“People like to party in Munich - the Isar is beautiful - and the result is we love it to death,” Heinz Sedlmeier from the Association for Protection of Nature and Birds told the Süddeutsche Zeitung.
Sedlmeier made his warning in response to news that a Munich events company plans to hold a Thai-style ‘full moon party' on the banks of the river on May 21st.
The event has free entry and 7,200 Facebook users have already said they will attend, with a further 54,000 saying they are interested.
The Isar flows through the English Garden, a beautiful 18th-Century park which stretches from the city centre to the northeastern limits of the Bavarian capital.
“Every loving relationship demands mutual respect. Commercial events like this full moon party they're planning or the huge quantities of rubbish which are left behind do not belong in such a relationship,” Sedlmeier said.
The environmentalist called for the city authorities not to allow any more parties to be held on the river banks.
He added that the areas where barbecues are allowed should be smaller and called for heavy fines to be imposed on people who dump rubbish in the river or its vicinity.
Christian Hiernis from the Munich Nature Association added his weight to the argument, saying that “the Isar cannot become the next party mile.”
Progress made in reviving natural habitats along the river is in danger of being lost, he warned, “ironically because of this very attractiveness."
Hiernis added that in the summer people pay little attention to where the are stepping, leading to the endangerment of some plant species.
City statistics show that in Summer 2015 authorities removed some 150 tonnes of rubbish from the river banks.