Im not sure if i remember it right, but currently the river segment has to satisfy some criteria to be included in outerra import. i think that one of them is that is has to be part of a connected water system of at least 20km in lenght (all the tributaries count, but the osm waterways have to be correctly connected). in the future when we have lakes we will probably relax this condition, but currently i had to find a way how to filter out segments that just dont make sense to render... Currently im experimenting with a lake fitler, that would cut the rivers on the border of large lakes, then maybe ill add some more rivers that are part of a smaller system that ends up in a lake, that does not have connection to any other rivers... In reality there should not be too many rivers, that are not connected to anything, but it depends on the quality of the data...

I will also try to relax the criteria to add river segments that have proper riverbanks defined or they just end up in a sea /ocean....