How to Delineate All Watersheds in QGIS
Delineating a single watershed is a common task, but what if you need a complete watershed inventory for an entire region or river network? Performing a watershed analysis in QGIS allows you to systematically map every drainage area within your study area. This is essential for comprehensive flood risk modeling, regional water resource planning, sediment load assessment, and environmental impact studies.
If you want to delineate one specific watershed, check out this guide: How to Delineate a Watershed in QGIS in 8 Steps
- Load DEM Data
- Enable GRASS tool
- Run r.watershed
- Convert raster to polygon/line
Step 1
Load DEM Data
Data you’ll need for this analysis:
High-Resolution DEM: You will need a high-quality Digital Elevation Model (DEM). For accurate aspect calculations, especially in developed or complex terrain, we recommend using a high resolution source. You can download LiDAR derived DEMs from public repositories or use Equator’s data menu to access curated, ready to use DEMs for your site.
10m resolution DEM of Horsetooth Mountain, Colorado
Step 2
Enable GRASS tool
GRASS is a free built-in plugin in QGIS. To enable it or check if it is already installed:
Plugins → Manage and Install Plugins → search for “GRASS,” enable it
If you previously already enabled GRASS, skip this step.
GRASS plugin
Step 3
Run GRASS – r.watershed
The r.watershed tool can generate a set of maps such as: 1) flow accumulation, drainage direction, the location of streams and watershed basins, and 2) the LS and S factors of the Revised Universal Soil Loss Equation
Processing Toolbox → GRASS → Raster (r.*) → r.watershed
For the inputs, most settings can be left as default. The most important settings are as follows:
- Elevation – the DEM
- Threshold (Minimum size of exterior watershed basin): This is the most important setting. It defines the smallest watershed area you want to identify.
- Smaller value (1,000 cells): Generates many small, detailed sub watersheds.
- Larger value (10,000 cells): Generates fewer, larger watersheds. Use this to generalize for regional studies.
- Guideline: For a 10m resolution DEM, a threshold of 1,000 cells represents a 0.1 km² contributing area. Adjust based on your project scale.
- Advanced Parameters – Only select “Unique label for each watershed basin” if you do not want other layers to also generate
r.watershed tool with example inputs
Watersheds generated with 10,000 min size
Watersheds generated with 100,000 min size
Step 5
Convert to Polygon
The generated watersheds are still in raster format, convert them into polygon.
Raster → Conversion → Polygonize (Raster to Vector)
Converting raster to polygon
Step 6
Convert to Lines
If you want the outline of the polygons, you can convert it again to lines.
Vectors → Geometry Tools → Polygon to Lines
The rest of the input could be left as the default settings.
Pro tip: Due to edge effects of the DEM, the lines generated along the outer edges might be inaccurate. You can smooth out the lines using the Simplify tool, but generally it is suggested to extend your DEM extent so that the watershed area is calculated correctly.
Converting polygon to obtain watershed boundary lines
Watershed boundary lines
Simplified (pink) vs non simplified (yellow) lines
