Unit 1 - Essays - Physical Question SIMPLE ENGLISH UPDATED Flashcards
With the aid of examples, discuss the view that velocity is the most important influence on sediment deposition in a river.
Paragraph 1: Velocity – The Main Factor
Key idea: The speed of a river decides if sediment stays in the water or settles on the riverbed.
Hjulström Curve (explain simply):
Fast water (>100 cm/s) keeps sand and silt floating—no deposition.
Slow water (<30 cm/s) lets fine sediments drop to the bottom.
🔹 Example: Mississippi River Delta
Near the river mouth, the water moves slowly, allowing fine silt and clay to settle.
This builds a delta (large landform made of deposited sediment).
🔹 Changes Over Time (Temporal Variation):
During floods → Water moves fast, carrying more sediment.
During droughts → Water slows down, so more sediment settles.
Example: Wadis in the Sahara Desert (these dry rivers suddenly fill with water and deposit sediment when the flow slows down).
🔹 Why is this important?
This shows that velocity is a key factor, but it changes depending on seasons and location.
Paragraph 2: Sediment Size – A Major Influence
Key idea: Different types of sediment need different speeds to move or settle.
🔹 How Sediment Size Affects Deposition:
Big rocks (gravel/pebbles) need very fast water (>150 cm/s) to stay floating.
Small particles (clay/silt) stay in the water even at low speeds because they are light and stick together.
🔹 Example: Kali Gandaki River, Nepal
Upper course (mountains): The water moves fast, so only big rocks settle.
Lower course (flat areas): The water slows, so fine sand and silt settle.
🔹 Scale (Small vs. Large Areas)
In lab experiments, scientists see that even when water speed stays the same, different-sized sediments settle at different rates.
This shows sediment size is important, no matter how fast the river moves.
🔹 Why is this important?
It shows that velocity alone cannot explain all deposition. Sediment size also plays a huge role.
Paragraph 3: River Gradient (Slope) – How It Changes Velocity and Deposition
Key idea: If a river flows down a steep slope, it moves fast and carries more sediment. If it flows over a flat area, it moves slowly and drops more sediment.
🔹 How River Slope Affects Deposition:
Steep slope → Fast water → Less deposition
Gentle slope → Slow water → More deposition
🔹 Example: Colorado River, USA
Grand Canyon (steep gradient) → Water moves fast, carrying sediment far.
Downstream near Yuma (flat gradient) → Water slows, and sediment is deposited, forming floodplains.
🔹 Changes Over Time (Human Impact):
Dams change river slopes and stop deposition.
Example: Aswan High Dam, Egypt → The dam slows the Nile, reducing sediment deposits downstream.
🔹 Why is this important?
Shows that velocity is influenced by the slope of the river, and humans can change this by building dams.
Paragraph 4: External Conditions – Other Factors That Influence Deposition
Key idea: Sometimes, other natural and human factors affect sediment deposition more than velocity.
🔹 Factor 1: Vegetation (Plants on the Riverbank)
Trees and plants slow down water, making it easier for sediment to settle.
Example: Sundarbans Mangrove Forest, Bangladesh → Mangrove trees trap sediments and create new land.
🔹 Factor 2: Water Chemistry (Salt and Flocculation)
When saltwater meets freshwater, tiny sediments stick together and settle quickly.
Example: Yellow River, China → The high salt content makes fine sediments settle faster, no matter how fast the water moves.
🔹 Why is this important?
These examples show that deposition can happen because of other factors (plants, salt levels), not just velocity.
Conclusion
Velocity is important, but it is not the only factor affecting sediment deposition.
Sediment size, river slope, and other natural conditions also play a big role.
Deposition patterns change over time and place, meaning velocity alone cannot explain everything.
