The Fourmile Run flood-control project will provide protection for the city of Alexandria and Arlington County, Va., from flooding of the Fourmile Run channel. The proposed plan for containing flood flows consists of eliminating existing constrictions in the channel, excavating and widening the channel, providing adequate bank-slope protection, and constructing energy dissipaters, grade control structures, and flow dividers. The investigation was conducted with a 1:30-scale model that reproduced approximately 5700 ft of the Fourmile Run channel and 400 ft of the Long Branch tributary. The existing bridges and vertical walls, as well as the proposed improved channel with bank-slope protection and hydraulic structures, were reproduced in the model. Tests encompassed flow conditions at the bridges and at the confluence of Long Branch, water-surface elevations, riprap and gabion stability, and performance of the hydraulic structures. Flow conditions through the hydraulic structures of the original design protection plan from the I-95 Bridge to just downstream of West Glebe Road Bridge were unsatisfactory for design discharges. Efforts to improve the hydraulic performance of these structures were not made because the sponsor considered development of the natural, soft-bottom channel a more feasible solution to problems. With design discharges in the improved natural soft-bottom channel, surface waves between I-95 and West Glebe Road Bridge caused failure of the riprap on the bank slopes. Failure of the riprap also occurred on the left bank slope downstream of West Glebe Road Bridge and along the left bank slope at the Long Branch confluence with Fourmile Run. Larger riprap was not considered feasible so gabion protection was tested in these areas and found to provide the optimum protection. Proposed gabion and riprap proctection plans were developed for the remaining reach of Fourmile Run and Long Branch. Discharges exceeding 8000 cfs overtopped portions of the left slope between West Glebe Road Bridge and Long Branch and both bank slopes downstream from Mount Vernon Avenue Bridge. A vertical wall was placed 2 to 5 ft from the crest of the left slope between West Glebe Road and Long Branch. This addition provided the necessary protection for the low topography. Additional protection was unwarranted for the low topography downstream of Mount Vernon Avenue Bridge based on a feasibility study by the sponsor. The proposed drop structure in Long Branch will lower the water-surface elevation to coincide with that in Fourmile Run and will reduce erosive velocities in the channel. The structure performed satisfactorily for low discharges; however, it was ineffective during peak discharges in Long Branch due to the tailwater effects from the higher flows in Fourmile Run.