Gaining central lines from buffer areas

Dear all,

normally buffer polygons are gained from centreal lines. Now I have a file with buffer polygons (of rivers) but actually need the central lines. Is there any chance to gain central lines from the buffer polygons?

Please see the attached screenshot for illustration. To make sure: I don't neet the outlines of the polygons but the central lines, the buffer polygons originally were made of.

Thanks a lot in advance for all hints.

Kind regards,
Matt

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