Which river is the longest in the world, stretching approximately 6,650 kilometers (4,130 miles)? For centuries, people debated which river deserved the title of "longest." This African river flows northward from the mountains of East Africa to the Mediterranean Sea, crossing 11 countries along the way. It has two main tributaries: the White Nile (which begins in Lake Victoria) and the Blue Nile (which begins in Lake Tana, Ethiopia). The ancient Egyptian civilization depended entirely on this river's annual floods for farming. Without it, one of the world's greatest ancient civilizations would never have existed. About 95% of Egyptians live within a few kilometers of its banks.

Which river discharges more water than the next seven largest rivers combined? This South American giant carries about 209,000 cubic meters of water per second into the Atlantic Ocean. That's more than the next seven largest rivers combined. Its width during the rainy season can reach 48 kilometers (30 miles). The river is so massive that its mouth is 240 kilometers wide. Ocean ships can sail 3,700 kilometers upstream. The river contains the world's largest freshwater rainforest and more fish species than any other river system – over 3,000 species, including the famous piranha and the pink river dolphin.

Which river flows through the most countries of any river in the world (10 countries)? This European river begins in Germany's Black Forest and flows southeast for 2,857 kilometers (1,775 miles) before emptying into the Black Sea. It passes through or forms the border of Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova, and Ukraine. Four capital cities lie on its banks: Vienna (Austria), Bratislava (Slovakia), Budapest (Hungary), and Belgrade (Serbia). The river's name comes from the Latin "Danuvius" and has been an important trade route since Roman times. Its delta is a UNESCO World Heritage site and is home to Europe's largest wetlands.

Why is China's Huang He River often called the "Yellow River"? This 5,464-kilometer river is the second-longest in China. Its name comes from the huge amount of yellowish silt (called "loess") that it carries from the Loess Plateau. The river carries about 1.6 billion tons of sediment annually – the highest of any river on Earth. When the river floods, it leaves behind a yellow deposit that has enriched farmland for thousands of years. However, these same floods have also earned the river the nickname "China's Sorrow" because historically, devastating floods have killed millions of people. The worst flood in human history (1931) killed an estimated 1-4 million people.

Which major world river is famous for flowing northward, contrary to the common belief that rivers flow south? Most people assume rivers flow south, but this 6,650-kilometer river proves otherwise. It begins in the mountains of East Africa (near the Equator) and flows northward all the way to the Mediterranean Sea. The ancient Egyptians noticed this and called the southern part of the river "Upper Egypt" because it was upstream, and the northern delta "Lower Egypt" because it was downstream. This northward flow is why the river was so predictable to ancient Egyptians – the monsoon rains in Ethiopia and East Africa arrived like clockwork each year, causing the river to rise and flood its banks in the same season.

Which river is the longest in the world that flows entirely within a single country? This 6,300-kilometer (3,915-mile) river begins in the Tibetan Plateau and flows eastward across China before emptying into the East China Sea near Shanghai. It is China's most important river, draining nearly 700,000 square miles – about one-fifth of China's land area. The river is also the world's third-longest overall. It has been crucial to Chinese civilization for over 7,000 years. The Three Gorges Dam on this river is the world's largest hydroelectric power station. The river is home to the endangered Chinese alligator and the critically endangered Yangtze giant softshell turtle.

Which river has the largest drainage basin (the area of land that drains into it) in the world? This South American river system drains an area of about 7 million square kilometers (2.7 million square miles) – roughly the size of Australia. Its basin covers about 40% of South America, including parts of Brazil, Peru, Bolivia, Colombia, Ecuador, Venezuela, Guyana, Suriname, and French Guiana. The basin contains the Amazon Rainforest, the world's largest tropical rainforest, which produces 20% of the world's oxygen. Over 1,100 tributaries flow into the main river, with 17 of those being over 1,600 kilometers long. The basin is home to 10% of all known species on Earth.

Which river carved the world's deepest gorge, the Yarlung Tsangpo Grand Canyon? This river begins in western Tibet near Mount Kailash and flows eastward across the Tibetan Plateau before taking a dramatic U-turn and cutting through the Himalayas. The gorge it carved is 5,500 meters (18,000 feet) deep – deeper than the Grand Canyon (which is about 1,800 meters deep). The river then flows into India and Bangladesh, where it becomes the Brahmaputra River. Until 1993, it was believed to be a tributary of another river, but modern exploration revealed it to be a separate major system. Its upper reaches are sacred in Tibetan Buddhism.

Which river is considered the most sacred in Hinduism, with over 500 million people living within its basin? This 2,525-kilometer river begins in the Gangotri Glacier in the Himalayas and flows across northern India into Bangladesh, where it merges with other rivers to form the world's largest river delta (the Sundarbans). Hindus believe bathing in this river washes away sins and helps achieve salvation. Millions of pilgrims visit holy cities like Varanasi (also called Kashi) to perform rituals. Unfortunately, it is also one of the world's most polluted rivers due to industrial waste, sewage, and religious offerings. Despite pollution, over 400 million people depend on it for drinking water, farming, and fishing.

Which country is home to the world's longest underground river, which flows through a cave system and directly into the sea? This 8.2-kilometer (5.1-mile) river is located in Puerto Princesa on an island in this Southeast Asian country. It flows completely underground through a limestone karst cave system before emptying into the West Philippine Sea. The cave features impressive rock formations, including stalactites and stalagmites, and is home to bats, swiftlets, and other cave-dwelling creatures. The lower portion of the river is subject to tidal influences, making it a unique ecosystem where freshwater meets saltwater. It was named a UNESCO World Heritage site in 1999 and is one of the New7Wonders of Nature.