Simplify the following text for a Grade 8 native English-speaking person.
Original: The Volga and its tributaries form the Volga river system , which drains an area of about 1.35 million square kilometres in the most heavily populated part of Russia .
Simplified: The Volga and its tributaries form the Volga river system , which drains an area of about 1.35 million square kilometres in Russia .