Interesting idea though not one in priority to appear. Currently, it's possible to make a ruleset with different rivers as you can see in Alien World, but they don't connect to each other like big and small rivers normally should. There should be some tweak that we first generate big rivers and then small rivers, and tell the river generator that big river tiles can hold mouths of small rivers.
Likely there was a scripted river generator somewhere in the forum?
Very interesting. So, it is possible to have the two kinds of river, just by making a sufficiently sophisticated ruleset+tileset? No need to patch the freeciv c code? I already have a terrain generator that can easily make small and big rivers; It simulates waterflow, so all I need for river sizes is another treshold.
Big and small rivers can probably be made to connect, by putting a small river on any tile having a big river. Adjacent small rivers will then connect to the small river existing under the big one.
I could not run an "alien" world. My freeciv did not like the alio tileset, for some reason. But I can still read the ruleset.
In my dino2 ruleset (see - viewtopic.php?t=91837&start=20) I have tile extras Waterways and Canals. Kind of a similar idea, but you build them with Workers/Engineers.