Thank you for the positive feedback, everyone!
@fracas: We are working on the gf-thingie ;-)
@BlackEd: Creating the mountains was surprisingly easy! We used styrodur, simply carved out the mountain and the rocks with a sharp knife und drybrushed it with various shades of grey.
The actual challenge was glueing the rocks to the styrodur base. We couldn´t find an adhesive which glues the parts together without dissolving the styrodur (we had the same problem with the artificial water. It has to be heated prior to being poured, so ultimately we had to coat the riverbed with green stuff to prevent contraction of the styrodur and resulting air bubbles). In the end we settled with pinning the rocks in place using sewing pins. Working with the styrodur was in fact so simple, cheap and yielded such nice results, that we can´t wait to build more warmaster terrain :-)