Alluvium Sediments, such as silt, sand and gravels, carried by a river that it has dumped on its bed, its floodplain or its delta as its current slows down. The deposit of alluvium made where a river or stream enters a plain or open valley may form the shape of a fan, and is called an alluvial fan.

Braid A small island in the middle of a river. Braids form when a river can carry no more sediment, and is forced to dump some on the river bed. Over time the sediment builds up into braids. These may split the river into smaller channels.

Channel The groove in the ground along which a river flows.

Confluence The point where two rivers meet.

Delta An area of land sometimes formed at the mouth of a river. Deltas develop when a river loses its speed and deposits its sediments. These form braids and divide the river into several channels.

Estuary The wide part of a river near its mouth where it meets the sea and fresh and salt water mix.

Floodplain A flat area to either side of a river's banks, extending to the base of the enclosing valley slopes, across which it may flood when its waters are high. When a river breaks its banks, it leaves behind layers of alluvium (fine gravel or silt). These layers gradually build up to create a level plain, which is also known as an alluvial plain.

Fluvial processes Erosion and deposition of sediment carried out by rivers and streams and the landforms (e.g. valleys, gorges, ridges, banks, channels, islands, etc.) created by them. Fluvial processes also include the motion of sediment in rivers and streams.

Gorge A deep, steep-sided valley, sometimes called a canyon. Gorges may be cut into the ground by a river or may form when a cave roof collapses.

Lake A large, still, pool of water. Many lakes are filled by rivers. Some are filled by rain, or melted glaciers.

Levee A bank of sediment, deposited by a river either side of its channel when it floods.

Meander A curve or bend in a river. Meanders form when a river erodes sediments from one side of a bend and deposits them on the other side. This occurs because the water on the outside of the bend flows more quickly than the water on the inside. Over time this action increases the size of the bend.

Mouth The point where a river flows into a sea or lake.

Ox-bow lake A curved lake that was once part of a meander. It is formed when a river cuts through the land between two meanders in order to shorten its course. The old meander is cut off by sediments and becomes an ox-bow lake.

Plunge pool The deep pool at the base of a waterfall. It is carved out by the force of the falling water.

Rapids A fast-flowing, turbulent stretch of river. Rapids occur where the river is shallow and the riverbed slopes steeply.

River A river is a natural channel of water that runs downhill. It starts as a spring, as melted water from a glacier, or as rainwater collecting on sodden ground. Near its source, river is a fast-flowing stream. Lower down it slows and widens as other streams join it. The river eventually flows into a lake or sea.

River erosion The gradual wearing down of the land by a river. A river does this in three ways: dissolving, carrying away and abrasion. Certain rocks, such as limestone, can be dissolved. The force of running water is sometimes enough to carry away loose sediment. This sediment may then scrape against, or abrade, the sides of the channel, acting like sandpaper. The sediments themselves are worn down by the constant collision of particles.

Sediment Particles of soil, sand and mud that are moved from one point to another by water, wind or ice. Fast-flowing rivers can transport larger pieces of rock, such as pebbles or even boulders.

Source The point where a river begins, often high in the mountains or hills.

Spring A point where water gushes up from underground.

Stream A small, shallow flow of water.

Tributary A small river that flows into a larger river.

Valley A long, low-lying area between hills or mountains, usually with a river running through it. A valley formed by the erosive power of flowing water, a river valley, is usually V-shaped. A fast-flowing mountain stream has steep-sided valley slopes, while a slower-flowing river generally has broader and gentler valley slopes with a flat valley bottom known as a floodplain.

Waterfall A cascade of water that occurs where a riverbed changes from hard to soft rock. The river wears away the softer rock more quickly, so a “lip” of hard rock forms on the river bed, gradually becoming a cliff over which the river waters plunge.

Consultant: Ian Fairchild