River Dolphins' natural habitat could easily (or easier than other cetaceans) replicated in zoos/aquariums. Many waterparks and amusement parks have lazy rivers/river rapids rides. Using this concept and technology, a zoo/aquarium could build a zoogeographic region with such a river flowing through the area. There could be several viewing areas, underwater tunnels, and covered bridges with windows (to discourage throwing items or jumping in) crossing the stream. Tributaries could be added to increase the river further, create isolation "pools", or seperate enclosures for other aquatic species. Some of these "tributaries" can be feeding areas for the dolphins to attract them and allow keepers for visual checks, training, and possible containment as needed for management situations. During varying times of the year, the water volume could be increased to flood set aside and enclosed low-lying areas to also replicate the flooded seasons that occur. Also water speed could be increased with water jets along the stream. This concept could also work for manatees and possibly marine mammals.
I've often thought about that. You could make it seem like visitors walk along one river for the whole exhibit but it would really be three seperate parts with giant otters, boto, and manatee. As well as various other south american species in exhibits(tapir, marmosets, macaws ect.)