A river is a naturally flowing course of water, usually freshwater, flowing towards an ocean, sea, lake or another source of water. 

Here's a list of the top 8 longest rivers of the world :

• River Nile : The river Nile is disputedly considered the longest river in the world as the Brazilian Government maintains that the river Amazon is longer than Nile. It flows through most parts of northern Africa through Ethiopia, Eritrea, Sudan, Uganda, Tanzania, Kenya, Rwanda, Burundi, Egypt, Democratic Republic of the Congo, South Sudan. There are two tributaries - Blue and White Nile.
• River Amazon : Some scholars do claim that this river is longer than Nile, this claim is still disputed to this day. It flows for more than 6,600km. The river's journey begins high in the Andes. It has more than 1,100 tributaries — 17 of which are over 1,497 km long — the Amazon River undoubtedly has the largest drainage system in the world.
• The Yangtze River : The Yangtze river is the third longest river in the world and the longest river in the Asian continent. It is also the longest river to flow within one country. It drains into the East China Sea in Shanghai.
• The Mississippi River : This river system actually comprises of three rivers: Mississippi, Missouri, Jefferson Rivers. The Mississippi river begins in northern Minnesota  and drains in the Gulf of Mexico. 
• The Yenisei River : This is the largest draining to the Arctic Ocean.  The Selenge River is  the headwaters of this river system. The Selenge River is about 992 km long and drains into Lake Baikal. The Yenisei drains into the Arctic Ocean. The total length covered is about 5,539 miles.
• The Yellow River : It is called so because of its color, it has large amounts of loose sediment in the water. It is also called as the Huang Ho river whose basin is said to be the birthplace of the ancient Chinese civilization. 
• The Ob-Irtysh River : It is also called as the Ob river and is one of the three Siberian rivers., along with Lena and Yenisei. It flows into the Arctic Ocean.
• The Parana River : This river is located in South America and is the second largest river in the continent after the Amazon river. The Paraná River is a river in south Central South America and it runs through Brazil, Paraguay, and Argentina for about 4,880 kilometres.