Flow accumulation is a hydrological raster derived from a digital elevation model (DEM). For each cell it counts the number of upstream cells that flow into it, based on a flow-direction grid. Cells with high accumulation values mark where water concentrates, so the layer effectively traces stream channels and drainage networks across a terrain surface.
Why it matters
Flow accumulation is the backbone of terrain hydrology: it underpins stream extraction, watershed and catchment delineation, drainage-density analysis, and erosion and flood-risk screening. Thresholding the accumulation grid (selecting cells above a chosen value) produces a synthetic stream network you can compare against mapped rivers or use where field data is sparse.
The workflow
The standard sequence is: (1) fill sinks in the DEM to remove spurious pits that would trap flow; (2) compute flow direction, commonly with the D8 algorithm, which sends each cell's flow to its single steepest downslope neighbour; (3) compute flow accumulation from that direction grid. Tools include QGIS (SAGA/GRASS providers), ArcGIS Hydrology tools, GRASS r.watershed, and WhiteboxTools.
Concrete example
In QGIS via GRASS, r.watershed takes a filled DEM and outputs accumulation directly. Selecting cells with accumulation greater than, say, 1000 upstream cells isolates the main channels for a small catchment; raising the threshold yields a sparser, higher-order network.
Common pitfall
Skipping sink filling produces broken or truncated stream lines, because flow gets trapped in depressions. Also, D8 forces flow into a single direction and can create unrealistic parallel lines on flat terrain; multiple-flow-direction (MFD) algorithms distribute flow more naturally on gentle slopes.