I'd picked up the technique from a gaming forum several years back, frankly I'm amazed I remembered the technique. It turned out really simple and quick.
So first off buy yourself some 2mm craft foam in navy blue; there are many stockists, I think I got mine in an arts & crafts store in Leeds, but of course online is an easy option: Such as Here
Then cut into riverine sections as suits your tastes. I think these are about 3 inches wide; make sure the ends are all the same width!
Don't let it dry, whilst wet add one drop of white paint (or for dirtier water, maybe use green or brown...)
Then spread this around unevenly, being careful to generally follow the flow of the river. Your looking to get an impression of movement.
As it's still wet it will blend quite nicely, but it will also need a good few hours to dry. Producing about ten feet of this required less than half an hours painting.
So, this used three or four quid of foam, some cheap acrylic paint from the art shop and took about an hour to do. Well within anyones capabilities to do. Perfectionists may want to add banks some how, but for normal club use I think these would do just nicely.
I reckon you could use the same technique with different colours to do roads as well.