Longest flow path in QGIS from single feature vector

Question

I have got a vector layer with one feature (MultiLineString) representing a river network (s. below) without bifurcations. In addition there is a point shapefile containing the outlets of different subcatchments and one polygon for each of them.

Example: in the following image, the length should be from A (outlet, point in shapefile) to B (should be found automatically). As the point at A is not exactly on the line, an ideal solution would replace the outlet onto the line.

Is there a simple way to calculate the longest flow path (one single branch) in QGIS? The final goal is to have one length for each subcatchment.

Answer 1

There is a tool in Whitebox Geospatial Analysis Tools called Vector Stream Network Analysis within the Stream Network Analysis toolbox. It will take a shapefile stream network, that does not contain topological information about flow direction among stream links, and a coarse resolution DEM as inputs. It will output a number of stream metrics for each stream link in the attribute table including:

Extracting each link containing 'true' for the MAINSTREAM attribute, will isolate your longest path in each network. Here's an example of the Grand River network in Ontario:

The tool isn't well documented because I am still in the process of publishing the paper related to the tool.