Preprocessing a DEM and calculate flow direction

1. Introduction

In this first tutorial, you’ll take your initial steps into preparing a Digital Elevation Model (DEM) for hydrological analysis. Before we can delineate streams or define catchments, the terrain data needs to be cleaned, structured, and transformed into layers that reveal how water would naturally move across the landscape.

By the end of this tutorial, you will be able to:
•    create meaningful subsets of larger raster datasets
•    identify and fill sinks in a DEM to ensure continuous flow
•    calculate flow direction and apply clear, informative styling to the resulting raster

These foundational steps form the essential base for the stream and catchment delineation workflows you’ll explore in the following tutorials.

This tutorial is based on a session in the QGIS Masterclass that is available as an on-demand course at the Australian Water School. Until 3 July 2026 you can get a discount of 20% with code AWS-FOSS-20.