Tutorial: Stream & catchment delineation using GRASS tools
4. Determine the flow accumulation threshold for channels
Now each pixel in the study area has a value for flow accumulation, i.e. the amount of upstream cells that drain through each cell.
Not all cells, however, are part of a river channel. We need to determine the threshold of flow accumulation above which we can consider the pixel as part of a river channel.
1. Install the QuickMapServices plugin: in the main menu go to Plugins | Manage and install plugins...
2. Install the QuickMapServices plugin.
3. In the main menu go to Web | QuickMapServices | OSM | OSM Standard.
4. Look for the Rur river on OSM Standard and inspect the accumulation layer with the Identify features tool from downstream to upstream.
You'll see that downstream the values go up to 2622554 while upstream they are much lower.
By comparing OSM Standard with the accumulation layer you need to determine the minimum flow accumulation for a river. Here we'll take 50000 cells as the threshold.
5. In the main menu go to Raster | Raster Calculator...
6. Type the following equation:
Which means: if flow accumulation is larger than 50000 cells, then give boolean True (1), else give boolean False (0).
"accumulation@1" > 50000
7. Save the result as channels.tif. Click OK.
8. Style the channels layer using the Paletted/Unique values renderer with blue for value 1 and remove value 0 to make it transparent.
Normally, you would repeat this for different threshold values until you get the best match with the rivers on a reference map or satellite image.
Here we'll continue with the value of 50000 cells.