Mississippi River background for a dio

I don't know how large a back drop you need, but Google is your friend. When I went looking for a backdrop for my Woodland Indians, I did a search for Adirondack pictures or something like that and chose several and printed one. Here's an example of a 54mm figure in front of the back drop.

When I chaired the IPMS North Central Texas' group build of ACW ironclads (mostly Lone Star Models) a few years back,  we contacted someone in an IPMS club along the Mississippi  to go out and take a picture across the river.   We got several,  far shore bank, tree line, and perhaps a white church steeple projecting above the treeline.  We really didn't want much cultural information in our pictures. 

Another in the club had access to a large format printer and he took the JPEGs and printed them approximately 3 feet long.

Then with some spray adhesive - attached them to a backdrop board.   The display went over well & we placed (first, IIRC -- part of our 4-peat run).   One of the backdrops is still on my wall inthe model room.

There's the idea for you -- ping for someone with access to the river to take the picture for you & mail it to you.   Then Photoshop it to the size & format you want.  Then take it to the local Kinkos to have it printed