It's difficult to photograph well from on the water, but this is the confluence between the Mississippi and Willow Rivers. That darkness in the upper right of the river is not a shadow. That's the Willow.
When a river meets another, they don't mix immediately. They run alongside each other, with a distinct line between the water from each that only disappears after a considerable distance - hundreds of yards at least.
The Mississippi, where it meets the Willow, is brown, somewhat turbid, light colored from the silt suspended in it. The Willow, on the other hand, is clear, tea-colored from peat upstream, and looks utterly black. These two strands of water carry on downstream, inhabiting the same bed but only touching at an eddy-laden line between them, until time and distance merge them into one thing.