How would I go about making this model

[kicker]

DS has to produce a 3d model of a river basin. How would he make this please?

www.3dgeography.co.uk/#!river-models/ctqu The fifth image down on that page.
Would it be too hard for a 10 year old and would the cost of materials be prohibitive Thanks

The 5th image looks quite advanced for a 10 year old on his own. The 6th one looks sort of easier; he could even lay strips of papier mache down over the hills/mountains to make them smoother?

Could you combine the ideas in photo 6 with Yippie's paper mâché suggestion to make the rough outline of photo 5? Or you could get some modelling paste (£6-12 for a small tub in hobbycraft) or cheaper still would be filler from B&Q to cover the card/paper mâché - would need to do it bit by bit so that it dries and then add a bit more etc?

Does it need to be such a difficult model?