Abstract: Estuary is a transitional area from river to sea, which represent an environment with complex essential factors, so they are highly different from each other in terms of the patterns of temporal and spatial variability of the processes. Generally, there are many parameters to define the boundary between the river and the sea. The relationship among the demarcation methods based on the geometric and geomorphologic character and other estuarine parameters was analyzed. For each of these, a geomorphic node may be the most suitable method to determine the location of boundary. At a point on the river centerline, a function of B=f(L) was defined, where B is the channel width at this point and L is the distance from this point to the entrance of estuary along the central axis of the river. By constructing a mapping function, the geomorphic node can be found and the quantitative segmentation of the boundary between the river and the sea can be realized. For different kind of estuaries, the B-L curve was constructed to defined the river-sea boundary based on the geomorphic node. The result show that for the blocked estuary the rates of river width variation at the geomorphic node are -0.5 to 0. For the channel-form estuary and interconnected estuary, the rates of river width variation at the geomorphic node are 0.5 to 1.2. For the firth estuary,the rates of river width variation at the geomorphic node are 0.6 to 1.5. For the funnel-shaped delta, the rates of river width variation at the geomorphic node are 1.3 to 4.5. For the bird-foot delta and fan delta,the rates of river width variation are always 0 to 0.2, and the entrance of estuary is regarded as geomorphic node. This B-L curve is applied to measure and calculate the length of estuary shoreline of China mainland, and some perfect results are acquired.

Key words: river-sea boundary, geomorphological parameters, geomorphic node, rate of river width variation