The answer is seafloor spreading and subduction.
Hope this ain't too late (this is the PLATO answer)
Answer:
A river forms from water moving from a higher elevation to a lower elevation, all due to gravity. When rain falls on the land, it either seeps into the ground or becomes runoff, which flows downhill into rivers and lakes, on its journey towards the seas.
The geometry term is actually "scale factor", which means the ratio of any 2 corresponding lengths in 2 similar geometric figures.
