Rivers

The number of rivers in a region depends largely on the amount of precipitation or melted snow and ice there. Rainforests of the Amazon plain, the wettest tropical plain in the world, produce hundreds of streams and rivers. Arid regions, including Saudi Arabia, Egypt, the southwest United States, and western and central Australia, have few rivers, and most are dry for a large part of the year.

Rivers may have large numbers of tributaries. In the case of large rivers, some of the tributaries are rivers in their own right. The Danube in Europe has about 300 tributaries, including the Rivers Sava and Drava. Seventeen of the Amazon’s tributaries are more than 1,600 kilometres (1,000 miles) long.

The ridges that surround and separate drainage basins are called drainage divides. In the western United States and Canada, the Rocky Mountains form the North American Continental Divide. Waters east of the Continental Divide eventually flow into the Atlantic or Arctic oceans, and waters from the western slopes flow into the Pacific.

River Gradient

Approaching its mouth, a river usually passes through a broad floodplain consisting of sedimentary material that the river itself has deposited. In this portion of its course, a river may have a gradient as gentle as 19 centimetres per kilometre (12 inches per mile).

Erosion

Rivers with a muddy appearance contain large quantities of suspended sediment and organic material. In northern China the Huang He, or Yellow River, carries tremendous amounts of yellow silt.

Rivers carve steep-sided, V-shaped valleys into mountainsides; large ones are called gorges or canyons. The Colorado River in the western United States shaped the Grand Canyon, one of world's most spectacular sights. This canyon is about 446 kilometres (277 miles) long, more than 1,600 metres (5,300 feet) deep, and reaches a maximum width of 29 kilometres (18 miles).

Humans make natural erosion worse by grazing livestock, ploughing soil, and destroying forests. Without vegetation to soak up water, run-off increases, carrying soil into rivers. Sediment raises the bottom of rivers, forcing them to overflow their banks, widen their channels, or adopt new flow paths. Reservoirs behind dams can also become clogged with excessive silt; for this reason, the useful life of many reservoirs is only a few decades.

Sediment Deposits

Deltas are large deposits of soil or silt formed where a stream or river empties into an ocean, lake, or slower river. Like alluvial fans, deltas may cause a river to split or migrate because of the buildup of sediment. Deltas usually contain highly fertile soil. The combined delta of the Ganges and Brahmaputra rivers in Bangladesh and India, and the delta of the Nile in Egypt, are two of the world’s largest.

Floods

Floods, however, do have one benefit: sediment left behind on a river’s floodplain forms rich, fertile land. The ancient Egyptian civilization depended on the flooding of the Nile to add nutrients to the soil.

Human Intervention

Building dykes or levees along a river’s banks forces the river into a confined path and usually prevents flooding in that particular area, but can cause increased flooding further downstream in regions without dykes. Flooding waters may, moreover, overflow or breach dykes.

Large dams, such as Paraguay’s Itaipú Dam and Egypt’s Aswān High Dam, have been constructed to store water for irrigation and to harness the water’s power for the production of hydroelectricity. The resulting reservoirs also provide areas for recreation.

Dams, although beneficial, have harmful effects as well. The creation of reservoirs floods vast areas of land and can force evacuation of the population. Archaeological sites, both known and undiscovered, are flooded under new reservoirs. In the filling of Aswān High Dam’s Lake Nasser, engineers had to relocate the temples of Abu Simbel to higher ground.

The large surface area of reservoirs leads to high water loss from evaporation. Dams adversely affect animal life in rivers. Some prevent fish, such as the sockeye salmon of the Pacific Rim area, from swimming upstream and spawning, although engineers can design structures, such as fish ladders, that allow fish to migrate past dams.

In addition, extensive use of water for irrigation may virtually empty a river of its water. Large-scale irrigation in the southwest United States virtually dries up the Colorado River before it reaches its mouth in the Gulf of California. Irrigation may also result in high concentrations of salt in the remaining water. Finally, several dams have ruptured because of earthquakes and improper construction and maintenance, creating devastating floods downstream.

Pollution