Discovery just published this video of how we can catch fish in a small river. For that you need to create a stone wall diagonally on the river, then create an enclosure with a small opening where the fish should go in. After this put some branches over that enclosure so that the fish would go there and hide.
After all this is done you need to go upstream and fish some fish. Scare them go to downstream into your trap and then block the entrance to the enclosure. Remove the branches and catch the fish. Sounds so easy, but I’m sure is complicated as hell.