Other contributors have said "What is a river that is brackish?" is the same question as "Where is brackish water found?" If you believe that these are not asking the same thing and should be answered differently, click here
Brackish water makes a habitat for fish like:
Arrowhead Puffer Fish
Black Fin Shark, White Tip Shark Catfish
Columbian Shark, White Tip Shark Catfish
Fahaka Puffer Fish
Fan Dancer Goby
Figure...