water retained by a cofferdam; a preliminary operation period; and then a normal operation period. In 2003, the Three Gorges Reservoir was impounded to elevation （El.） 135 m, and power generation began with the water being retained by a cofferdam. In 2007, the reservoir was impounded to El. 156 m, and preliminary operations began. By 2009, as the main hydraulic structures were completed, the reservoir was fit to be impounded to its normal level of El. 175 m. However, it was still kept at the pool level during the preliminary period. Based on considerations of the resettlement progress, observed results of sedimentation at the reservoir tail area, the influence of sedimentation on Chongqing Port, and so forth, the duration of the preliminary operation period was determined at six years. Subsequently, in 2013, the Three Gorges Reservoir was impounded to its design pool level of El. 175 m; hence, the normal operation period began.