Here is how I created the 1ft elevation river valley bottoms.
I set a brush to 1-1.5 miles diameter at 1ft elevation. I drew in with green first where I wanted the river. Then went back with the 1ft elevation and leveled the length of the river valley bottoms due to irregularities left behind doing the macro topography texturing. Those valley bottoms had already been mapped relative to all the field\vbase objects on the terrain. Then I laid in the rivers and made sure they passed 2 miles from each field laid down to have bridges from the spawn. I touched up with 1ft if while setting river curves I logically needed some more width. That is how a number of random cliffs ended up on the sides of river valleys.
At every field with bridges I set the focus for the circle tool on "Group Master" 3 mile radius. Only inside of that 6 mile area did I micro terrain texture which resulted in micro gully's down to the river from each valley side. I respected the river and 1ft elevation while adding 660ft diameter micro hills or ridges just short of the river to maintain the 1ft elevation under the river object. This does not take a lot of time for me now to do this, after the time I spent doing it to all the feilds in the first place.
If you want to programmatically elevate the river valley ground under all river objects by 19ft with say 660ft to 1 mile to each side of the river object, I can take it from there with the roads, bridges and all the feilds affected with micro gully's for the GV players. Including any distortion of macro topographical features which take minutes to convert to something else and minutes to randomize some paint to blend it in.
I have relied on extensive application of the micro texturing in the 6 mile diameter circle at all feilds with GV spawns to enhance the GV experience for your customers after spending MANY weekends watching the competitor's tank videos to understand how they were building their micro terrain for tanks. I want to do as little repainting or any major re-sculpting as I can get away with not doing. I developed an embossing texturing technique for flat ground that may well keep me from touching any higher slope elevations near the fields and spawns. The GV driver thinks there is a field of small hills between the bridge and the field or the spawn. In some cases where a field is at 35-100ft, I may not need to touch a thing. I had to map all of this out to create the micro terrain in the first place.
Or, if you only need my rivers to be minimum global elevation 20ft, I will run back over them with a 20ft elevation 1 mile brush, raise the bridges and fix the roads. Then micro texture and adjust any cliffs or rock formations this will impact. "I do not want to touch anything else."
