FYRIS

'Fyrisån' is a river in the Swedish province of Uppland, which passes through the city of Uppsala and ends in Lake Mälaren.
It was formerly called the 'Full' or the 'Sala' river — ''Sala'' referred to the halls (Up-Salir) of the Swedish kings at Old Uppsala — but its name was changed in the 17th century in memory of the Fyrisvellir, marshy plains which had long since been turned to agriculture, but were famous from Norse mythology.
Boats can sail up the river from Lake Mälaren all the way to central Uppsala where two weirs make further progress impossible. On the last day of April every year, students attempt to navigate the weirs on homemade rafts with predictable results.