Don't know about your river clay but I've used clay from my property and "fired" clay. The clay I dug here, I dried it then ground it up to a fine powder and added about a 1/4 part to my ingredients. "Fired" clay can be added up to 50% of the portland cement. If you add too much of native or "fired" clay the pieces will crumble.
Fired clay can be bought from pottery supply house. Billie