It was later determined to be an algal bloom, which was made possible by the discharge of saline industrial wastewater into the river on the Polish side, is now considered the probable cause of the disaster.

Nowa Ruda (the 21st most polluted city in the European Union in 2015) with another coal mine lies in the mountains in the basin of Eastern Neisse, another of Oder's tributaries.

Further west of Wrocław, in the basin of the Kaczawa tributary, lies Legnica with large copperworks KGHM and factories, whereas copper ore is mined in nearby Lubin.

[1] On 11 August 2022, volunteers and anglers removed at least 10 tonnes of dead fish from the 200 km (120 mi) stretch of the river north of Oława in southwest Poland.

[18] According to the Brandenburg Environment Minister, Axel Vogel, German laboratories believe the fish deaths may have been the result of large amounts of salt in the water.

"[20] Data gained via an automatic water quality measuring station in Frankfurt an der Oder (over 100 km (62 mi) downstream) indicated that from 4 August 2022 the electrical conductivity had climbed anomalously.

On 7 August 2022, dissolved oxygen, electrical conductivity, and UV absorption almost doubled; nitrate levels plummeted and chlorophyll increased by a factor of ten.

The government media has compared the situation to other previous minor and unrelated events in Warsaw and Gdańsk, where the opposition Civic Platform holds power.

[30] On 12 August, the deputy minister of infrastructure, Grzegorz Witkowski [pl], blamed the opposition and the ecologists, and stated that the river was safe to enter, and fishing is allowed.

[33] Some anglers fear that fish might never return to the river,[34] and Steffi Lemke, the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection, called the situation a catastrophe and a shocking ecological disaster.