River Notes

What is a River?

• A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake, or another river.

River Components and Features

• River Source (Headwaters): The starting point of a stream.
• Watershed: High-lying areas that separate individual drainage basins.
• Confluence: A point where two rivers meet.
• Waterfall: A cascade of water falling from a height, formed when a river or stream flows over a steep incline.
• Tributary: Streams or small rivers that feed into a larger river.
• Floodplain: An area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding.
• Marsh: An area of low-lying land which is flooded in wet seasons or at high tide, and typically remains waterlogged.
• Meander: A bend in a river.
• Levee: Natural flood wall.
• Oxbow Lake: A loop formed by a horseshoe bend in a river.
• Delta: Wetlands that form as rivers empty their water and sediment into another body of water.
• Estuary: The tidal mouth of a large river, where the tide meets the stream.
• River Mouth: The end of the river, in the ocean.

River Stages

• Youthful Stage
• Mature Stage
• Old Age Stage

River Profile

• Long Profile: The change in gradient with distance; it starts off steep and reduces with distance from the source, resulting in a concave profile.
• Cross Profiles:
  • Upper Valley
  • Middle Reaches
  • Lower Reaches

River Characteristics & Processes in Different Reaches

• Upper Valley:
  • Height above sea level: 500
  • Vertical erosion with hydraulic action, abrasion, and attrition dominant processes.
  • Traction and saltation at high flow.
  • Load size is large and angular.
  • V-shaped valleys.
• Middle Reaches:
  • Channel is deeper and wider.
  • Vertical erosion decreasing in importance; more lateral erosion and deposition.
  • Suspension is the main transportation type.
  • Load becomes smaller and less angular.
• Lower Reaches:
  • Channel is at its widest and deepest and may be tidal.
  • Deposition is more important than erosion.
  • Fine material is deposited.
  • A large amount of load, but the size is very small and very rounded.

River Transportation

• Suspension: Fine, light material is carried along by the river.
• Solution: Minerals are dissolved in the water (a chemical change).
• Traction: Large boulders and rocks are rolled along the river bed.
• Saltation: Small pebbles and stones are bounced along the river bed.

Waterfalls

• Develop when a river flows over resistant rock.
• Occur in the upper course of the river, which is steep and where erosion happens quicker.
• The river erodes the softer rock faster than the harder rock.
• The hard rock will create a waterfall with a plunge pool.
• In time, the overhanging hard rock will break off and collapse.
• The deep plunge pool forms because of the hydraulic action of the water and abrasion by the rocks swirling around in the plunge pool.
• The waterfall will retreat over time (and disappear).

Waterfall Formation Stages

1. Undercutting.
2. Overhang collapses.
3. Plunge pool develops.
4. Waterfall retreats upstream.
5. Steep, gorge-like valleys.

Rapids

• Smaller waterfalls that result when water flows over harder, more resistant rock and softer rock.
• These become steeper over time as erosion takes place.
• They create white waters, which are called turbulent flow.

Potholes

• A pothole is a circular or cylindrical hole in the riverbed, produced by the force of water and abrasion.

Meanders

• A meander is a bend in the channel of a river.
• It is produced as a river erodes the sediments of an outer bank and deposits sediments on an inner bank.
• Lateral erosion occurs.

Cross Section Through a Meander

• Outer bend: Fastest velocity, river cliff, undercutting.
• Inner bend: Area of deposition, slip-off slope.

Levees

• A ridge of sediment deposited naturally alongside a river by overflowing water.
• Thickest and coarsest sediments deposited at channel edges.
• Thin and fine sediments deposited over outer parts of the floodplain.
• Natural levees are built up by many floods.

Braided Stream

• When the river's energy decreases, so does its capacity to transport the river load, and so deposition occurs.
• The river deposits its load in the channel and so chokes itself.
• This forces the river to flow around the deposited sediments.

Floodplain

• A generally flat area of land next to a river or stream.
• It stretches from the banks of the river to the outer edges of the valley.

River Delta

• A delta forms when a river flows into the sea.
• Deposition occurs due to reduced speed and lower gradient of the river.
• The coarser sediments are deposited first, and the finer particles away from the river mouth.
• The classic shape of a delta is triangular or arcuate (Greek letter D).
  *Distributary Channels.