The work concerns a laboratory study to investigate the effect of changing the bed roughness of the main channel on the branching discharge and separation zone formed in the main channel as a result of discharging flow through a branch channel.The ratio ofmain channel roughness to the roughness of the branch channel bed has been changed between (1.7 – 3.9) for two branching angles (30˚ , 90˚ ) and for different values of main channel discharge ranged between (7 – 17) L/sec. Laboratory result shows that branching discharge ratio increases by the increasing the total discharge through the main channel at roughness ratio less than (2), where's the branching discharge decreases at roughness ratio greater than (2), and stay constant at roughness ratio about (2) for all discharge passing through the main channel.The result shows that the length of the separation zone decrease with the increasing in the roughness ratio and total discharge through the main channel.