A river bifurcation is critical for distributing water, sediment and nutrients to the downstream branches of deltaic river networks. However, the downstream branches of a bifurcation can be linked by a connecting channel cutting through deltaic floodplains. The floodplain connecting channel as a downstream control can affect water partitioning at the river bifurcation and hence the hydrological connectivity of the river network. However, its effects are still largely elusive. In this study, we explored how a connecting channel linking downstream branches affects water partitioning at the upstream bifurcation and water distribution along the two branches. The investigation was conducted through idealized numerical simulations using Delft3D, followed by analysis of the cascading effects on the hydrological connectivity of river networks using graph theory. The results show that connecting channels can mitigate asymmetric water partitioning at the upstream bifurcation. However, this happens at the expense of inducing more uneven flow at the downstream outlets. The flow adjustment is due to the altered spatial water surface slope in the two branches associated with the flow exchange from one channel to the other via the connecting channel. Further analysis of hydrological connectivity shows that connecting channels can generally reduce the vulnerability of the channel network to hydrological alterations, especially changing inflow, by enhancing flow exchange between the two branches. Our results suggest that connecting channels are critical paths for hydrological connectivity, which have important implications for the management of deltaic river networks and their floodplains.