Streams: network extraction
Once you have a flow accumulation raster, the drainage network is already in it. This sub toolbox turns it into an explicit stream layer: extract the channels, remove the artefacts, repair the topology and convert the result to vector lines.
Start here
Section titled “Start here”Extract Streams
Section titled “Extract Streams”The tool that defines your network, and the one decision that shapes everything downstream.
| Parameter | What to set |
|---|---|
| Flow accumulation | The accumulation raster from Flow Routing. |
| Threshold | The number of upstream cells above which a cell counts as a channel. |
| Zero background | Writes zero instead of NoData outside the streams. Useful when the result feeds another raster operation. |
| Output | The stream raster. |
The threshold is the whole tool. A low value gives a dense network reaching into every hollow, a high value keeps only the main rivers. There is no correct value: it depends on your terrain, your resolution and what you call a stream.
The practical method is to try. Run it with a threshold, compare the result with what you can see on the basemap or on aerial imagery, and adjust. Remember that the threshold counts cells, so its meaning in surface area depends on your resolution: 1000 cells is 1 square kilometre on a 30 metre DEM and 1000 square metres on a 1 metre DEM.
Remove Short Streams and Prune Vector Streams
Section titled “Remove Short Streams and Prune Vector Streams”Two cleaners. Every extraction produces short spurious branches, especially at low thresholds. Remove Short Streams drops the raster branches below a length you set, Prune Vector Streams does the same on a vector network.
Run one of them before ordering the network, otherwise the spurious stubs will count as first order tributaries and distort every ratio.
Raster Streams To Vector
Section titled “Raster Streams To Vector”Converts the stream raster to vector lines, which is what you need to style the network by order, to measure lengths, or to export it.
Repair Stream Vector Topology fixes the connection problems that conversion can leave behind, such as tiny gaps at junctions. Run it before any analysis that follows the network from link to link.
All the tools
Section titled “All the tools”| Tool | What it does |
|---|---|
| Extract Streams | Extracts the channel network from a flow accumulation raster, above a threshold you set. |
| Extract Valleys | Extracts valley bottom lines from a DEM, without flow routing. |
| Remove Short Streams | Removes raster stream branches shorter than a given length. |
| Prune Vector Streams | Removes short branches from a vector stream network. |
| Raster Streams To Vector | Converts a stream raster to vector lines. |
| Rasterize Streams | Converts a vector stream network back to a raster aligned with your DEM. |
| Repair Stream Vector Topology | Fixes gaps and connection errors in a vector stream network. |
| River Centerlines | Extracts the centreline of wide rivers mapped as polygons. |
Use a D8 pointer. The tools that follow the network, ordering and watersheds, expect a network derived from a D8 flow pointer. A network extracted from a dispersive accumulation will be several cells wide in places, and will not thread correctly.
Burn in what you know. If you already have a trusted stream layer, from a national dataset for instance, Rasterize Streams and the burning tools in Depressions & Storage let you force the DEM to agree with it rather than fighting it.
Where to go next. Streams: ordering metrics to classify the network, or Watersheds & Basins to delineate what drains into it.