I want to create a center line for a river (polygon). I thought about simply creating a new line manually, but it would be a long and imperfect process. If there is a way to automate this process, that would be much better.
Can anyone help?
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You can do so by following the procedures expressed in this answer. Here's how to implement it in QGIS:
This method is a bit rough on the edges, depending on how big your features are it may be faster to vectorize it by hand, but it gets the job done. It also has room for improvement, especially on step 5.
I would try converting your river polygon to lines to extract each bank, then using the QChainage plugin. It will allow you to create a set of points at fixed distances along both of your river bank lines. Then average the x and y coordinates of those points to get center points for the river. After that, simply use the Points2One plugin or something similar to create a line from the average points that you calculated in the previous step.