ASIA - Yangzee
AFRICA - Nile
EUROPE - Volga
SOUTH AMERICA - Amazon
Well done Gadjo

NORTH AMERICA - Mississippi system
Well done BleAivano

AUSTRALIA - Murray system
Well done Storm

Just the Antarctic left, If no one gets it today I guess gadjo with four right gets the next question.

Here is a clue it is 15km (yes just 15km) long and is named after a semi-precious stone.