I am not sure if there is any fabric that has been produced of just a river. I searched on eBay and this is what I found:
http://www.ebay.com/sch/i.html?_from...abric&_sacat=0
Depending on your skill level, you could make a river flowing using a Bargello pattern and shades of blue fabrics.
http://www.quiltingboard.com/main-f1...g-t262619.html
Good Luck.