In playing with a few of the new layout mode line tools, I all of a sudden realized that Layout mode could be extraordinarily useful for defining rivers. My quick 10 minute attempt below isn’t fantastic but gets the point across:
Quickly and easily, you can draw paths that hug the terrain elevations.
However, more importantly you can bring up the lofting curve for the river and adjust its vertical profile so that the water never flows uphill. The river will appropriately dig through the terrain to achieve the elevation profile you set.
In the image above you can see this at work; the right fork of the river originally ran up and over the ridgeline, so I set the profile to instead cut through the mountain instead.
It actually works pretty nicely as is; a custom River shape could do much better and open up a whole host of other possiblities. Something for me to think about.