I have read the river is polluted, and seen pictures of turbidity.  Bioindicators should be easy to sample, but perhaps just difficult under your circumstances.  If you know of a favored food and suitable sampling substrate, such as a favored algae on a piece of plexiglass, perhaps it can be dropped with net, and pull up net with plexiglass and feeding fish after a certain amount of time, such as 10 minutes.  In 1984 I provided report which used aquatic insects as indicators In other studies, we have documented fish, often using electric shock treatment.  In a river condition, shocking is a challenge, and at best you can only sample small areas or habitats and lack of visibility is a challenge unless they float to surface.  Under the conditions, scuba diving seems inappropriate.  Perhaps baiting in something like a crabpot or crab basket might attract and capture them.  Consulting with your fishery, marine and/or aquatic experts or University professors may be best to also assure you have permits if needed and consider there expertise and experience.