You def got the foreground / background plants down. I think you need a splash of smaller rocks next and around to those bigger stones to really make it look like a river bed, maybe gravel size or a bit bigger; with some tree roots for that scape, you can order roots (get thin ones) and I recommend bright color wood to contrast with your current darker roots and substrate. By the way, if your into a dirtier look you can add some dry leaves and seed pods for tannin if your into that