If this is a game map, then convert your destructible rock chunks to a ramp/bridge. Then make a couple three four copies modify their tinting a bit, a light (very light) green, a light (very light) blue, a slightly darker and a slightly more darker one, perhaps increase the range of size a wee bit. Then mix it all up, the over all look of various slight hues and tones will make the rocks look better.
especially if you are going to use green rocks with the rock chunks, its kind of odd to have clean gray rock with green rocks sticking out.
The light is blinding - due to the glare I can not see what else is behind the arch of the river. Move the light source (sun) to the left corner of the camera view, slightly lower behind the hill you have there, or remove that hill (lower it) and place darker versions of your trees (they are shadowed).
That is a big problem with using a light source behind objects, for terrain images only you can tinker with the tinting, darkening the object to make it look like you are looking at the shadow side of the object. This does not work well with games since the player is moving around and can see the other litup side which is dark due to the tinting.
Since you are using river plane, I assume you already know about the waves that come with it:
Water model - Wc3campaigns
If not then make an individual model for each wave using the custom pathing, that will allow you to place waves. If you are going to stick with the whale, then you need waves around him/her to indicate movement through the water.
You might also want to modify the waterfall doodad as well, makes its smallest scale 0.01 this way you can decrease the z to nearly flat, and you can use it to indicate water flow around the bars of the Gate of the River.
Also you need some wave action against the base of those rocks - while the water has waves, it appears that the waves never hit anything at the shore.
Break up that texture on the hills (right hand side), grassy dirt 'lines' of single point brush size running along the grid diagonally tends to work rather well at 'breaking up' large rock terrain.
If I was rating on a scale of 1-10 I would rate this an 8, you have a wonderful idea, a very good base, now you just need to fiddle with the details a little.