as eastern leaves you with a couple of days of from work I bought this excellent stuff called:
Great stuff for building larger things with a nice carved out side so you can easily stick more then one of these together. My local DIY store had a special offer regarding 5 of those sheets (measuring 120cm by 45cm, TF80 means its 8cm high or thick). You do not need any special hot metal to cut it as a normal kitchen knife will do the job as well without you getting to sweaty. Love this stuff.
So first of all the plan:
Get a gaming table of approx 120cm by 135cm
Get some hills on that and carve in a river section.
Paint the stuff upfront
here is the actual look alike of a sheet 120cm by 45 with the hills at the highest point raised to 16cm. For flocking....you can either buy an overextent of flock from your local hobby store and get your wife angry because you always will lose some of the flock later on and it will be transported through the whole house or you take a look at Lidl Ireland and get your hands on this:
Artificial Grass Mat (100cm by 200cm):
Next steps include the finetuning of the stone areas and then cutting the mat to fit here :)