Some problem in modelling

I am trying to recreate toonish version of one of the other images posted in the forum. Please look at the attached file, in which the green lands are just planes and the land in middle is a curve profile extruded along another curve path. I applied a wood texture to col,ref,spec. The river again is a profile extruded along a path with a cloud texture applied to nor and ref.

Note that there are some unwanted yellowish lines in the middle region. I see it when rendered with yafray, but do not see with blender renderer. I am pretty much sure it is the problem with my model, but could not figure out what is the problem and how to fix it. Can somebody help me?

I am also attaching the blender render… Also please give me other feedback on improving the model.