Soper's Mill is an lesser known whitewater play spot in Iowa.  It is located north of Ames.  There is a public park right next to the rapids where you can park.  Watch you footing on the rocks and boulders that is lining the river shore it is an easy way to hurt yourself.  This rapid is created by an old rock dam.  The remnants of the dam is what creates the nice wave to surf on.  There is another spot at this park further down the park road.  It creates a decent wave to surf on and practice some basic moves.  This waves is formed by a pipe I am told.  This area is quite safe for there is a nice pool behind each rapid spot to recover on.