Why does particle size in river alluvium generally decrease downstream?

Correct answer: Because heavier particles get deposited earlier

The direct answer is A: heavier particles get deposited earlier. River water can carry particles only while it has enough energy and velocity. Large, heavy, or coarse particles need more carrying power. When the river slows, loses energy, or encounters a gentler slope, these particles settle first, often in upper or nearer areas. Smaller and lighter particles can remain suspended for longer and travel farther downstream. As a result, downstream alluvium generally contains finer material, although real rivers may vary because of floods, tributaries, dams, and local conditions. Option A correctly describes the main process. Option B is absurd because water does not turn into stone; this is not a sedimentation explanation. Option C is wrong because moving away from the sea does not explain particle sorting, and rivers generally flow toward lower levels and often toward the sea. Option D is wrong because wind does not normally stop a flowing river or explain the pattern. Exam cue: high energy carries coarse load; low energy deposits it.