I believe there are still free ID grids and the rivers were united for future modders that might want to add more provinces.
I doubt there is some space in Europe's ID grid. The grid is quite large, limit quite small, and it does not take much work to fill it completely
EDIT:
Every modder wants as detailed map as possible, including rivers. Every modder would love to have each river separated and bear its real name, and not a general label "River". But when he is drawing the map, he realizes that he is limited by 256 province IDs for a quite large area, and with a shock he notices that his realistic rivers are taking 50 province IDs from that already very small limit. At first you can think that 200 provinces can be enough for a sector, but after you design your borders, then you reach a killing fact, that just land provinces themselves are above that ID grid limit. :wacko:
And what you'll do when you have 300 land provinces, 70 sea zones, and on top of that another 50 as rivers, while you're allowed 256 at max?
Rivers are the least important thing on map, so you'll merge them under one ID, reducing the overload by 49 IDs, then you reduce sea zones from 70 to about 20, but in the end still you need shot down 100 land provinces.
And when you reduce it all to 260 provinces, there is no more stupid job, than to decide which 4 more provinces you'll kill to finally reach the limit.
And in that situation, try to come to the map maker, and ask him to release some IDs to make FREE slots for the future modders.
I don't think you'll return alive, or at least not on your own legs :rofl: