Rivers: Sources and Processes

Rivers: Sources and Processes

Overview

  • Introduction to rivers, their processes, and features.

Introduction to Rivers

  • Approximately 97% of Earth's water is in oceans.

  • The remaining 3% is freshwater, mostly locked in glaciers, pack ice, or underground.

  • Rivers and lakes contain only about 0.3% of Earth's water.

  • Rivers are vital in the water cycle and human life.

  • Definition of a river: A large body of water flowing in a channel, draining excess water from the land.

  • Source: The beginning of a river, which may be a spring, lake, or glacier.

  • Mouth: The end of a river, where it empties into a sea, ocean, or lake.

  • Tributary: A river that flows into another river.

  • Confluence: The point where a tributary joins a larger river.

  • Drainage System: A main river and all its tributaries.

  • Drainage Basin: All land drained by a river and its tributaries.

  • Watershed/Divide: High land separating drainage basins.

River Processes

  • Rivers shape the landscape through erosion, transportation, and deposition.

Erosion

  • Erosion: Removal of weathered material by agents like wind, water, and ice.

  • Running water is a powerful erosion agent.

  • Rivers erode in four ways:

    1. Hydraulic Action

    2. Solution/Corrosion

    3. Corrasion/Abrasion

    4. Attrition

Hydraulic Action

  • The sheer force of water.

  • Water splashes against banks, entering cracks and openings.

  • This loosens material along the riverbed and banks, wearing away rocks.

Solution / Corrosion

  • Chemical action of water on rocks.

  • Some rocks (e.g., limestone) and minerals are soluble.

  • They dissolve slowly in river water containing carbon dioxide from the air.

  • Examples: salt and calcium carbonate.

Corrasion / Abrasion

  • The river grinds its load against the bed and banks.

  • This loosens material that the river carries downstream.

Attrition

  • Materials carried by the river (rocks, stones) collide with each other.

  • Over time, they become smaller, smoother, and more rounded.

Transportation

  • Transportation: How a river carries material; the material is the river's load.

  • A river carries its load in four ways:

    1. Solution

    2. Suspension

    3. Saltation

    4. Traction

Solution

  • Dissolved materials are carried downstream.

Suspension

  • Tiny particles (e.g., silt) are suspended in the water.

  • Heavy rainfall often turns river water brown due to suspended material.

Saltation

  • Small stones bounce along the riverbed.

Traction

  • The heaviest material (large rocks) rolls along the riverbed.

Deposition

  • Deposition: When a river drops its load.

  • Occurs when the river lacks energy to carry the load due to reduced velocity or water volume.

  • Heavier material is deposited first, while lighter material is carried farther downstream.

River Features

  • River valley sections: upper, middle, and lower course, each with distinct features.

The Upper Course

  • Closest to the source; steep land; fastest flow.

  • Erosion is dominant.

  • Features:

    1. V-shaped valleys

    2. Interlocking spurs

    3. Potholes

    4. Rapids

    5. Waterfalls

V-shaped Valleys

  • Rapid river flow cuts down into the riverbed (vertical erosion).

  • Large boulders and fast flow cause rapid erosion.

  • Steep V-shaped valleys form.

  • Soil and loose rock from the valley sides are added to the river's load.

Interlocking Spurs

  • River meanders around hard rock obstacles.

  • Erosion concentrates on the outside of bends.

  • Vertical erosion occurs simultaneously.

  • Ridges of high land (spurs) project towards the river, decreasing in height.

  • Spurs alternate on either side of the river.

Potholes

  • Uneven riverbed causes water to swirl quickly.

  • Pebbles and rocks erode circular depressions (potholes).

  • Potholes increase in size over time.

Rapids

  • Sudden increase in channel slope.

  • Water flows more quickly and roughly.

  • Can occur due to changes in rock type.

Waterfalls

  • River descends vertically (e.g., off a cliff).

  • Often found where hard rock lies over softer rock.

  • River erodes the softer rock.

  • Hard rock remains as a cap.

  • Plunge pool forms at the base due to the force of falling water.

The Middle Course

  • Less steep land; lateral erosion more prominent.

  • More open valley.

  • Some deposition occurs.

  • River transports much of its load.

  • Features:

    • Meanders

    • River cliffs

    • Slip-off slopes

Meanders

  • River winds across the land in large loops.

River Cliffs and Slip-off Slopes

  • Faster water flow on the outside bend of a meander leads to erosion.

  • River bank is undercut, forming a river cliff.

  • Slower water flow on the inside bend leads to deposition.

  • Deposited material creates a slip-off slope or point bar.

The Lower Course

  • River is at its largest, carrying the greatest water volume.

  • Nearly flat land reduces river speed but still moves quickly.

  • Deposition is the dominant process.

  • Features:

    1. Ox-bow lakes

    2. Meanders

    3. Flood plains

    4. Levees

    5. Braiding

    6. River bars

    7. Deltas

    8. Distributaries

    9. Alluvial Fans

  • Features 1-3 are found in both middle and lower courses, but are more common in the lower course

Ox-bow Lakes

  • Crescent-shaped lakes formed when a meander is cut off from the main river.

  • Over time, they become marshy and eventually dry up.

  • Meander becomes very deep, and the two sides get closer.

  • River cuts straight across instead of following the meander.

  • Deposition seals off the ends, creating the ox-bow lake.

Meanders

  • See middle course

Flood Plains

  • Low-lying areas around the river covered by water during floods.

  • Flat and covered with sediment (alluvium).

  • Vary from a few hundred meters to several kilometers wide.

  • May be marshy with small lakes.

Levees

  • During floods, the river deposits heaviest particles closest to the bank.

  • These particles accumulate, building up the river banks above the floodplain.

Braiding & River Bars

  • Braiding occurs when a river splits into smaller channels due to sediment clogging.

  • More efficient to erode smaller channels than the entire deposit.

  • Intertwining channels resemble braided hair.

  • Areas of sediment that the braids travel around are called river bars.

  • River islands form when river bars are stabilized by vegetation.

Deltas & Distributaries

  • Deltas: Sediment deposition at the mouth of a river entering a lake, sea, or ocean.

  • Only form under specific conditions:

    1. Large sediment amount and low velocity.

    2. Coastal waves must not remove sediment faster than deposition.

  • Large rivers transport a great deal of material.

  • Reduced speed at the river's mouth leads to deposition.

  • Sediment accumulates until it reaches the surface, creating a delta.

  • Braiding through a delta creates channels called distributaries.

Alluvial Fans

  • Alluvial: Relating to alluvium (fine sediment).

  • Alluvial fan: Fan-shaped depositional feature of fine sediment.

  • Forms where a fast-flowing mountain river suddenly slows down upon meeting flat land.

  • Sudden energy loss causes sediment deposition.

  • Deltas form at river mouths entering oceans or seas, while alluvial fans form when rivers slow down at the base of mountains.