The navigation system currently in use at Bonneville Locks and Dam, Columbia River, was placed in operation in May 1943. Navigation problems have resulted in damage to the guide wall and the wing wall of the lock. A new 86-ft- wide by 675-ft-long lock has been proposed for construction immediately south of the existing lock. It was recognized that major channel modification 5 could be necessary to develop satisfactory navigation conditions for tows using the new lock. This report documents a comprehensive model study that was conducted to investigate conditions that could be expected with the proposed design and to develop the modifications required to ensure satisfactory navigation conditions. The effects of various plans and modifications on water-surface elevations and current directions and velocities were studied, as were the effects of the resulting currents on model towboat and tow behavior.