The river Oder is important. Rising in Czechia and flowing 500 miles to the Baltic, along the way it becomes Poland's second-longest river, including forming more than 100 miles of the country's western border with Germany. 

But at this border, death is piling up. In a horrifying embodiment of humankind's pollution of the planet, fish corpses are floating to the surface – and officials on both sides of the watery frontier are warning residents to stay out of the river due to a possible chemical contamination. 

"We do worry about this. This is terrible, it is simply terrible," said local resident Edelgard Ahrens on a bridge spanning the Oder from Poland to Germany. "And especially that a lot – not just in this case, but generally, a lot – can be dumped into the water without regulation." 

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The State Criminal Police Office in Brandenburg has launched an investigation. Water samples have been taken after reports of a possible hazardous situation in Poland. 

German freshwater expert Christian Wolter said he is not surprised by the mounting number of dead fish in the Oder as they are burdened with lack of oxygen and a surfeit of toxic substances. 

"The fish are under stress and what exactly came in on top – a toxic substance, organic matter or construction work, dredging, the turbidity, anything can be the straw that breaks the camel's back", said Wolter.

According to the state Ministry for the Environment, no current report from Polish authorities has been issued. A ministry spokesperson said they had only learned of a possible incident from Polish media.