On June 7, 1976, the United States Supreme Court unanimously extended the Winters doctrine¹ of implied federal reservation to apply to subterranean water as well as surface water. In addition to the fundamental significance of such an extension, the Court's decision in Cappaert v. United States² tacitly recognizes an in situ use (leaving water in place) as part of a governmental purpose which may be served by the implied reservation doctrine,³ a recognition which promotes continued expansion of the arguments of environmental advocates that natural resources are entitled to legal protection on the same basis as economic interests. This Article will attempt to explicate salient aspects of the Cappaert decision and anticipate the extent of Cappaert's impact.