Rivers — a major source of water and food, as well as a means of transport, hydropower, waste disposal, and political boundaries — have been integral in the development of human civilization, and continue to play a fundamental role in modern economic and cultural life.
Due to off-gassing from contaminants such as fertilizers and human waste, rivers and streams are now also a major source of greenhouse gas emissions. Accurate documentation of the world’s water features — their location, speed of flow, and overall size — is important in the study of climate change and the Earth’s biological processes.
The exact number, size, and overall length of the world’s rivers, however, is difficult to determine. The definition of where a river starts — its source — and where it ends — its mouth — can depend on varying approaches to river topography as well as regional differences in naming and classification. According to a study published in June 2018 by NASA-funded researchers Dr. Tamlin Pavelsky and Dr. George Allen, the global surface area covered by rivers and streams is 44% greater than previous studies had estimated.
To identify the longest rivers in the world, 24/7 Wall St. reviewed various expert sources, primarily the Encyclopædia Britannica. Length was generally defined as the distance from a river’s source to its mouth or confluence with another major river.