Which principle is exemplified by studying a modern river to learn about how ancient rock layers form

There are two different answers, 1, water wears away the rock slowly over time, allowing you to see the levels odsilt, sediment, ash, Ect, over time, it's a time machine.
a river is also a natural rock tumbler, it breaks, polishes and carries minerals down stream.

Magma is outside the volcano  and lava is inside the volcano.