If you want to make a river for real water what I would do is get some sort of PVC piping or other plastic container and cut it in half longways. Then depending on what kind of plastic container/pipe you use I would attach brackets or something to the side so that when you dig out the place you will put the river/pond you can drive garden pins or metal stakes through it to secure it in place. Then you can cover the bottom with pebbles or even moss! That would be fun!!
If you want an artificial pond the easiest way I have found is two part clear cast resin like the "Amazing" brand at craft store. Color it with few drops of alcohol ink or mix with blue glass pebbles and then pour resin into silicon mold or in a translucent container like a 2 liter bottle, milk jug, etc.
FYI I actually retail and make handmade fairy miniatures and love doing custom work. If you ever have an idea and want something made give me a shout.
