Drainage Basins

Introduction

A drainage basin is the area from which rain drains into a body of water. The hydrologic cycle involves the circulation of water that evaporates from the oceans, and other bodies of water, through precipitation and travels back into the water supply. Drainage basins transport water from precipitation back into streams, rivers, and lakes through various means. Their nature is influenced by the underlying geologic features because these control how water flows.

Drainage basins comprise the land and the various overground and underground water channels running through it. A drainage basin is classified according to how its water channels are distributed in the surrounding land area. The output of a drainage basin is the input to the water supply. Therefore, what happens in a drainage basin is important for managing water resources. Water inputs may carry pollutants such as sediments and agricultural runoff with them as they drain toward their output. Excess water input and activities like deforestation may also cause flooding in a drainage basin.

Historical Background and Scientific Foundations

All significant bodies of water, from streams to oceans, are served by an area known as its drainage basin, which occupies an important position in the hydrological cycle. It is local topography that determines how water flows from one place to another. A drainage basin acts like a funnel, conveying water that originates as precipitation to a stream, lake, river, sea, or ocean. The input is in the form of groundwater, runoff, or throughflow, which are the three main ways in which precipitation in the form of rainwater or snow moves along or through the ground. A certain amount of sediment and other substances, which may be polluting, also enters the drainage basin along with its water input.

A drainage basin can be seen as an open and dynamic system with its input, as described earlier, and an output, which is the water supply to a body of water. The factors affecting the movement of water within the basin include its topography, soil type, bedrock type, local climate, and nature of vegetation cover. Sometimes the term watershed and drainage basin are used interchangeably. However, the watershed actually denotes the topographic barrier that divides one water basin from another. Drainage basins are often considered as a nested system with larger basins draining into smaller ones.

A drainage basin comprises the land through which the water drains and the water channels that flow through it. Drainage basins come in all shapes and sizes, and can be classified according to the pattern of channels within the basin, which is often influenced by the underlying geology. For example, a dendritic or branched drainage basin is typical of one built on sediment that is vulnerable to erosion. Basins built on tectonic faults or bedrock joints will cause water channels to take up a gridlike or rectangular pattern. Parallel drainage patterns are found where there is steep relief and deranged drainage, with no obvious pattern, that occurs in areas where there has been glacial or volcanic activity.

Impacts and Issue

The way a drainage basin functions can have an impact upon the quality and quantity of water entering a local supply. As water flows through the basin, it will pick up nutrients, sediments, and pollutants and these will end up in the water supply. Management of water resources, therefore, concentrate on the state of the local drainage basins, and sources of pollution such as agricultural runoff are controlled at the source wherever possible.

For instance, one of the world’s most important drainage basins is that serving the River Ganges in northern India. This area is often flooded because of melting snow from the Himalayan Mountains along with monsoon rains occurring between April and September. Deforestation in Nepal, Bhutan, and Northern India has contributed to the flooding of the basin through soil erosion and the lack of takeup of water by tree roots. Although this situation has helped with rice crops in the area, it has also resulted in flooding that destroyed homes, livelihoods, and caused many deaths.

See Also Precipitation; Runoff; Streamflow; Water Resources

BIBLIOGRAPHY

Books

Kaufmann R., and C. Cleveland. Environmental Science. New York: McGraw-Hill InternationaEdition, 2008.

Web Sites

Physicalgeography.net. “The Drainage Basin Concept.” http://www.physicalgeography.net/fundamentals/10aa.html (accessed).

WORDS TO KNOW

BEDROCK: Solid layer of rock lying beneath Earth’s surface.

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

FAULT: A fracture in the continuity of a rock formation resulting from tectonic movement.

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

SEDIMENT: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth’s surface. Sediments can also result from chemical precipitation or secretion by organisms.

THROUGHFLOW: The horizontal flow of water through soil.

TOPOGRAPHY: The surface features of an area, such as hills or valleys.

WATERSHED: The boundary of a drainage basin.