This would be just gold in Terragen, I cannot describe how something like this could be used for so much, even beyond rivers.
It looks similar to perlin billows, but it's voronoi based I guess, and works with the underline voronoi used for the continents.
https://simblob.blogspot.com/2018/10/mapgen4-river-representation.html
Here is the river system thingy in use in his map system: https://www.redblobgames.com/x/1843-planet-generation/ It's pretty convincing. I don't know if
@Matt would know what's going on here, but something like this in TG to finally fake rivers would be amazing.
Here is the Mapgen4: https://www.redblobgames.com/maps/mapgen4/