Summary of River and Coastal Landforms

## Introduction Fluvial landforms are shapes on Earth’s surface created by rivers and streams as they erode, transport and deposit sediment. Rivers change their course and shape the landscape as they flow from their source in the highlands to their mouth at the sea or a lake. Understanding fluvial processes helps explain valleys, floodplains, deltas and many features important for farming, engineering and settlement. ## How rivers change along their course Rivers are commonly divided into three stages: **upper (youthful)**, **middle (mature)** and **lower (old)**. Each stage is defined by gradient, energy and the dominant processes. > **Definition:** A river’s **source** is where it begins (usually in the mountains). The **mouth** is where it ends (into the sea, lake or another river). ### Comparison table: Upper, Middle and Lower course | Feature | Upper Course | Middle Course | Lower Course | | --- | ---: | ---: | ---: | | Valley shape | V-shaped | U-shaped | Wide, flat floodplain | | Gradient | Steep | Medium | Gentle, almost flat | | Main processes | Vertical erosion (downcutting) | Lateral (sideways) erosion and transportation | Deposition | | Typical landforms | Waterfalls, interlocking spurs | Meanders | Floodplains, levees, deltas | Fun fact: Did you know that many major cities developed on floodplains because the soil is fertile and transport by rivers was easy? ## Key processes: Erosion, Transportation, Deposition - **Erosion**: The wearing away of rock and soil by river action (hydraulic action, abrasion, attrition). - **Transportation**: The carrying of sediment (load) in solution, suspension and as bedload. - **Deposition**: When the river loses energy and drops the sediment it carries. > **Definition:** *Load* is the material carried by a river; *sediment* is material deposited by the river. ### How process varies with energy - Steep gradient (upper course) → high energy → strong vertical erosion. - Moderate gradient (middle course) → lower energy than upper → lateral erosion and formation of meanders. - Low gradient (lower course) → lowest energy → deposition and building of floodplains and deltas. ## Meanders and oxbow lakes ### What is a meander? > **Definition:** A **meander** is a large bend in a river created by erosion on the outer bank and deposition on the inner bank. - On the outer bend the current is fastest so **erosion** forms a river cliff. - On the inner bend the current is slower so **deposition** forms a slip-off slope with sand and shingle. Diagram idea: outer bend (erosion) vs inner bend (deposition). ### How an oxbow lake forms (step-by-step) 1. Meanders become more pronounced as lateral erosion and deposition continue. 2. Neck of a meander narrows until during a flood the river breaks through and takes the shorter course. 3. Deposition seals off the old meander loop, forming an **oxbow lake**. 4. Over time the oxbow lake may silt up and become a **meander scar** (dried-up oxbow). Fun fact: Did you know that oxbow lakes can be useful habitats for wildlife and are often richer in biodiversity than the adjacent river channel? ## Floodplains and levees - **Floodplain**: Flat area beside a river built by repeated deposition during floods; very fertile and used for agriculture. > **Definition:** A **floodplain** is the flat land on either side of a river that becomes covered by water during floods. - **Levees**: Raised banks formed naturally by deposition during floods or built by people to reduce flooding. Practical example: Farmers plant crops on floodplains because regular flooding deposits nutrient-rich silt. ## Deltas - **Delta**: A depositional landform at a river mouth where the river’s flow slows and drops sediment faster than the sea can remove it. - Deltas often split the river into distributaries and create fertile land used for agriculture (e.g., rice cultivation in some deltas). > **Definition:** A **delta** is a fan-shaped deposit of sediment at the mouth of a river