I am trying to use QGIS to assess what areas will flood if the river floods a specific height. I have a DEM of the area as well as a vector file of the lines of the river of interest. I cannot get a raster file of the river height. Is there any way to do this?

  • You might check with these folks: hatarilabs.com
    – Stu Smith
    Commented Apr 6, 2021 at 1:44
  • 1
    Could you please clarify what you mean by "areas will flood if the river floods a specific height.". If it is a flat water level, you can simply just mark the area in the DEM that fills to that level. However most river systems are a bit more complex than that, youll likely have a slope and varying levels in tributaries.
    – itsgupta
    Commented Apr 6, 2021 at 3:21
  • For example if the river level rises 1 foot, what will flood, I can't just fill the DEM to a set level because the river does not run at a constant elevation Commented Apr 6, 2021 at 18:22
  • Search for HAND - height above nearest drainage.
    – FelixIP
    Commented Sep 17, 2021 at 2:48

2 Answers 2


If you have GRASS enabled in your processing toolbox, you can use r.lake. This method takes a DEM, a water level and a starting point as input. The output is a raster map with the flooded area.

  • 1
    It is clear from tool name that this tool is wrong one for river flooding.
    – FelixIP
    Commented Sep 17, 2021 at 2:45

The usage of r.lake can be a way to forward, however r.lake will result in substantial impacted areas of flooding due to its static water filling algorithm. In my option, r.lake would be a good solution in sea level rising. However, in this fluvial inundation case, you can also use IDW tool to extrapolate the water level to the surround terrains.

You need to first identify the overtopping position along the river polyline then make point feature there. Thereafter, you can use IDW method to obtain a raster with distributed water level. Finally, by substracting the raster with distributed water level from DEM, you can obtain an inundated area.

Yet, the suggested method above is only for rough estimation, since momentum of overtopping water is not considered at all. To get more accurate prediction results, 2D hydrodynamic model should be used.

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