Moriches Inlet is located on the south shore of Long Island, New York, approximately 45 miles (72 kilometers) west of Montauk Point and 80 miles (130 kilometers) east of New York City. The inlet forms the primary outlet through the barrier island between Moriches Bay and the Atlantic Ocean. The inlet is protected and stabilized by two stone jetties approximately 800 feet (245 meters) apart . During 3anuary, 1980, a severe northeast storm resulted in the breaching of the barrier island immediately to the east of the existing Moriches Inlet. By the fall of 1980, the breach had expanded to nominally 2900 feet (885 meters) in width with a maximum depth of around 10 feet (3 meters) and the U.S. Army Corps of Engineers was requested to affect its closure. The method adopted by the Corps consisted of the placement of beach fill in the opening to develop a cross-section with a center line elevation of +13.25 feet (4 meters) MLW and side slopes of 1V:25H. Initiated in October 1980, the closure operation was succesfully completed in February, 1981. The formation of a significant breach immediately adjacent to the existing inlet and the artificial closure of the opening afforded a unique opportunity to study the dynamics of a tidal inlet under the influence of relatively rapid changes in tidal prism and cross sectional area. The purpose of this paper is to present the results of a field measurement program and subsequent analyses of the dynamics of the inlet/breach system. The analyses were based on data obtained before, during and after closure of the breach.