Abstract: During the field work for Sediment Dynamic System (SDS) four box-core samples were taken in a tidal channel of Skgit Bay, Puget Sands Washington U. S. A. By using the X-radiography equipment and the conventional analysis in laboratory, some results have been got from sedimentary structures as follows; 1. The tidal active layer (or reactivity layer) is defined, its thickness of 1.5-2.5 cm, and its influential factors are examined critically. 2. According to the features of the sedimentary structures, the layers of flood current and ebb current can be differentiated, the layers of bedload and suspended load discriminated, too. 3. There is a dominant current found along the bottom of the channel, and it is defined as the flood current 4. Based on the granulometric data of the sediment, the speed of the tidal current ups to more than 40 cm/sec. The judgments above are confirmed by spot data measured by SDS. 5. Using the X-radiographied films, the information of the channel sedimentation is got, for example net deposition rate is 0.5-1.0 cm, erosion rate is 1.0-1.5 cm in a tidal cycle. 6. A simple sedimentation model of a tidal channel in estuarine environment is set up finally