Riverbank areas are prone to landslides, where one of the causes is an erosion of river wall soil by river flow. Pesanggrahan River, one of 13 large rivers that cross the city of Jakarta, is also threatened by landslides on its banks and slopes in several places. To protect the riverbanks from landslides, riverbank reinforcement has been built at several points in the Pesanggrahan River. However, it is necessary to analyze the impact of bank reinforcement which will affect the stability of the river channel, especially in the downstream part of the protected river channel. The flow profile was modeled using HEC-RAS software for conditions before and after the construction of bank reinforcement. Analysis of river channel stability emphasizes changes in roughness values due to changes in riverbank material, which originally came in the form of soil with wild floras to impermeable structures. The analysis results show that riverbank protection causes instability of the river channel in the downstream part due to changes in roughness value and flow shear stress along the river channel. If the value of flow shear stress (τ0) exceeds the critical shear stress (τc), erosion and deposition will occur. Novelty in this study, especially in the Pesanggrahan River, is the relationship between the construction of riverbank reinforcement and channel instability as a result of changes in roughness values (n), which affect flow shear stress value, causing soil particle movement or bed channel erosion and deposition at the downstream of riverbank reinforcement structure.