A.1.4 River Landforms Notes

Waterfalls Are Created by Differential Erosion

1. Waterfalls are created by differential erosion, which occurs when a river flows over layers of rock with varying resistance to erosion.
2. Here’s how it works:
   1. Hard and Soft Rock Interaction
      1. A river flows over a layer of resistant hard rock (e.g., granite) underlain by softer rock (e.g., shale).
      2. The softer rock erodes more quickly due to the processes of hydraulic action(the force of water breaking rock apart) and abrasion (rock particles scouring the surface).
   2. Undercutting
      1. As the soft rock erodes, the hard rock above it becomes unsupported and eventually collapses.
   3. Waterfall Retreat
      1. Over time, this process causes the waterfall to retreat upstream, leaving behind a steep-sided gorge known as a gorge of recession.

Floodplains Are Flatlands of Fertility

1. Floodplains are the wide, flat areas on either side of a river, often rich in nutrients and ideal for agriculture. But how are these fertile lands formed?
   1. Overbank Flow
      1. During periods of flooding, a river overflows its banks, spreading water and sediment across the surrounding land.
   2. Deposition
      1. As the floodwaters slow down, they lose energy and deposit their load.
      2. Coarser materials settle closer to the river channel, while finer particles (like silt and clay) are carried further, creating a flat, fertile plain.
   3. Repetition
      1. Over time, repeated flooding builds up layers of sediment, forming the floodplain.

Meanders Form The Curves of Rivers

1. Why do rivers twist and turn instead of flowing straight?
2. The answer lies in the interplay of erosion and deposition, which creates meanders, curved sections of a river.
   1. Initiation of Meanders: Small irregularities in the riverbed cause water to flow faster on one side of the channel and slower on the other.
   2. Erosion on the Outer Bend: On the outer bend of a curve, the river’s velocity is higher, leading to lateral erosion through hydraulic action and abrasion. This creates a steep bank known as a river cliff.
   3. Deposition on the Inner Bend: On the inner bend, where velocity is lower, the river deposits sediment, forming a gentle slope called a slip-off slope.
   4. Migration: Over time, meanders migrate laterally across the floodplain, creating a dynamic, ever-changing landscape.

Levees: Natural Embankments

1. Levees are raised banks that form along the edges of a river channel, acting as natural barriers to flooding.
   1. Flood Events: When a river floods, it carries a large load of sediment.
   2. Deposition Near the Channel: As the floodwaters spread out, they slow down and deposit the heaviest and coarsest material closest to the riverbank. Finer materials are carried further onto the floodplain.
   3. Build-Up Over Time: With each successive flood, the deposited material builds up, forming raised embankments known as levees.

Deltas: Where Rivers Meet the Sea

1. Deltas are iconic features that form where rivers meet standing bodies of water, such as oceans or lakes.
2. They are created by deposition as the river slows down and loses energy.

Key Factors in Delta Formation

1. Sediment Load: Rivers must carry a significant amount of sediment to form a delta.
2. Velocity Reduction: As the river enters the standing water, its velocity drops sharply, causing sediment to settle.
3. Coastal Conditions: Low-energy environments (e.g., sheltered coastlines) favor delta formation, while strong waves or tides may disperse sediment, preventing a delta from forming.

Types of Deltas

1. Arcuate Delta: Fan-shaped with many distributaries, such as the Nile Delta.
2. Bird’s Foot Delta: Long, narrow projections extend into the water, like the Mississippi Delta.
3. Cuspate Delta: A pointed shape, formed where wave action is dominant, such as the Tiber Delta.

Inland Deltas: When a River Never Reaches Body of Water

1. An inland river delta is a specific delta formation that occurs where a river spreads out and distributes its waters and sediment in a landlocked area, away from the sea or ocean.
2. Unlike coastal deltas, inland river deltas develop in regions where a river fan out over a flat plain, typically resulting in a complex network of channels, lakes, and islands.
3. These deltas can create rich ecosystems and support diverse wildlife and plant species, often serving as vital habitats and agricultural areas.
4. An example of an inland river delta is the Okavango Delta in Botswana.