The journey, which started at the source of the Rhine in Tomasee in the canton of Graubünden almost two weeks ago, would have taken the swimmer along 1,230 kilometres of river to its mouth by the North Sea near Hoek van Holland.
The project, which was being sponsored by Swiss Tourism, proved more challenging than Bromeis first realized.
At the very start, Bromeis was confronted with metre-thick ice to swim through. He was also hailed on once, and had to resort to travelling by kayak more often than he had intended.
“I prepared well for this project with the basic information available to me,” Bromeis said in a statement.
“I was sufficiently fit, and my experience from swimming in Swiss lakes gave me reason to be justifiably optimistic. Swimming the complete river became impossible in the end due to the changed general conditions.”
In the end, the sheer exhaustion from swimming in such freezing temperatures proved too much.
Bromeis’s efforts were intended to highlight the importance of water as a natural resource that should be treasured. He also wanted to draw attention to Switzerland’s many places of natural beauty in what is being called the Year of Water.
See also: Why I'm swimming the length of the Rhine