River Processes and Landforms: A Comprehensive Guide

Posted on Sep 2, 2024 in Geology

Rivers

Erosion

Corrosion: When sand and pebbles are dragged along the river bed, wearing it away.

Attrition: When rocks and stones wear each other away as they knock together, becoming smaller and more rounded.

Solution: Rocks such as limestone are dissolved in acidic water.

Hydraulic Action: Fast, flowing water forces into cracks, breaking up the bank over time.

Transportation

Solution: Where dissolved chemicals are carried along, invisible to the eye.

Suspension: When tiny particles are carried in the river’s current.

Saltation: Smaller stones or pebbles are picked up and then dropped again in a ‘skipping’ motion.

Traction: Large stones are dragged along the river bed.

Bed Load: Heavier materials carried along the bottom of the river.

Factors Affecting Transportation

  • Energy of Water: The faster the flow, the larger the materials that can be transported.
  • Volume of Water: More water has a greater volume of load.
  • Bedrock along the Course of the River: Hard rocks are slower to erode, while some are soluble by river water.

Deposition

Deposition happens when the river loses energy. This can be caused by:

  • Decrease in gradient
  • Decrease in river flow
  • River meeting the sea or a lake
  • River flowing more slowly on the inside of bends

Discharge: The volume of water flowing down a river at any given time, measured in cubic meters per second.

River Profiles

Cross Profile

Distinct changes occur between the source and mouth of a river and its valley, involving the cross-section and energy of the river. In the source or upper course, the river has more energy due to the steep gradient. Because it is high above base level, it has enough energy to erode and transport materials vertically, creating a V-shaped valley. Further downriver, the river uses most of its energy to transport load, so there is less energy to erode. This is why valleys get wider due to lateral erosion.

Long Profile

A cross-section of the river from its source to its mouth.

  • Upper Course: Steep gradient, so erosion is greatest. Sediment load is limited because little material is available. The energy of the river is at its highest, so vertical erosion happens.
  • Middle Course: More material to transport, so there is less energy to erode. Less vertical erosion occurs, and lateral erosion dominates. Material is deposited because of the lack of energy.
  • Lower Course: Where deposition mainly happens because the gradient is low, so erosion and transportation are less.

Hydrological Cycle

The continuous transfer of water from the ocean into the atmosphere, then to the land, and finally back to the oceans.

Processes:

  • Precipitation: Deposition of moisture, usually from clouds. It can be in the form of rain, snow, hail, or sleet.
  • Interception: Capture of rainwater by leaves.
  • Surface Storage: Water stored on the surface (can be in lakes, puddles, oceans).
  • Infiltration: Soaking of rainwater into the ground.
  • Soil Moisture Storage: Water stored in the soil.
  • Percolation: Water passing through lower layers of soil.
  • Groundwater Storage: Water stored in spaces of porous, permeable rocks.
  • Surface Runoff: Rainwater that runs across the surface of the ground as rivers.
  • Throughflow: Movement of water within the soil sideways.
  • Groundwater Flow: Movement of water through saturated ground.
  • Evaporation: Change in state from liquid to gas.
  • Transpiration: Liquid water evaporating from vegetation. (Both evaporation and transpiration are collectively called evapotranspiration.)

River Landforms

V-Shaped Valley

Formed from vertical erosion in the upper course, creating a deep and narrow channel. The sides of the channel are weathered, leaving them unstable, so soil material slides down to the river.

Waterfalls

Waterfalls happen when a river crosses a band of hard rock. The softer rock gets eroded by abrasion and hydraulic action, creating a ledge. Erosion in the softer rock continues and undercuts the hard rock, leaving a hollow bottom called the plunge pool. The plunge pool is full of rock, so abrasion and hydraulic action continue due to the force of the falling water. Eventually, the ledge collapses, and the waterfall moves backward, leaving a steep-sided gorge.

Example: Angel Falls, Venezuela (known for its tourism, scenic beauty, attraction for scientists, and facilities like hotels built for tourists)

Potholes

Smooth, rounded hollows formed in the bedrock of a river by vertical erosion. They are formed by stones trapped in hollows on the river bed. Eddies in the water swirl the trapped stones and cause abrasion, which drills down into the rock. The hollows become deeper and wider and eventually join.

Rapids

Commonly formed where water is shallow and the river bed is rocky and irregular, making the water rough. The gradient is often steeper in these areas. Rapids are a barrier to river navigation and can be caused by bands of hard rock in the river bed.

Middle and Lower Course Landforms

Floodplains

Flat land beside a river that is liable to flood. Floodplains are often marshy and poorly drained. The river often flows above the level of the surrounding land but is enclosed by embankments or levees.

Types of Deposition that Help Build Floodplains:

  • Deposition on the inside of meanders
  • Deposition of gravel in the river bed
  • Deposition of fine silt and mud

Example: River Ganges, Bangladesh

Advantages of Floodplains:

  • Fertile land due to alluvium deposits
  • Flat land, easy to build on, suitable for high population density
  • Good water supply for irrigation
  • Water source for fishing
  • Good transportation routes using boats

Disadvantages of Floodplains:

  • Land and crops can be flooded and destroyed
  • Houses and buildings can be destroyed during floods
  • Infrastructure (electricity, water supply, roads) can be damaged
  • Risk of loss of life and displacement of people

1998 Flood in Bangladesh:

  • Some areas experienced flooding for 70 days
  • Over 1,000 people died
  • 70,000 homes were destroyed
  • 250,000 people were left homeless

Levees

Levees form naturally when a river floods. When water overflows the river channel, it slows down, causing the coarsest part of the river’s load to be deposited at the channel edges, making the banks higher. Levees can also be raised or strengthened artificially to prevent flooding.

Example: Mississippi River (fourth largest drainage basin in the world, 150km wide floodplain, 230 dams)

Meanders

Lateral erosion occurs on the outside of a bend, and deposition occurs on the inside. The fastest current is always on the outside of the bend. A new shape of meander forms, and the neck of land between loops becomes narrower. The river eventually cuts through the neck during a flood, creating an oxbow lake. Deposition then occurs on both sides of the river.

Deltas

Conditions for Delta Formation:

  • The river must carry a large load of sediment.
  • There should be no strong tide currents that would wash away sediments quickly.

Delta Formation Process:

  1. The river is slowed down by entering the sea or lake.
  2. River flow is blocked by deposition and is forced to divide into distributaries.
  3. Sediments are deposited along the distributaries and extend outward into the sea or lake.

Example: River Rhine delta in the Mediterranean Sea in southern France