SUMMARY

Rivers are natural systems that carry water falling on land, to areas lower in altitude, usually the sea. Energy is lost from the water as it moves from higher to lower altitudes. This energy allows the water to perform a number of actions, including the erosion and deposition of sediment. Erosion occurs when water picks up sediment from the surrounding landscape, the riverbank or the river bottom. Deposition occurs when water drops sediment on the river bottom or on the river lowlands during floods.

The erosion-deposition cycle depends on water interacting with different sizes of particles. The faster the water, the larger the particles it can carry. When their speed slows down, larger particles can no longer be carried and are deposited.

Headwater streams are steep and torrential with alternating pools and riffles where water flows more rapidly. They are mainly dominated by erosion. Wood falling from the forest canopy is an important element in the structure of these streams.

The middle river represents a balance between deposition and erosion. Much of the heavier material is deposited but lighter silts and sands can still be picked up by the flow and transported further downstream. At intermediate slopes, rivers can form braided channels; that is, have numerous channels separated by islands.

In the lower reaches of the river where the slope of the landscape is low, sediments are much finer, ranging from sands to fine mud. This zone is influenced mainly by deposition, but there is also localized erosion with local reworking of the sediments. Here the main channel or channels of the river form a sinuous snake-like path across the landscape, that is refered to as a meander. Individual meander bends are constantly changing shape because of the processes of erosion and deposition. During the floods, water rises in the channel until it reaches a level refer to as bankfull beyond which it overflows the riverbank and begins to spread across the low-lying floodplain. The floodplain consists of two main components:

• An area of land that is submerged in the wet season and dry in the low water season.

• A number of water bodies (lakes), some of which retain water throughout the year and some of which dry out. These can be small or large.

When water first crosses the bank, the speed of its current diminishes rapidly, and it deposits the coarser sediments to form a raised area known as a levee. Levees usually flank the main river channel but may also be present on the banks of channels feeding the floodplain water bodies. Levees provide areas that are relatively suitable for human settlement.

The mouth of the river or estuary is often in the form of a delta with numerous channels and lagoons. The slowing of current in the coastal flatlands causes massive deposition of silt. The main channels of the river split and the river reaches the sea through a number of arms. The various channels are separated by flat floodplains that are often covered with lakes and marshes. The lower part of the river has three zones:

• A permanently freshwater zone inhabited by freshwater species;

• A fluctuating middle zone that is sometimes salty and sometimes fresh (depending upon flood and tide conditions). This is inhabited either by freshwater or marine species depending on the time of year. There are also some species that are specialised for living in zones of changing salinity; and

• A permanently salt water zone inhabited by marine animals and plants.

Nutrients are carried by the water from upland to lowland sections. As they travel they are continually absorbed and released by animals and plants along the river course. This process of successive transport and storage is known as "nutrient spiralling" and is important to ecosystem function and productivity.

Because rainfall is seasonal in most parts of the world, all rivers exists in two forms, a dry season form where it is confined to its main channel and a wet season form where it covers adjacent land known as the floodplain. Flooding of rivers is a natural event that is a very important aspect of the ecosystem.

Seen from above, a river has a tree like pattern with many small streams feeding into fewer larger rivers and eventually into one very large river. The various levels of branching create various levels of streams known as orders.

The river system includes the main river channels from headwaters to the sea, and its associated permanent or temporary wetlands including floodplains, lakes, pools and delta. All of these features are interconnected and inter-dependant.