Abstract: The separation zone develops immediately downstream of the open-channel junction when branch flow enters,resulting in contraction of channel cross-section This particular hydraulic characteristic influences significantly the local sedimentary processes,channel scour and sidewall erosion Based upon the experimental investigation and theoretical analysis,the shape and size of the separation zone are studied,and the formulations for both contraction coefficient and energy loss coefficients are presented,together with the comparisons with experiment results Both the analytical solutions and the experiment results indicate that for the given junction angle the shape of the zone remains essentially const ant The fine relationships show the both width and the length of the separation zone increase either as discharge ratio or junction angle increases.