Simplify the following text for a Grade 8 native English-speaking person.
Original: It rises in the mountainous district known as the Pireneus , west of the Federal District , but its western tributary , the Araguaia River , has its extreme southern headwaters on the slopes of the Serra dos CaiapÃ s. The Araguaia flows 1,670 km before its confluence with the Tocantins , to which it is almost equal in volume .
Simplified: These can be found to the west of the Federal District . Its western tributary , the Araguaia River , has headwaters very far south on the slopes of the Serra dos Caiapós .