A project of river channel restoration has been promoted in the Shibetsu River, that attempts to restore the natural meander channel remained as a oxbow lake by letting the river water run through. In this project, it is intended that water streams into both the restored meandering channel and the straightened current river course by a small dam constructed downstream of the bifurcation point. But there are few papers treating the relationship between bifurcated flow ratio and the height of a dam in such complex channel system. This study is devoted to investigate the regulation function of a dam for bifurcated flow rate into an oxbow channel. Theoretical solutions for the flow rate are derived from the momentum balance of the several controlvolume sections of the bifurcated area. The results are compared with observed data in model tests and in field survey at a study section of the Shibetsu River to show good agreement.