The question asks to identify what a "knickpoint" in a river's long profile represents. This is a geomorphological term related to river erosion and landscape evolution. Understanding the characteristics and causes of knickpoints is key to answering this question.
A) A break in the smooth concave long profile — marked by a waterfall or rapid — caused by rejuvenation, rock resistance change, or river capture. This option accurately and comprehensively defines a knickpoint, including its appearance (waterfall/rapid) and common causes (rejuvenation, rock resistance, river capture). It correctly identifies a knickpoint as a disruption in the typical graded profile of a river.
The question asks for the definition of headward erosion. We need to understand the process by which a river or stream extends its valley. Headward erosion specifically refers to the erosion that occurs at the source of a stream, causing it to lengthen upstream.
C) A stream extends its valley backward (upstream) toward the watershed by erosion at its source
This option accurately describes headward erosion. The term "backward (upstream)" clearly indicates the direction of erosion, and "at its source" specifies the location where this erosion takes place, leading to the lengthening of the stream's valley.
The question asks about the dominant type of erosion in the middle course of a river. To answer this, we need to understand the characteristics of a river's middle course and the types of erosion associated with it.
Correct Option: B) Lateral (sideways) erosion — the river swings from side to side widening its valley
The question asks about the characteristics of the outer bank of a meander. A meander is a bend in a sinuous river or stream. Understanding the dynamics of water flow in a curved channel is key to answering this question.
B) Active erosion forming a steep river cliff (cut bank) — water velocity is highest here due to centrifugal force. This accurately describes the processes occurring at the outer bank of a meander. The centrifugal force pushes the water with higher momentum towards the outer curve, increasing its erosive power and leading to the formation of a steep bank.
The question asks about the location where point bars (also known as slip-off slopes) develop in a river meander. This is a fundamental concept in fluvial geomorphology, relating to the processes of erosion and deposition in meandering rivers.
C) Inner convex bank — water velocity is lowest here causing deposition. This option correctly identifies the location (inner convex bank) and the geomorphic process (lowest water velocity leading to deposition) responsible for the formation of point bars.
The question asks about the formation sequence of oxbow lakes. This is a common geomorphological feature associated with meandering rivers. We need to identify the option that correctly describes this process.
Option A accurately describes this sequence of events.
A) Meander→neck narrows by lateral erosion→river cuts through the neck during flood→old meander loop sealed by deposition→oxbow lake
This option correctly outlines the sequential process of oxbow lake formation, starting from a meandering river and ending with the isolated lake.