It is important to study the internal relationship between Jingjiang River (the middle reach of the Yangtze River) and Dongting Lake in theory. This research includes the runoff variation and rules of erosion and deposition in Jingjiang River, and the fluvial process of three out-flowing streams into the Dongting Lake. First of all, the runoff variation and erosion/deposition in Jingjiang River is presented in the following aspects the characteristics of river cut at the reach of braided channel of Jingjiang River; the discharge flow increment in Jingjiang River has intensified the erosion and has affected the three out-flowing streams; erosion exists simultaneously in the Jingjiang River and the out-flowing streams; the erosion in a certain cross-section of Jingjiang River and its middle-lower reaches achieves equilibrium (which means erosion stops) after the completion of Three Gorges Reservoir construction, and the time of the equilibrium is discussed; the sediment load variation at Jianli station is estimated. Furthermore, in terms of the fluvial process in the three out-flowing streams, the properties of out-flowing streams as channels located on delta is described; the mechanism of zero flow and formation of seasonal stream is analyzed; deposition caused by out-flowing, deposition equilibrium, and deposition of bifurcation channels of the three out-flowing streams are presented; sediment-carrying capacity reduces significantly during the erosion of out-flowing streams; and the decrease of runoff mitigates after the operation of Three Gorges Reservoir.