Sounds like a fun project, are there any updates on how it is coming along?
There are a number of streams and rivers that are naturally quite clear (meaning greater then 15-20ft visibility.) North Georgia has many that naturally hold trout and similar species. There is a number of lakes that have 40-50ft visibility up in the lake of the woods region and out in Colorado. I have also seen pictures from similar type waters both in the Amazon basin and further south in Argentina and Chile. The biggest thing is that they are typically entirely spring fed rivers, and natural suspended particles settle out or are removed from the water column.