Tutorial: Stream and Catchment Delineation using GRASS tools

In the previous section we've determined the flow accumulation threshold for determining the streams. Here we're going to delineate the streams using that threshold value.

1. In the Processing Toolbox choose GRASS | Raster (r.*) | r.stream.extract.

2. In the r.stream.extract dialogue choose DEM_filled as Input map: elevation map and accumulation as Input map: accumulation map. Change Minimum flow accumulation for streams to 50000 cells (that's the value that we've determined in the previous sections). Save the Unique stream ids (vect) to streams.gpkg. We don't need the other outputs.

3. Under Advanced parameters change the v.out.ogr output type to line.

4. Click Run. Close the dialogue after processing. Ignore the red warnings in the log.

Note 1: this only works with the GeoPackage output format for the line vector. A shapefile will result in an empty layer.
Note 2: the Input map: accumulation map is optional. If you don't provide this it will calculate the flow accumulation again, but the result will be slightly different then what we've calculated in the previous section.
For more info about r.stream.extract check the manual.

5. Style the streams vector layer.

Now we have delineated the streams we can determine the outlet point on the stream for which we want to delineate the catchment.