If you zoom in you see a length in km next to each river. For small ones it looks like it is the total length of that waterway. For bigger rivers that I'm familiar with I was not able to make sense of that number. It is not the length, it's too big for that. Maybe it's the total length of that waterway and all connected waterways? It also does not change when I travel along that waterway. My example is the river Neck…
Exactly this.