At least on my current terrain becasue I scaled it for tanks upwards in profiling all the topography, my rivers are on the bottom of river valley's.
That means at any elevation the river is at where I'm putting in a bridge, the flat area is a minimum of 2x660ft wide. That is how close you can run an elevation brush set for 20-25ft higher next to the center line of an average 50ft wide river to cut micro hills dropping down to the river. Some river bottoms have small rises just back of the river side then a low area back of that due to regular flood periods. Makes for good tank hiding areas. Also, no one in a tank or flying over will be able to tell that the 3 mile segment of river running past a field with bridges over it is a flat section of land to create an illusion for the tank drivers. That is why I leave the Group Master 3mile circle on while I do the micro terrain around a field. I micro texture starting just outside of that because very few if any tank will ever drive outside of that 6mile diameter area. Saves me having to micro texture 625,000sq miles. The medium amd macro texture is all that's needed for the rest of the terrain becasue only planes will see that. Since topo features are scaled to tanks, on the ground the tank driver sees them as mostly distant elevations of land with no detailed features like in real life. It's the 3miles from the spawn that is detailed for them.
Read an interesting forum post from some terrain builders at ARMA. It takes a team of up to a dozen terrain builders to create terrains smaller or almost as large as ours over about 6 months or longer. Some of the terrain is flat and painted to look like a representation of a 3D world from the air that vehicles are not expected to drive around at. With all the 3D objects and functions they have to stuff into their terrains, one team of 9 couldn't complete a terrain about the size of our 1943 Europe terrain. I got the impression they have a toy box of 3D objects and functions that can be picked and choose'd from which have to be assembled into villages and other civilization artifacts. And then a more liberal policy on custom objects which may be due to most of their terrains are smaller than ours with less players. Still, look what we produce while still adding in all the toys Hitech allows us. And their terrain building tools are supposed to be more sophisticated than our old school terrain editor. Something like my lakes and reservoirs, once you know how to produce them, if you prep the space they go into properly, that earthen dam area 30 minutes. The foundation for that big dam area waiting for water, 40 minutes becasue I kept changing my mind on how the polygons looked in the dam itself. Something that small in a mega structure like a canyon reservoir stands out to the eye.
When making lakes with branching arms, just like when "Y" branching a river. You will have to kink a tiny segment of one river then set it to not show so you can blend the end of another segment into it creating the "Y". In my reservoir screenshot, it may not work past something 100ft wide. And to get it to work, I may be running a bunch of short segments and blending them. There is a problem when you bring together two different segments. As they come closer together, one end segment or the other will tend to be the only segment that will respond to your mouse. Means you have to prep the meet face of one segment and prep the other's face a few miles away then pull that up to the other river's meet face. At least I hope I can make that work with segments 5000-10,000ft wide.
I turned in a DMP from a CTD working on this test terrain. We will have to wait for Hitech's results. I'm not unsure pulling river segments wide into lakes is not causing a CTD after say 30-40 minutes while the convoys and trains are running. But it may be more likely the CTD happened in response to while I was letting the trains and convoys run while testing, leaving the game minimized to the desktop was the source of the problem. I was testing that along with the trains and convoys because some number of players always minimize the game to go look at other things. A habit I picked up from the two years of AH3 alpha\beta testing.