Welcome to the forums, pepito.
Considering The River II is an extremely common expansion, sold both independently and as part of Expansion 6, and Corn Circles I is extremely rare, only sold as an exclusive in a German version of the Carcassonne base game, I very much doubt anyone would be willing to make this trade. Most people don't have Corn Circles I and those that do, such as myself, paid fairly high prices for it and are not likely to let it go easily. Good luck in your campaign, but I wouldn't hold much hope for someone willing to part with the expansion.