Here's why:

* Weathering: When rock is weathered, it breaks down into smaller pieces (sediments). This process doesn't destroy the original rock material, it just changes its form.

* Transportation: As the sediments are transported downstream by water, the total amount of material remains the same. It might be spread out over a larger area, but the total mass is conserved.

In essence, the total amount of rock material remains constant throughout the weathering and transportation process. It's simply redistributed and transformed.