The prehistoric practice of diverting and bailing is so simple that it may strike the modern fisherman as ingenious. Instead of removing the fish from the water that surrounds it, the water that surrounds it is removed from the fish.
This method is still practiced in small ponds in Asia and perhaps elsewhere.
Fishermen dig a low ditch up to one shoreline of the water, and then let gravity drain away as much water as possible. Bailing water out of the pond with buckets helps accomplish this goal.
After some hours, the now vulnerable fish can be caught with plunge baskets, cast nets, and hand nets. This technique is best performed during dry season, using the head start provided by nature.
Streams are more difficult than ponds for this approach. If the goal is to catch fish by subtracting water, doing so in the endless flow of a stream makes little sense. Diverting the water of a stream is most easily accomplished where two channels pass around an island.
Here the fishermen either deflect water into only one channel by building a temporary dam or dig a new diversion channel. This causes the water to flow outside of the fishing area. They then block the partly de-watered fishing channel and hunt the fish in the shallows with the usual assortment of nets, spears, and even their bare hands.