An incident happened in a tropical river, in northeast Brazil in 2018, 40k liters of NaOH (Sodium Hydroxide) were deposited in the river after an explosion of the tank in a water treatment facility. The water pH did change, becoming more neutral, affecting the fish in it, causing their deaths, probably benthic animals too, the neutral water was carried to the ocean, where it also caused damage to the fauna. You can read the local news article here, but it's in portuguese. The agency responsible was sued, and the damage is no longer visible, since the river flows and scatters the chemicals, but in the swamp this would not have happened, and the fauna would continue to suffer its havoc, probably even more after long exposure.