A full processing chain for inundation modelling/mapping is implemented in different specialised software like HEC-RAS or Flood Modeller. Alternatively, inundation water level can be estimated separately, and then can be mapped in desktop (universal) Geographic Information System (GIS) software. Last approach can be demanded in a complex already-formed GIS-based mapping processing chains, when inundation area mapping appears only as a step of analysis and mapping technology. This context is usual for cadastral accounting of inundation areas. However, such a processing chain have a lack of computation tools, as currently used desktop GISs (e.g., QGIS, ArcGIS, etc.) capable originally to map only a lake-type inundation areas (certainly, in the case of regular software assembly), while along-the-river inundation should have water surface sloping along the river body. Addressing to the filling the gap of along-the-river inundation mapping in desktop GIS software, we have developed an approach to mapping of river-type inundation area, and implemented it in a test mode to support cadastral accounting of river flood inundation areas. The approach is based upon geometrical modelling of the water surface only, and is not concerned with consideration of hydrogeological conditions and ground water regime. Being simplified, this technique may be rough tool for the cases when hydrogeological aspect have to be respected, however it is simple for implementation and is useful when estimating topography impact only.