Beginning in 2003, China's Three-Gorges Reservoir will start water impoundment in phases. By 2009, it will be holding 40 km3 of water, flooding a stretch of middle Yangtze River about 600 km in length. The water impoundment process represents a geophysical controlled experiment offering a unique opportunity for detailed studies of a classical forward/inverse modeling problem of surface loading. While Wang <2000> studied the large loading effects on a local scale, we aim for longer spatial scales upwards from several hundred km, specially on the time-variable gravity signals that can be detected by the newly-launched GRACE satellite mission, whose 5-year lifetime (until 2007) will span the major impoundment period. Our results using the Green's function method adopting the PREM elastic Earth model indicate that the per-year geoid height increase is above the GRACE observational sensitivity out to harmonic degree 20, and to degree 50 (corresponding to a wavelength of 800 km) when integrated over the 5-year period.