Final answer:
Sediment size changes from head to mouth as the river widens and the current slows down, resulting in smaller and finer sediment particles.
Step-by-step explanation:
As a river or stream flows from its source to its mouth, the sediment size changes. At the head or source of the river, the sediment size tends to be larger and coarser. This is because the river is fast-moving and has a steep gradient, allowing it to transport larger particles. As the river flows downstream towards its mouth, the width of the channel gradually widens and the current slows. This slow-moving water has more sedimentation, but the sediment size becomes smaller and finer. This is due to the decrease in gradient and the increase in volume as tributaries join the main river, allowing it to transport smaller particles.