Craig over at Minimods has a really nice article up on building rivers from foam.
The foam he uses seems to be a high density variety, akin to the type used to make children’s foam play tiles and the like. He gets it from Hobbycraft in the UK, a chain that does not seem to have outlets in Belgium, but I think you can get similar stuff at De Banier stores in Belgium.
Anyway, Minimods in general is shaping up to be a great miniature wargames blog (with a tighter focus than our eclectic selves), so by all means go over and have a shufti.