That is not a simple explanation. The basic rule is that when an adjective is restrictive it goes after the noun, but when it is not restrictive it goes before. So what does that mean? The idea is that intrinsic properties of something are not restrictive. Adjectives that put a noun into a specific category of the main group are restrictive. La verda hoja = the green leaf Leaves are generally green so it is not a restrictive adjective la manzana agria = the sour apple Apples can be sweet or sour or have many different flavors, so this would be restrictive. It further classifies as the apple as in the sour group. So how does this apply to river? Well it's not really always very clear. I just looked up what placement is common because I'm not a native speaker. Usually there is a preference for one position over the other. I believe it is more common in front because it could be seen as a general trait of rivers to be beautiful. However, it can be placed behind also. There are some adjectives that change meaning depending on their position. viejo amigo = long time friend amigo viejo = friend of old age