River Landforms: Erosional & Depositional Features

Fluvial Dynamics: Understanding River Landforms and Evolution

Table of Contents

• The Long Profile and the Three Stages
• Upper Course: Erosional Landforms
• Middle Course: Meanders and Migration
• Lower Course: Deposition and Flooding
• Check for Understanding

In fluvial geography, we define landforms as the physical features on the Earth's surface created by the processes of erosion, transportation, and deposition. As a river travels from its source to its mouth, its energy levels change, leading to a distinct sequence of landforms across three primary stages.

The Long Profile and the Three Stages

The long profile of a river is a line representing the gradient of the river from source to mouth. Generally, this profile is a concave curve, showing a steep gradient in the upper course and a very gentle gradient near the sea.

Valley Cross-Sections

• Upper Course: Narrow, steep-sided V-shaped valleys. The river occupies most of the valley floor.

• Middle Course: Wider valley with a developing floodplain. The river begins to meander.
• Lower Course: Very wide, flat valley with extensive floodplains and large levees.

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Upper Course: Erosional Landforms

In the upper course, vertical erosion dominates. River curve from side to side as they head downhill for reasons. Firstly, to avoid obstacles and secondly, due to gravity, water moves to the lowest ground, which might not be straight on. The water will erode downwards into bedrock through the processes of hydraulic action and abrasion. This leaves interlocking spurs - ridges of harder rock that "lock" together like the teeth of a zipper.

The V-Shaped Valley

V-shaped valleys are formed primarily in the upper course of a river through rapid vertical erosion and weathering on the valley sides. Fast-flowing, turbulent water uses hydraulic action and abrasion to cut deeply into the riverbed. Rain, frost, and gravity break down the valley walls, which collapse into the river to be carried away, creating the characteristic "V" shape.

Possible resource: https://youtu.be/nIMYAs--O5o?si=7lZrhToKM9zdoncy

Waterfalls, Plunge Pools, and Gorges

A waterfall occurs when a river flows over a layer of hard rock followed by softer rock. The softer rock erodes faster, creating a step. Over time, the falling water creates a deep plunge pool via hydraulic action and abrasion. As the hard rock is undercut, it eventually collapses, causing the waterfall to retreat upstream, leaving a steep-sided gorge in its wake.

Potholes and Rapids

Rapids form where the river passes over alternating bands of hard and soft rock, or where the gradient increases. Potholes are cylindrical holes in the riverbed formed by "drilling" action as pebbles are whirled around in turbulent eddies (corrasion).

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Middle Course: Meanders and Migration

As the gradient decreases, the river gains more lateral (sideways) energy. This creates meanders—large loops in the river's path.

Meander Cross-Section

The flow of water in a meander is asymmetrical:

• Outside of the Bend: Fastest flow (thalweg), high energy leads to lateral erosion, creating a steep river cliff and deep water.
• Inside of the Bend: Slow flow, low energy leads to deposition of sediment, creating a shallow river beach (or point bar).

Oxbow Lakes

Over time, continuous erosion on the outer bends brings the "neck" of the meander closer together. During a flood, the river takes the shortest path, cutting through the neck. Deposition eventually seals off the old loop, forming a crescent-shaped oxbow lake.

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Lower Course: Deposition and Flooding

In the lower course, the river carries a massive load of fine sediment. Its primary work is deposition.

Floodplains and Levees

A floodplain is the wide, flat area of land surrounding a river that is covered in water during a flood. When a river overflows, it loses energy and drops its heaviest sediment first right next to the channel. Over many floods, these deposits build up to form natural embankments called levees.

Deltas

When a river reaches the sea or a lake, it slows down abruptly. It deposits its remaining load, which can block the main channel and force the river to split into smaller distributaries, forming a delta.

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Check for Understanding

Test your knowledge on the concepts covered in this lecture.

1. Which process is primarily responsible for the retreat of a waterfall?
A) Deposition
B) Undercutting and Collapse
C) Vertical Weathering

2. In a meander, where is the water deepest and the flow fastest?
A) The inside bend
B) The river beach
C) The outside bend

3. Natural embankments formed by the deposition of sediment during floods are called:
A) Gorges
B) Levees
C) Interlocking Spurs

Answers:

• 1. B - The waterfall retreats as the soft rock is undercut and the overhang collapses.
• 2. C - The thalweg (fastest flow) is forced to the outside, creating a river cliff.
• 3. B - Levees are built up on the banks through repeated flooding.