The problem may be sludge. In Norway, we had fish deaths due to emissions from rock blasting / tunnel work. The water was supposed to be cleaned of silt, but the treatment plant was overloaded and the fish died.

We were planning on testing the water. As we can't do non target, it would be interesting to reduce down what we would test for. There is no waste treatment in the area I can think of.

You have already identified that wastes released from the residential and industrial areas could be the case for mass die-off. I suspect industrial wastes. The nutrients you mentioned could be poisonous to the fish. Test the water samples from the streams.

For context, dead/ sick crows and herons have also been seen in the vicinity.

An easy way to trace the source can be to examine benthic animals. Try to find a place upstream, which is not polluted, and compare with a polluted one. Repeat the test somewhere between the two stations. If this test is "normal", do a new test further down. If the sample is bad, go higher up. It should not be necessary with a lot of species knowledge. Often there is a difference between a lot of life and almost nothing. Or a mass occurrence of tolerant species. Usually one or two types of animals.

With ongoing assessment, the source was found to be wastewater from an overflow pipe. The river contained toxic levels of ammonia. Sadly all life in the river has been killed (invertebrates (even sediment-dwelling), aquatic life) and some birds were seen in great distress. As I have seen wastewater in rivers before I'm surprised by the extent to which the river has been wiped out. Are there any other chemicals you would suggest testing?

Ammonia in high concentrations is deadly to all life in water. As with all toxins, dose is crucial. In Norway, we have had fish deaths due to ammonia released into the water due to tanker accidents.

Yeah it was devastating to see. Hopefully we can find out what else could have been in it.

Dylan--you say a point source has been ID'd as flow from an overflow pipe. What are the potential sources of inflow into the overflow pipe? It seems to me it may be possible to identify the constituents in the overflow pipe if you trace back to the sources of water in the overflow pipe--however I guess if the overflow occurred during a large flood event, there may be many potential sources to investigate. Perhaps start with finding the source(s) of the high ammonia loading (likely some specific industrial/chemical industry business such as fertilizer; or a waste site?); that same source area(s) may also have contributed other pollutants.