"The rivers Mura, Drava and Danube constitute a 700 kilometre green belt that networks nearly one million hectares of one of the most important European river basins," the two associations said in a press release recalling that the region is also called the European Amazon.
The activists also warn about perils for the rivers stemming from the construction of stone embankments and hydro-power plants and about threats to animal and plant species, notably endemic ones.
Saturday's race will be accompanied by other events to entertain participants in the regatta.