River systems: drainage basins, erosion, transport, deposition, landforms：練習問題

In the upper course of a river, which landform is characterized by a narrow, steep-sided valley often formed by intense vertical erosion and the absence of a floodplain?

The diagram shows a cross-section of a river channel and its floodplain. Why is coarser sediment deposited on the channel banks to form levees (Feature X) during a flood event?

Based on the relationship between channel efficiency and the hydraulic radius \( (R = A/P) \), which of the following scenarios would result in the highest potential for a river to perform vertical erosion in its upper course, assuming the discharge volume remains constant?

Which erosional process occurs when the load carried by a river, such as pebbles and sand, wears away the river bed and banks through a grinding action?

What is the primary reason for the formation of braids (braided channels) in a river system?

Define the term river discharge and state the mathematical formula used to calculate it, where \( Q \) is the discharge, \( A \) is the cross-sectional area of the channel, and \( v \) is the mean velocity of the water.

During which specific river event are natural levees primarily formed, and what characteristics of the floodwaters facilitate their depositional build-up along the river banks?

Examine how variations in rock resistance and geological structure influence the development of gorges and rapids in the upper course of a river, providing a more complex explanation than simply a 'steep gradient'.

The longitudinal profile of a river illustrates the change in its altitude and characteristics from its source to its mouth. Fluvial processes shift in dominance along this profile.

(a) Describe how river gradient and total discharge typically change as a river moves from its upper course to its lower course. (2 marks)

(b) Explain two physical reasons why a river is more likely to deposit its load in the lower course. (2 marks)

(c) Briefly describe one difference in the appearance of a river channel in the upper course compared to the lower course. (1 mark)

The following data represents observations of river discharge (\( Q \)) and suspended sediment concentration (\( C \)) at a specific gauging station during a single heavy rainfall event:

At 0 hours: \( Q = 2.5 \) \( m^3/s \), \( C = 8 \) mg/L
At 6 hours: \( Q = 45.0 \) \( m^3/s \), \( C = 180 \) mg/L
At 12 hours: \( Q = 15.0 \) \( m^3/s \), \( C = 35 \) mg/L

(a) Based on the data, explain the relationship between river discharge and the river's capacity to transport sediment. (2 points)

(b) Using the concepts of hydraulic action and abrasion, account for the sharp increase in sediment concentration observed at the 6-hour mark. (3 points)