Just so you guys know over time the smaller graines no mater what it is will settle to the botom. I have had this happen in my 55 and it has been going for 5+years. I know have perty much 1 mixed substraight no differentaion any more. It had florite black, eco complete, Black sand an tahitian moon sand black.
I have found this to be true also. I have 2 tanks I did in black sand and florite mixed. Looked great! After about a year it just looks like florite. I'm also interested in learning any trick that may avoid this.
I would put sand from floor to top where you want your sand river design. Maybe fill with the amazonia then scoop out the path till bare glass shows and fill with sand. Some sand may mix under horizontal and need topping off later, but would be better than being stacked on top.
That's what I would try, but I haven't tried, so just a suggestion. I have no physical experience with such a design.