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I made two river bank lines and I want to associate them.

I want to make a centerline using two rows as a base. I managed to make the centre line with a lot of work creating several lines from one margin to the other of the river and then using the centroid, but this solution is very tiring and I have to do this in several areas.

The surest method for solving your question from the point of view of the cartographic approach is to use the Voronoi Polygons.

I offer one of the solutions to your question using QGIS tools.

So, the initial data is a river as an areal object, see the figure below.

2) Vector > Geometry processing > Convert polygons into lines, see image below

3) Open the closed line QGIS "Split Features" tool and run the GRASS v.split tool, so that the points are evenly distributed along the line and run the Vector> Geometry Processing> Voronoi Polygons tool on them, see the figure below

4) Cut the original Voronoi polygons from the result with Vector > Geoprocessing > Trim and then select only one half of the river area, see the picture below using the Ctrl key and the selection tool more
Vector > Geoprocessing > Association by Basis > By all indications.

5) Cut from the source layer the result of the selected left or the right area, see the picture below.

6) Then from the result of cutting create a line, blow it up, remove unnecessary parts and leave the centre line of the river, see figure below

This is the main approach to solving your question and it is clear that a variety of tools allows you to solve your question in various ways, but the idea remains the same, good luck ...

Try the GRASS algorithm v.centerline. You may have to tweak around with the parameters, but as long as your river polygons are longer than they are wide it should work for a large portion of your inputs.

The HCMGIS plugin says it does this:

Spatial Processing: Create Skeleton/Medial Axis/ Centerline for road/ river networks and similar linear structures