Final answer:
The velocity of a stream tends to increase from the head to the mouth due to factors such as river shape and size, gravity, and obstacles. However, local conditions can cause velocity variations along the river.
Step-by-step explanation:
The velocity of a stream changes from the head to the mouth due to various factors such as the shape and size of the river. In general, the velocity tends to increase as the stream moves from the head to the mouth. This is because the river widens and the depth decreases, allowing the water to flow faster. Additionally, factors like gravity, friction, and obstacles along the river can also affect the velocity.
For example, when a river starts at a higher elevation (head) and flows downhill towards the sea (mouth), gravity helps to increase the flow velocity. As the river widens and the water depth decreases, there is less resistance to flow, resulting in an increase in velocity. However, it's important to note that the velocity may vary at different points along the river depending on local conditions and obstacles.
Overall, the stream velocity tends to increase from the head to the mouth, but it can be influenced by various factors along the way.