This one sits on a rock bed. As it can't really dig into it (especially with some types of rocks), so the stream is larger and not very deep. There are still no banks but rocks are really visible. I guess it's almost impossible right now to handle these two types of rivers, because it will need a lot of local parameters. But something between these two seems good. The most important is: almost no banks/transitional area, and visible rocks, low depth water.
What you call banks is actually border in the road system, where the material changes. It's not yet supported well with the rivers, together with proper profile.
On the picture above you can see the transitional area, or an extended bank, where the profile changes because of the river but the material and the vegetation not much so.
Another problem is that you could place a river or a stream anywhere, and we have to modify the terrain in a wider area to create the extended bed for it. The same thing happens with the roads, and right now there's one limitation - these extended banks are not procedurally refined, and they look artificial. The limit is because of HW limits in older shader models, once we stop supporting older OpenGL version (< 4.x) we can use a better mode for road/river blending.
Also, once we generate rocks in river beds, smaller streams will automatically expose them and they will also appear on the banks.