 Hello, this is Hans van der Kwas, Senior Lecturer at IHE Delft Institute for Water Education. In this video I'm going to explain first how strahler order works and then how to calculate the strahler orders from a digital elevation model in QGIS. The strahler ordering method starts with the smallest tributaries from the source and gives them order 1. When two of order 1 join, they become order 2. And when two of order 2 join, they become order 3. So when two of the same order join, we increase the order. However, when a smaller order joins a higher order, we don't increase the order. So in this case, when order 1 joins order 3, it remains 3. And then when two of order 3 join, it becomes order 4. In this way, the orders increase towards the outlet. It's easy to calculate the strahler order from QGIS. When you have loaded a digital elevation model, you can go to the Processing Toolbox. There you can search for the strahler function. Strahler order is a tool under the SAGA tools. Here you can choose the filled DEM. I choose the output file name. Let me call it strahler. And I click Run. Now it will calculate all the strahler orders. It's done. Click Close. And in order to make sense out of the calculation, we can style the strahler order layer. So I go to the Layer Styling Panel. And strahler orders is in fact an ordinal scale. So it's not continuous data, it doesn't contain decimals. It is also not nominal like classes that we can mix. There's a certain order in it. So the best thing to do is to use then the Pelleted Unique Values Renderer. We have here. And because it represents river and the higher the order, the bigger the river, we choose here blues and then I click Classify and it uses all the unique values that it finds in this raster layer from 1 to 11 in this case, where 1 is the smallest order at the sources and 11 is the largest order towards the outlets. So basically that's how you calculate the strahler orders in QGIS. If you'd like to get updates of new videos, please subscribe for my YouTube channel. You can find more free materials on our GIS OpenCourseWare website, which is GISOpenCourseWare.org.