Flow Length function

Available with Spatial Analyst license.


Creates a raster layer of upstream or downstream distance, or weighted distance, along the flow path for each cell.

This is a global raster function.


A primary use of the Flow Length function is to calculate the length of the longest flow path within a given basin. This measure is often used to calculate the time of concentration of a basin. This is done using the Upstream option. The function can also be used to create distance-area diagrams of hypothetical rainfall and runoff events using the weight raster as an impedance to movement downslope.

The value type for the Flow Length output raster layer is floating point.


Parameter nameDescription

Flow Direction Raster

The input raster that shows the direction of flow out of each cell.

The flow direction raster can be created by running the Flow Direction function.

Direction of Measurement

The direction of measurement along the flow path.

  • Downstream—Calculates the downslope distance along the flow path, from each cell to a sink or outlet on the edge of the raster.
  • Upstream—Calculates the longest upslope distance along the flow path, from each cell to the top of the drainage divide.

Weight Raster

An optional input raster for applying a weight to each cell.

If no weight raster is specified, a default weight of 1 will be applied to each cell.

Related topics

In this topic
  1. Overview
  2. Notes
  3. Parameters