Koji_Kabuto: well, yes, you should be able to use it on backdrop that is white with transparent pixels where river shoul not be visible, a.k.a river edge.
- make transparent image, then add white circle to it;
- create backdrop, attach previously created image to it;
- add water effect to that backdrop with all other effect parameters as they are in example;
- run application and check result - if that is what you want, then cool. if not, tell me what you want to make, or make picture with visual description :0