And anyway, taking a few train rides in a sandbox is hardly an onerous task. I'm getting pretty good at rivers now, having sorted a fair few miles of them last night. The lake tool is very useful of course, but having done some experiments I'm now finding the terrain up/down tool can be good for doing rivers, in some situations, as long as it is used with great caution.
Say you have sloping terrain with a river drawn on it, and want the river sunk down a bit. You can use the "Change height" option when drawing the river, but that can be problematic if you need to revise things on complex terrain. I find it's generally best to not use that option, and to draw the rivers on top of the terrain without digging them in automatically. So, I figured out another way of tackling it.
The trick is to set the brush size (or area effect if you prefer) to #2, and to go up the middle of the river one click at a time, at even spacings (by eye), overlapping the area effect so the centre of the next click is within the limit of the last one's area (about halfway to the edge works pretty well). This will sink the river a bit, with a pretty natural slope to the banks. If you want it a bit deeper, just repeat the process.
You will probably have introduced some minor lumps and bumps to the river bed. These can often be fixed with the smoothing tool (bulldozer icon in terrain modification menu) set to either #1 size (for dragging along the middle of the river) or #2 size (for discrete clicking here and there).
The lake tool is good here too, because you can quickly use it to level the river bed if you have a dip or a lump in the wrong place. The lake tool only does levels it can grab from another tile, so the smoothing tool can be used carefully to smooth out such changes in level. It's best to check from several angles, using both the river tool view and normal game view, if you want them really slick.
