any advice on making flat and preferably flexible roads and rivers?

[Mindenbrush]

I believe I saw someone using Duct Tape as a backing on canvas to stop the edges curling.

[cram]

Appreciate all the impute guys! Loads to work with here.

Good point about the flexible river. My idea is to have only one or two river sections flexible, these would be used to flow down hills that have been positioned at the boards edges.

I've not as yet ruled out crafting paths, dirt roads and rivers onto terrain board sections, but then their set permanently in place.

[Munindk]

Could glueing washers at the corners be a solution?

Or maybe applying a layer of caulk or thinned down PVA on the opposite side, to create some pull there? I've seen it done on mdf and similar.