A physical model test for the diversion project of the Fu River into Qinglan Lake was conducted to investigate the siltation and adjustment law of a channel connected to a lake with a gradually widening boundary. The results show that there exists a spatial variation for deposition styles and cross-section adjustment along the channel in the initial siltation state. A stable single channel develops with natural levees in the upper reach. A relatively stable braided channel develops with transverse bars, point bars and longitudinal bars in the middle reach. In the lower reach, siltation bulks of different scales and siltation location adjustment appears accompanying with the phenomena of cut-bank and channel diversion. The reason for such spatial variation has been discussed in the perspective of hydrodynamic and sediment dynamics. The relationship between the mainstream flow velocity and the incipient velocity of median size sediment is an important indicator to determine the adjustment of river bed erosion and deposition, which could possibly be a key reason for the variation of initial siltation along a widening river channel.