Your river does seem wide, like Bruno said, but there are really wide rivers in nature, much wider than 18 meters, so I wouldn't say your river is necessarily wrong.

There's nothing like looking at the real thing. Did you do that? If I'm after realism, I always do a research before. I look at reality around me and I look at lots of photos. I found these ones:

Comparing these photos with your images, I'd say:
a) Your image has very little haze. Either increase the haze percentage or increase the aerial perspective.
b) The grass and river margin are too smooth and uniform, with no details. Details are important to give the notion of scale and realism. There should be bushes, areas without grass, etc.
c) Your trees end too soon. There's no background, only foreground and midground. To give a sense of scale, the trees should fade away in the distance.

Not regarding scale but rather realism only:

Your margin doesn't seem natural. If it's sand, it should be flatter. If it's rock, it seems too smooth. Anyway, I didn't find any reference where a winding river with so many tight turns runs through a vertical rock wall so close to water. I guess that's not very common. In the images I found, the trees always grow closer to the water.
The water is too transparent
There's not enough variety in the trees.

Just my two cents. :-)

Forum: Vue

Subject: Vast Scenes?