Most of a river's sediment is deposited where the river empties into lakes or the ocean. This deposition commonly builds a delta at the river's mouth. In arid rigions, many streams deposit their loads as alluvial fans at the base of steep slopes. The origin and evolution of the major rivers of the world are controlled by the tectonic and hydrologic systems. We can manipulate river systems by constructing dams and flood control projects, and such construction can result in many unforseen and unwanted side effects.
Erosion is both ubiquitous and varied.
It forms gullies and canyons. The movement of ground water is controlled largely by the porosity and permeability of the rocks through which it flows. The water table is the surface below which all pore spaces in the rocks are saturated with water. Inartesian systems, it is moved by hydrostatic pressure. The natural discharge of ground water is generally into streams, marshes, and lakes.