actually I didn't - but I figured out how it works! the rivers are basically a subtraction from the very basic land-shape layer, so I just have to edit that layer again and it works. No harm done, since that layer is not visible anyway, it only functions as the general area to which all other layers are restricted.
I guess those are the dangers of blindly following a tutorial without understanding what exactly is happening there...

I edited the rivers a little bit again, so that even the top one looks more natural and also adjusted the colors.
Anyhow, the most important question for me is now: How big does that land look to you? Does it seem big enough to be a continent?

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