This invention relates to water-retaining barrier means for a watercourse such as a river. The barrier means includes at least one flexible membrane forming an inflatable enclosure and secured to or otherwise incorporated in said enclosure as a rigid member which extends along the whole length thereof. The ends of this rigid member are arranged to bear against the banks of a watercourse in such a fashion that any stresses exerted on the barrier are transferred to the watercourse banks. In practising this invention rigid blocks may be built on the sides of the watercourse to form abutment members against which the ends of the rigid member are applied. The inflatable enclosure may be in the shape of a ''V'' with the apex of the ''V'' pointing upstream, and in this case two rigid members are incorporated with the enclosure the one ends of these rigid members being connected together at the apex of the V. In such a case also the inflatable enclosure may be in two parts particularly in the form of two frustra of a cone joined together by their major bases so that they also form a V shape. The invention also provides a method of positioning such a barrier means and also a method for damming a watercourse using such a barrier means.