Combining flows often occur in open channel networks of drainage systems and river engineering. Open-channel junction flows were analyzed by solving the depth-averaged two-dimensional, elliptic Reynolds-averaged Navier-Stokes equations with the Hanjalic-Launder (H-L) modification to the k-ε turbulence model without the free surface “rigid lid” approximation with an efficient finite-volume procedure. The model can also analyze flows with separation. The model was used to analyze the relative importance of various factors and was compared with laboratory measurements. The H-L modification produced much better simulations of the separation zone size with 20% better accuracy than the standard k-ε model. The H-L modification was then used to study the characteristic of junction flows and the separation zones with different discharge ratios. The simulational results show that separation zone size decreases as the discharge ratio of the upstream main channel to the downstream channel increases. Open-channel junction flows are often solved in depth-averaged two-dimensional, elliptic Reynolds-averaged Navier-Stokes equations with the Hanjalic-Launder (HL) modification to The k-ε turbulence model without the free surface “rigid lid” approximation with an efficient finite-volume procedure. The model was used to analyze the relative importance of various factors and was compared with laboratory measurements . The HL modification produced much better simulations of the separation zone size with 20% better accuracy than the standard k-ε model. The HL modification was then used to study the characteristic of junction flows and the separation zones with different discharge ratios. The simulational results show that separation zone size decreases as the discharge ratio of the upstream main channel to the downstream channel in creases.