3D cross-section diagram illustrating the different stages of a river, from its mountain source down to the delta at the mouth.

To label the parts of the river in the provided image, we need to identify each section based on its characteristics and location along the river's course. Here is the step-by-step explanation and solution:

Key Terms and Their Characteristics:

1. Source: The starting point of a river, often in highlands or mountains.
2. Upper Course: The initial part of the river where it flows rapidly through steep terrain, forming features like V-shaped valleys and waterfalls.
3. Middle Course: The part of the river where it begins to flow more smoothly, carving out wider U-shaped valleys, and developing meanders.
4. Lower Course: The final part of the river where it slows down and deposits sediments, forming floodplains and deltas.
5. Mouth: The point where the river meets a larger body of water (e.g., sea, lake).
6. Delta: A triangular area at the mouth of a river where sediment is deposited as the river slows down.
7. Tributaries: Smaller streams or rivers that flow into the main river.
8. Floodplain: The flat land alongside the river that is prone to flooding during high water levels.
9. Valley: The depression or channel through which the river flows.
10. Watershed: The area of land that drains into the river system.

Labeling the Image:

Now, let’s match these terms to the labeled points (a, b, c, d, e, f, g) in the image:

#### Point a:
- This is located at the top of the image, in the mountainous region.
- Label: Source

#### Point b:
- This is near the source, flowing through steep terrain.
- Label: Upper Course

#### Point c:
- This is further downstream, showing a wider valley with some meandering.
- Label: Middle Course

#### Point d:
- This is even further downstream, showing a wide, flat area.
- Label: Lower Course

#### Point e:
- These are smaller streams flowing into the main river.
- Label: Tributaries

#### Point f:
- This is the flat area alongside the river, likely prone to flooding.
- Label: Floodplain

#### Point g:
- This is the triangular area at the mouth of the river where it meets the sea.
- Label: Delta

Final Labels:

- a: Source
- b: Upper Course
- c: Middle Course
- d: Lower Course
- e: Tributaries
- f: Floodplain
- g: Delta

Additional Observations:

- The valley can be inferred from the shape of the river's path throughout its course.
- The watershed is not explicitly marked but would encompass the entire area draining into the river.

Final Answer:

\[
\boxed{
\text{a: Source, b: Upper Course, c: Middle Course, d: Lower Course, e: Tributaries, f: Floodplain, g: Delta}
}
\]