TheWerra– a natural river – had and up to a certain degree still has at least a brackish water body. A portion of the salt content of the river comes from natural leaching, the greater part is passed by nearby potash industry.
The legal threshold value in the 1940s was a salinity of about 0.5%. The value was often exceeded. Later in the 1960's the underground dumping of the by-products was ceased and all the brine ended in the Werra. The salinity rose up to 4.0%.
The ocean like salinity terminated the former (almost) freshwater habitat and changed it to brackish water habitat with a very low biodiversity: e.g. only one invertebrate (of former 60 -100) survived and two invertebrate halophile aliens were introduced.
After the German reunification potash mining (the mine was in the former GDR) declined and stricter limits were introduced. The salinity limits now are ~0.5 % and will be reduced to ~0.3% until 2019. The actual salinity seems to be ~0.33%.