Discovery Science: Earth – Rivers- Catchment Areas and Stream Courses

Earth Science: Rivers

Rivers are the lifelines of landscapes. Prerequisite for river formation is an excess of precipitation in relation to evaporation and seepage, as well as a certain topographic gradient through the area.

Although only a small part of the Earth’s entire water volume circulates via rivers, all water courses—from rapid mountain stream to leisurely flowing river—contribute significantly to the structure of the Earth’s surface. Additionally, they provide the source for drinking water, transportation routes, and energy.

Catchment Areas and Stream Courses

All types of water courses on Earth serve as natural drainage systems for surrounding land areas. They absorb excess surface water and transport it to deeper-lying areas by gravitational force.

The area drained by a river is its catchment area (basin). It is separated from the catchment area of another river system by watersheds, typically mountain ranges. River density and stream flow are dependent upon topography, climate, and vegetation cover. Main rivers that discharge into a lake or an ocean differ from tributaries that flow into main rivers. In the direction of its flow, a river is divided into upper reaches, middle reaches, and lower reaches.

At a length of nearly 4,163 miles (6,700 km), the Nile River in Africa is the longest river on Earth. The South American Amazon River, although slightly shorter at 3,978 miles (6,400 km), and with its approximately 15,000 tributaries, carries the largest amount of freshwater. The Yangtze River is Asia’s longer river, spreading across 3,915 miles (6,300 km). A river can spring from a standing body of water or form from a glacial melt. Most rivers have their source in mountains, where cool, oxygenated water emerges from a spring.

Through water influx and precipitation an initial rivulet gradually grows into a rapid-flowing mountain creek running straight through a mountain range with a steep gradient. Along sharply inclined stretches rapids develop; on rocky banks, which do not erode readily, water gushes down as a waterfall. At the edge of a mountain range, where gradient and current velocity abruptly decrease, a mountain creek deposits most of its carried sediments as a scree slope.

Further downstream a river accumulates more water from tributaries and then be- comes a calm yet large flowing stream. Under normal conditions when a narrow mountain valley broadens a river becomes wider and meanders in sweeping bends. The sediment load of the river is reduced and dissolved as sediment is carried to the ocean in larger amounts during periods of high water flow.


PERMANENT (PERENNIAL) RIVERS continuously carry water at all times.

PERIODIC RIVERS are dependent upon seasonal changes between rainy and dry seasons

EPISODIC RIVERS carry water only occasionally, such as after heavy precipitation.

ALLOCHTHONOUS RIVERS (i.e., not formed locally) originate in humid areas and flow through arid regions, thereby losing significant amounts of water