An offensive by U.S.-supported Syrian Democratic Forces against Islamic State-held positions on the south shore of Lake Assad, the reservoir impounded by Tabqa Dam, on March 22 has allowed them to secure their first beachhead south of the Euphrates River. Attackers borne by boats and U.S. helicopters took the Islamic State by surprise, and the SDF has made rapid progress in spreading out from the initial positions they captured. For the past few months, the SDF, with heavy U.S. support, has been involved in an operation to cut off Raqqa, which sits along the northern banks of the Euphrates, as allied forces prepare an offensive against the Islamic State-held city . As part of that effort, the SDF moved rapidly south across a broad front toward the river, reaching its banks and covering more than dozens of kilometers on either side of Raqqa.