The whole pond, stream, waterfall and filter are all concrete base with block sides, rendered and painted with G4. That allows me to link them together with no leaks or chances of leaks developing. You could make one out of liner as well too but may have to get one professionally joined to make sure it doesn't leak if you want the fish to swim up from the pond like mine.
If you just want a stream then a liner will work fine.
If you just want a stream then a liner will work fine.