Thanks for the feedback, that all makes sense. I've got lots of battle games in middle earth magazines, which have some cool scenery articles, never got around to making many of the things they show though!
The wood approach makes sense. I agree that wood would be more durable than papier mache, although harder to work with (for me anyway). I've more or less decided to go with printed plastic, since I can make as many variations as I need to in the computer to make sure that everything works. Whereas once I start cutting wood I need to be sure where I'm going with it.
4 inches sounds sensible. I'm gradually coming to the conclusion that 10 cm (so about that) will work best for the majority of the pieces, and then I'll have some single bank elements which can be combined to make wider channels. As you say, they will need a large table to be practical, and will only work for certain scenarios, but it would be cool to have the capability for that sort of scenario as well. I like the idea of doing narrower streams too! It should be easy enough to make tapering pieces which can connect channels of different widths.
What I want to do, which I haven't seen done elsewhere, is make a mixture of square and hexagonal tiles, where the edges have the same length. That way straight pieces and gradual curves can use a straight element, while the hex tiles give more degrees of freedom for bends and confluences. Now I need to get to work in sketchup and see what i can design.