Creating a dam for the creek in our back yard:
The location of a waterfall, rapids etc. is not choosable by humans, it is defined by the lay of the land.  You need to start with an elevation map of the land, i.e. what the elevations are at each point.  That will tell you a waterfall/rapids will be possible. 

Once you have located a viable location, then just move your house there :)   Seriously, see if you can re-grade the land so it is favorable for your water feature.

Just don't trifle with water projects...