Sediments eroded from mountainsides and transported in streams are an example of the conservation of

Matter  

Explanation:

The eroded materials are carried downstream and deposited as sediment at a different place.

The matter does not disappear. It is transported elsewhere.

In the picture below, you see erosion from an unvegetated stream bank.

Sediments eroded from mountainsides and transported in streams are an example of the conservation of Matter

Explanation:

Sediments are eroded deposits of soil while erosion is transportation by wind, water, and soil. Sedimentation happens when eroded rock fragments that are being transported settles onto the surface.

When mountainsides are eroded down to form small sediments by natural water bodies like streams, rivers or winds, the same forces carry it to some other places and deposit it. This action of carrying of the sediments and depositing and transporting them somewhere else aids in the conservation of matter.