This picture is the Chippewa River, as seen from Hwy 35. See all the sand?
This is the first time I've seen a confluence from the perspective of the river joining, rather than the river being joined.
It's also an unusual one, because the Mississippi coming out of Lake Pepin is just not at all the river it was going in. It's much clearer, for one thing.
I'm not any kind of expert in this sort of thing, but I think that the Chippewa's confluence, and all of the sand it introduces to the Mississippi there, may have helped build up Lake Pepin. It seems to have kind of blocked the flow, which could help explain such a huge amount of water dammed up, before we started damming the whole upper river.
The Chippewa reminds me of the Rum River, upstream of Anoka in the Twin Cities metro. They're both sandy, tend to be rather shallow, and have pretty clear water. I checked the Chippewa near the confluence, and it was at about 35cm, turbidity-wise.