Watersheds & Basins

This is where the workflow arrives: the area of land draining to a point. A watershed for a gauging station, subbasins for a modelling grid, hillslopes for an erosion study, or every basin in the raster at once.

All these tools need the D8 pointer raster from Flow Routing, and most of them also need the stream raster.

The one mistake to avoid

A watershed is delineated from a pour point, the outlet you place. Digitised by hand or taken from a gauge coordinate, that point almost never falls exactly on the cell the algorithm considers to be the channel. Off by one cell, and instead of the whole basin you get the few hectares of the hillside your point landed on.

Always run Snap Pour Points first.

Parameter	What to set
Pour points	Your point layer of outlets.
Flow accumulation	The accumulation raster.
Snap distance	The search radius, in the raster’s units. It should be a little larger than your positional uncertainty and smaller than the distance to the next channel.
Output	A corrected point layer, each point moved onto the cell of highest accumulation nearby.

Then feed that layer to Watershed.

Jenson Snap Pour Points is a variant that snaps onto a stream raster rather than onto the accumulation maximum. Use it when you already have a stream network you trust.

Start here

Watershed

The tool itself, once the pour points are correct.

Parameter	What to set
D8 pointer	The flow direction raster.
Pour points	The snapped outlets.
Output	A raster where each cell carries the identifier of the pour point it drains to.

Cells that drain nowhere in the set come back as no data, which is the normal result outside your basins.

Watershed From Raster Pour Points takes outlets already in raster form, for instance the result of another analysis.

Basins

Delineates every basin in the raster at once, one per outlet at the edge of the grid, with no pour points to supply. The quickest way to see the overall drainage organisation of an area, and a good check that your flow routing is sound.

Subbasins and Hillslopes

Two ways of cutting a basin into pieces.

Subbasins splits it at every stream junction, giving one polygon per link of the network. This is the standard unit of hydrological modelling.

Hillslopes goes finer, separating for each link the land draining to its left bank, to its right bank and to its head. The natural unit for erosion and sediment delivery studies.

Isobasins

Divides the terrain into basins of roughly equal area, at a target size you set, regardless of the network structure. Useful when you need a regular tiling for sampling or reporting rather than hydrologically meaningful units.

All the tools

Tool	What it does
Snap Pour Points	Moves each outlet onto the nearby cell of highest flow accumulation.
Jenson Snap Pour Points	Moves each outlet onto the nearest stream cell.
Watershed	The area draining to each pour point.
Watershed From Raster Pour Points	The same, from outlets supplied as a raster.
Basins	Every basin in the raster, one per edge outlet, with no pour points needed.
Subbasins	Splits the basins at every stream junction, one polygon per network link.
Hillslopes	Splits each link’s catchment into left bank, right bank and head.
Isobasins	Divides the terrain into basins of roughly equal area at a target size.
Unnest Basins	Separates nested watersheds into independent, non overlapping units.
Longest Flowpath	The longest flow path within each basin, the hydraulic length.
Max Branch Length	The longest branch upstream of each cell.
Flood Order	Orders cells by the sequence in which they would flood from the outlet upward.

Notes

Check the area. The first thing to do with a delineated watershed is to check its surface area against what you expect. A basin ten times too small means the pour point was not snapped, a basin that runs off the edge of the raster means your extent is too tight.

The output is a raster of identifiers. Each basin is a group of cells sharing a value, not a polygon. Style it as a categorical raster to give each basin its own colour, the way you would style any classified layer, in Style.

Nested basins. When several pour points sit on the same river, their watersheds contain each other. Unnest Basins separates them into the incremental areas between successive outlets, which is usually what a water balance needs.