When the boundaries of nations cover the same watershed, who owns the border water? The answer to that question is the premise of this paper. A discussion of international water law is given. Implementing the law is portrayed in an outline for (1) administration, (2) a procedure for data collecting, and (3) a computer system for data processing and determining ownership of water for each country. Special situations are considered, and a conclusion developed. After international law between two countries has been established, determining the ownership of boundary water is both feasible and practical. A computer method is shown whereby this is accomplished. The unique aspect of the method is the fact that cooperation and mutual esteem between individuals of both countries are necessary in order to obtain results.