Looking really good. I'd might want to break up the near ground texture a little with some larger patches of earth/sand, to get away from that just dumped pile of gravel look. The brighter stones in you last render look really interesting in the water, but too much out of it, so maybe a mix between the second and third ground colour, with the bias toward the second. I also think you could do with some shadows from the rocks and grasses, by slightly shifting your sun position.
Something I've found really usefull when constucting scenes, is turning off the parts I'm not working on, to speed up rendertimes, and allow me to focus on areas being worked. Or just use the crop, lol, but sometimes things like water tend to get rendered even if unseen, fake stones I've found being a bit of a render pig too, etc.
