2.3.1: A longitudinal profile is a diagram that shows the gradient of a river from its source to its mouth. It illustrates the changes in elevation along the river's course.

Here are the answers to the questions based on the provided sketches of longitudinal river profiles:

• 2.3.1: A longitudinal profile is a diagram that shows the gradient of a river from its source to its mouth. It illustrates the changes in elevation along the river's course.

• 2.3.2: Sketch B shows a graded longitudinal profile.

• 2.3.3: In sketch B, the river profile is smooth and concave, without any irregularities such as waterfalls or dams. This indicates that the river has reached a state of equilibrium where erosion and deposition are balanced.

• 2.3.4:

  • Waterfall: Natural
  • Dam: Human-made

• 2.3.5: The sea is indicated as the permanent base level because it is the lowest point to which a river can erode. A river cannot erode below sea level, making it the ultimate limit for downcutting.

• 2.3.6:

  • Cross-profile C (upper course): It is a narrow, deep V-shaped valley.
  • Cross-profile D (lower course): It is a wide, shallow valley with a broad flood plain.

• 2.3.7:

  • At C (upper course), the river has a steep gradient and high energy, leading to dominant vertical erosion (downcutting). This process carves out the characteristic V-shaped valley.
  • At D (lower course), the river has a gentler gradient and lower energy. Lateral erosion (sideways erosion) becomes dominant as the river meanders, eroding its banks and depositing sediment, which creates a wider, shallower valley and a flood plain.