Hydrologic flood plains are referred to areas around the streams where restrictions to the use of the land are imposed. Traditionally hydrologic flood plains has been defined as an area contained between two parallel lines to the main stream at a distance determined by maximum flood conditions. For natural streams that definition has some important practical problems for its determination, regulation and surveillance due to the stream flows dynamics and its continuous transformation, the asymmetric form of the river beds, and that the alignment has bi-dimensional considerations. In this work a methodology to define hydrologic flood plains in urban areas is proposed as a function of variables such as: flow channel capacity, river bed and scarps stability, recreational areas, areas for public utilities network, circulation areas, roads and walks areas, areas for ecosystems protection, and others.