The initial space settings of suitable environments for plants strongly affect the mutual feedback evolution of the river landscape and terrestrial plants. Thus, based on the morphological characteristics of newly-defined systematic out-branching channels in nature, this study performs an effective simulation of the flow of a designed moderately curved bend with a single short branch. The practice-based channel curvatures and branch on–off conditions are controlled in ANSYS FLUENT. The results show that: (1) the core zone of the depth-averaged primary velocity excess is approximately inversely equivalent to the channel migration potential; (2) the existence of the branch can strongly promote the formation of a new core vorticity zone and the conflicting development of the inner-negative and outer-positive vorticity zone after the bifurcation site at the cross-sections; (3) the free-flowing branch can greatly diminish the downstream helical flow strength; overall, the variation tendency of the ratio of helical flow strength to discharge squared is immune to the small range of change in stable inflow; and (4) the downstream channel is a strongly erosive region with the branch outlet closed, judging by the shear stress distribution; otherwise, it is a deposition region. The findings lay the groundwork for harmonious optimization of branch and plant configurations in river-bend systems.