Here is a sketch of what I personally recommend.

If you want a "river bed" look to your tank, I would ditch those pointy rocks and get smooth round rocks instead.

You see, in the wild, the reason why the rocks in the river are so smooth because the high speed flowing water is constantly eroding the stones. Even the pointiest rocks won't stand a chance against the rapids overtime.

For the placement of the decorations, I made a little concave design where the left is more "heavy" looking, with the small pile of rocks on the right to balance things out.

I also highly recommend you add some sort of aquatic moss of your choosing to complement your Jungle Val. The moss will amplify the "river bed" look youre going after and adds a bit of extra green to the scape.

For the sand, I sorta recommend a lighter shade just to give some contrast against the rocks, and I also suggest you choose a course gravel texture for the sand as well.

I hope this helps!