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The Rivers Portal

Introduction

Elwha River in the Olympic Peninsula

A river is a natural flowing watercourse, usually a freshwater stream, flowing on the Earth's land surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland, or another river. In some cases, a river flows into the ground or becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to by names such as creek, brook, and rivulet. There are no official definitions for these various generic terms for a watercourse as applied to geographic features, although in some countries or communities, a stream is customarily referred to by one of these names as determined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and Northeast England, and "beck" in Northern England. Sometimes a river is defined as being larger than a creek, but not always; in English the language is vague compared to some languages like French, where a fleuve flows into the sea and a rivière is a tributary of another rivière or fleuve.

Rivers are an important part of the water cycle. Water from a drainage basin generally collects into a river through surface runoff from precipitation, meltwater released from natural ice and snowpacks, and other underground sources such as groundwater recharge and springs. Rivers are often considered major features within a landscape; however, they actually only cover around 0.1% of the land on Earth. Rivers are also an important natural terraformer, as the erosive action of running water carves out rills, gullies, and valleys in the surface as well as transferring silt and dissolved minerals downstream, forming river deltas and islands where the flow slows down. As a waterbody, rivers also serve crucial ecological functions by providing and feeding freshwater habitats for aquatic and semiaquatic fauna and flora, especially for migratory fish species, as well as enabling terrestrial ecosystems to thrive in the riparian zones.

Rivers are significant to humankind since many human settlements and civilizations are built around sizeable rivers and streams. Most of the major cities of the world are situated on the banks of rivers, as they are (or were) depended upon as a vital source of drinking water, for food supply via fishing and agricultural irrigation, for shipping, as natural borders and/or defensive terrains, as a source of hydropower to drive machinery or generate electricity, for bathing, and as a means of disposing of waste. In the pre-industrial era, larger rivers were a major obstacle to the movement of people, goods, and armies across regions. Towns often developed at the few locations suitable for fording, building bridges, or supporting ports; many major cities, such as London, are located at the narrowest and most reliable sites at which a river could be crossed via bridges or ferries. (Full article...)

The Mississippi drainage basin includes the two longest main-stem rivers in the U.S. (the Missouri and the Mississippi) as well as 18 more of the rivers on this list.
The Mississippi drainage basin includes the Missouri and the Mississippi rivers, the two longest main-stem rivers in the United States, as well as 18 more of the rivers on this list. The Mississippi main stem is highlighted in dark blue.

The longest rivers of the United States include 38 that have main stems of at least 500 miles (800 km) long. The main stem is "the primary downstream segment of a river, as contrasted to its tributaries". The United States Geological Survey (USGS) defines a main-stem segment by listing coordinates for its two end points, called the source and the mouth. Some well-known rivers like the Atchafalaya, Willamette, and Susquehanna are not included in this list because their main stems are shorter than 500 miles.

Seven rivers in this list cross or form international boundaries. Three—the Milk River, the Red River of the North, and the Saint Lawrence River—begin in the United States and flow into Canada; two do the opposite (Yukon and Columbia). Also a segment of the Saint Lawrence River forms the international border between part of the province of Ontario, Canada, and the U.S. state of New York. Of these seven rivers, only the Milk River crosses the international border twice, leaving and then re-entering the United States. Two rivers, the Colorado and the Rio Grande, begin in the United States and flow into or form a border with Mexico. In addition, the drainage basins of the Mississippi and Missouri rivers extend into Canada, and the basin of the Gila River extends into Mexico. (Full article...)
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Cherish these rivers. Witness for them. Enjoy their un-improvable purpose as you sense it, and let those rivers that you never visit comfort you with the assurance that they are there, doing wonderfully what they have always done.
David Brower, Foreword to Oregon Rivers, 1997

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Photograph: Ansel Adams

The Tetons and the Snake River (1942)

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Looking north up the Lower Adams River

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