A 1:75- scale (undistorted) hydraulic model of Newburyport Harbor, Massachusetts, which included the lower 5,700 ft of the Merrimack River, approximately 2,300 ft of the Atlantic coastline on each side of the harbor entrance, and sufficient offshore area to permit generation of the required test waves, was used to investigate the arrangement and design of proposed improvements with respect to riverbank erosion control, wave protection, and river flow conditions. The proposed improvements consisted of (a) changes in the length of the north jetty, (b) changes in the crown elevation of the north jetty, and (c) addition of groins at two locations. An 80- ft-long wave generator, a water circulating system, crushed coal tracer material, and an automated data acquisition and control system (ADACS) were used during model operation. It was concluded from model test results that : (1.) For moderate to large incident waves, existing conditions are characterized by turbulent wave conditions in the entrance channel and strong longshore currents in the area between the south jetty and Plum Island Point , resulting in continued northeasterly movement of tracer material along the eroding portion of Plum Island . (2.) Of the improvement plans tested involving raising the north jetty crown elevation (plans 1-1B), plan 1 (crown elevation +11.0 ft) was selected as the optimum with respect to wave protection and construction costs. (3.) Of the improvement plans tested involving the north jetty extension (plans 2- 2E), plan 2B (900-ft extension) was selected as the optimum with respect to wave protection, prevention of erosion on Plum Island, and construction costs. (4.) Of the south groin plans tested, plan 3A (north jetty crown elevation +11 .0 ft) provided better wave protection than plan 3 (north jetty crown elevation +8.3 ft). (5.) The plans tested involving both north and south groins (plans 4 and 4A) resulted in extremely large current velocities in the entrance channel, and might cause erosion of Plum Island and navigation problems. (6.) Of the plans tested, plans 2B, 3, and 3A all provided adequate erosion protection for the northern end of Plum Island. Plan 3 requires the least volume of rock but does not improve entrance wave conditions. Plan 2B provides the optimum entrance wave conditions but requires the greatest volume of rock. Plan 3A offers adequate erosion protection while improving entrance wave conditions and appears to be the optimum plan with regard to protection provided and cost.