In preparing for a game recently, I realized that I would need a larger river than I had available. I needed a river that was 15cm to 20cm across (about 6 to 8 inches).
Someone suggested to me that I could make a quick-and-easy river from a plastic tablecloth at Dollar Tree. To my good fortune, I found a rectangular blue plastic tablecloth at my local store, although a check of the company website shows this item has been discontinued.
I needed a 4' length of river to match my 4' gameboard, and the short end of the tablecloth was just a few inches longer than that. So I used my roller cutter to cut a slightly curvy stretch of river.

Above, you can see some Giant Cockroaches drinking at the riverbank.

This is probably not the best terrain I've ever seen, but it's practical and inexpensive. One problem is that it is lightweight – if you have a fan going, it might blow away. While the material is thin and easily conforms to the tabletop, the edges sometimes have a bit of a twist or curl to them.

I did not try to remove the creases from the tablecloth. Instead, I wadded the river up and then straightened it out again, hoping the wrinkles would look something like water.

How to store it? I salvaged the cardboard tube from a roll of toilet paper…

…rolled the river around it, and secured it in place with a rubber band.
This project reminded me of being a kid, and making your own stuff, and having fun without spending a lot of money.