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A river is typically defined as a large body of water that flows toward the ocean, another river, or an inland sea. What constitutes a river, as opposed to a stream, for example, is imprecise. Some definitions suggest that a river is at least partly navigable by large boats and ships — there are, however, many flowing bodies of water designated as rivers that are not navigable by anything other than small canoes and rafts. Other definitions have set volumetric thresholds for what does and does not constitute a river, but those measures are also subject to broad interpretation. Likewise, a river system is a broadly inclusive term used to describe all of the streams and rivers that drain a river basin.
For example, several major rivers flow into the Mississippi River. The rivers include the Ohio River, Missouri River, Minnesota River, Illinois River, and the St. Croix River. Altogether, the Mississippi River system encompasses approximately 1,200,000 square miles (about 1,931,213 square km) in 31 states and two Canadian provinces — more than a third of all of North America.
River systems begin with the watershed and headwaters areas that feed the initial systems of streams that eventually become rivers. They also include the environments directly impacted by rivers, also known as riparian areas. These areas can include various forms of swamps, marshes, wetlands, flood zones, reservoirs, river deltas, canyons, and other geological formations. Man-made structures, such as dams, locks, and canals, are also considered parts of river systems.
In that sense, the Grand Canyon, Lake Mead, and Hoover Dam can accurately be considered part of the Colorado River system. On an even grander scale, the entirety of the Brazilian rain forest is considered part of the Amazon River system. This systems is the most abundant and varied ecosystem on earth.
Rivers and river systems have played significant roles throughout the course of human history. The annual flooding of the Nile in Egypt, for example, creates a vast and rich agricultural zone, which in turn led to the early rise of Egypt as one of history’s earliest dominant world powers. Control of the Mississippi River played a crucial role in the United States (US) Civil War. Hydroelectric power derived from rivers has spawned the development of modern societies the world over. Many of the world’s greatest cities, such as Rome, London, New York, and Beijing, are all located on rivers and are themselves considered by some people to be part of river systems.