Plains and Rivers
Complete Notes & Free Quiz

A plain is an extensive, nearly level stretch of land. Plains usually have very little change in height or elevation. They can be found along coastlines (coastal plains) or far inland (inland plains).

Plains are generally lower than the land surrounding them. Their flat, open nature makes them very suitable for human settlement, agriculture and transport.

Plains are among the most densely populated regions in the world for two main reasons.

First, the soil in plains is usually fertile and suitable for agriculture, especially where rivers deposit alluvial soil over thousands of years. This allows farmers to grow large quantities of food to support large populations.

Second, the flat land makes it easy and inexpensive to build roads, railways and cities. Transport networks can expand quickly on flat land, encouraging trade, industry and population growth.

The Indo-Gangetic Plain is the most densely populated region in the entire world. It is home to approximately one-seventh of the world's total population.

It comprises the floodplains of the Indus, Ganges and Brahmaputra river systems. Geographically, it covers most of northern India, almost all of Pakistan and nearly all of Bangladesh.

The primary reason for its enormous population is the extremely fertile alluvial soil deposited by these rivers over thousands of years, which supports intensive agriculture and dense human settlement.

Alluvial soil is fertile soil that is deposited by rivers as they flow over flat land and flood their banks. When river water slows down on a plain, it drops the fine sediment it was carrying, building up layers of rich, nutrient-filled soil over time.

Alluvial soil is important because it is very rich in nutrients and is ideal for growing crops such as wheat, rice and cotton. It is the main reason why river plains like the Indo-Gangetic Plain can support such enormous populations.

Plains can be classified in two ways:

First, according to their physical features: for example, whether they are covered by grass, ice, sand or forest.
Second, according to their formation: that is, the geological process by which they were formed, such as erosion, deposition or the uplift of land.

Grasslands are plains that are mostly covered with grass. The height and type of grass depends on the amount of rainfall the region receives. In areas with little rain, only short grass survives. Where rainfall is higher, tall grasses and scattered trees can grow.

The Great Plains of central North America are one of the world's most famous grasslands.

A tundra is a type of plain found in Arctic regions where the temperature is extremely cold for most of the year. The ground is permanently frozen (called permafrost) and only mosses, low shrubs and some grasses can survive in these harsh conditions.

Tundra plains are found in the northern parts of Canada, Russia and Scandinavia.

Structural plains are formed when a section of the ocean floor or a continental shelf is uplifted by geological forces. This process raises formerly submerged land above sea level, creating a flat plain.

Structural plains are commonly found at the borders and edges of major continents.

Erosional plains are formed over millions of years as agents of denudation: including rivers, rain, glaciers and wind: gradually wear down highland areas and carry the material away.

The resulting surface is not perfectly smooth because not all rocks erode at the same rate. These slightly uneven erosional plains are called peneplains, a word that means almost a plain.

Depositional plains are formed when material carried by rivers, glaciers or wind is deposited over a flat area, building up layers of sediment over time.

Alluvial plains are the most common type, formed when rivers deposit sediment as they slow down at the base of mountains or on flat land. The Indus and Ganges plains of South Asia are alluvial plains.

Lava plains form when volcanic lava flows spread across a flat surface and solidify. They are dark in colour because of a mineral called basalt.

Coastal plains form along coastlines when sea levels fall or when rivers deposit material into the ocean over thousands of years. The Indus Delta in Pakistan is an example.

A river system is the complete network formed by a main river and all the streams and channels connected to it. Its main components are:

Source: The starting point of a river, usually in mountains, glaciers, lakes or springs.
Course: The path the river follows from source to mouth.
Tributaries: Smaller streams and rivers that flow into the main river, increasing its volume of water.
Mouth: The point where the river enters a sea, lake or ocean.
Distributaries: Smaller channels that branch off from the main river near its mouth and flow separately into the sea.

A drainage basin is the entire area of land that is drained by a single river and all its tributaries. All rainfall that falls within a drainage basin eventually flows into that river.

The boundary of a drainage basin is called a watershed. A watershed is typically formed by a ridge, hill or mountain range that separates one drainage basin from another. Rainwater falling on one side of a watershed flows into one river system, while rainwater on the other side flows into a completely different river system.

Upper Course: The river begins at its source with a steep gradient and fast-flowing water. The river is narrow and cuts downward into the rock, forming V-shaped valleys, gorges and waterfalls. There is more erosion than deposition in this section.

Middle Course: The gradient becomes gentler and the river moves at a moderate speed. The river begins to flow sideways (lateral erosion), widening its valley. Bends called meanders begin to form as the river erodes its outer bank and deposits material on its inner bank.

Lower Course: Near the mouth, the gradient is nearly flat and the river flows very slowly. The river is wide and carries large quantities of sediment. Meanders are large and the river deposits most of its load, forming wide floodplains and a delta where it meets the sea.

A meander is a wide, sweeping bend in the course of a river. Meanders develop in the middle and lower courses of a river where the gradient is gentle.

On the outer bank of a bend, the water moves faster and erodes the bank, cutting into it and making it steeper. On the inner bank, the water moves more slowly and deposits sediment, forming a gentle slope. Over time, this process makes the bend more and more pronounced, creating the characteristic S-shape of a meander.

Rivers provide many essential benefits to human communities:

Drinking water: Rivers are a primary source of fresh water for millions of people.
Irrigation: River water is used to irrigate farmland, especially in dry regions like Pakistan.
Hydroelectric power: The energy of flowing river water is used to generate electricity in hydroelectric power stations.
Transport: Rivers have been used as transport routes for goods and people throughout history.
Fishing: Rivers provide fish as a source of food and income for communities.
Industry: Factories use river water for cooling and manufacturing processes.
Recreation: Rivers are used for sports such as swimming, boating and fishing.