The problem of delimitating the peninsula boundaries is based on modeling the watersheds’ matrix for different river basins in the difficult conditions of the boggy and lacustrine territory of the north of the West Siberian lowland and the Yamal and Gydansky peninsulas. It is shown that the main difficulties in carrying out the land boundaries of the peninsulas are associated with the complex arrangement of the estuary areas formed by large rivers, containing three sections, the estuary, the delta and the mouth – in each of which a gate serving as the initial and final point of the peninsula boundary may be located. In this case, the optimal method to implement the proposed algorithm is associated with the solution when a natural boundary is adopted for the river gate, where the mouth area passes into the delta. The algorithm considered allows us to establish the peninsula as a whole physico-geographical region and a large geosystem, united by the flows of matter and energy, and, consequently, as a single area of nature management.