The Lena River, some 4,400 kilometres (2,800 miles) long, is one of the largest rivers in the world. The Lena Delta Reserve is the most extensive protected wilderness area in Russia. It is an important refuge and breeding grounds for many species of Siberian wildlife.
The steady flow of clean, fresh water is an essential element for vast ecosystems and the health and survival of billions of people.
A river may have its source in a spring, lake, from damp, boggy places where the soil is waterlogged, from glacial meltwater, or simply from rain flowing off impermeable rock or man-made surfaces. Almost all rivers are joined by other rivers and streams, termed "tributaries', the highest of which are known as headwaters. Water may also be recruited to a river from ground-water sources.
Throughout the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain. For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow.
The area drained by a river and its tributaries is called a catchment, catchment basin, drainage basin or watershed. The term "watershed" is also used to mean a boundary between catchments, which is also called a water divide.
Floodplains and deltas A river's water is generally confined to a channel, made up of a stream bed between banks, but in larger rivers there is also a wider floodplain shaped by flood-waters over-topping the channel.
Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and floodplain can be blurred especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry. Rivers that carry large amounts of sediment may develop large deltas at their mouths, if conditions permit.
The flora and fauna of rivers have developed to utilise the very wide range of aquatic habitats available from torrential waterfalls through to lowland mires
Although many organisms are restricted to the freshwater in rivers, some, such as Salmon and Hilsa have adapted to be able to survive both in rivers and in the sea.
Flooding is a natural part of a river's cycles. The majority of the erosion of river channels and the erosion and deposition on the associated floodplains occur during flood stage. Human activity, however, has upset the natural way flooding occurs by walling off rivers, straightening their courses and by draining of natural wetlands.
Rivers have played an important and life-sustaining role in human societies for thousands of years, which is why many of the world's great cities sit on the bank of a great river.
We love our rivers and we abuse them. We have used them as a source of water, for food, for transport, for recreation, as defenses, as a source of power to drive machinery, and as a means of disposing of waste.
The earliest evidence of river navigation is found in the Indus Valley Civilization, in Northwestern India around 3300 BC and riverine navigation is still used extensively in major rivers of the world like the Ganges, Nile, Mississippi, and Indus.
Rivers continue to be a very important source of food for societies around the world. Apart from being a rich source of fish, rivers indirectly aid in cultivation with its supply of water for the crops.
For millenia, humans have used rivers to dispose of waste. Untreated sewerage pours into a river in Bangkok, Thailand. Creative Commons licence
Many of the great rivers of the world snake through different countries and different states and have historically been used to identify borders. For this reason their proper management and protection requires a high level of cooperation amongst governments.
Rivers are often managed or controlled to make them more useful and less disruptive to human activity.
Dams or weirs may be built to control the flow, store water, or extract energy.
Levees may be built to prevent run-off of excess river water in times of flood.
Canals connect rivers to one another for water transfer or navigation.