Although a mere
0.03% of all freshwater in the world is found in river systems
, running water is
probably the most important geomorphic agent in shaping the face of the Earth.
Review of the hydrologic cycle
is a model of the pathways of water movement in the environment.
exists in several states (conditions) and moves from one state to another in a cyclical manner.
The main pathways of movement are:
is the water supplied to the Earth's surface as rain, snow, sleet, etc.
is the movement of water away from a surface into the atmosphere. When this
surface happens to be a plant, this process is called
When precipitation reaches the surface,
the water can either enter the soil by
over the surface.
The amount of water infiltrated into the soil depends in large part upon
the soil texture.
Remember this from the last lesson?
Animation of the five processes of the hydrologic cycle.
"Together, these five processes -
condensation, precipitation, infiltration, runoff, and evapotranspiration
make up the Hydrologic Cycle. Water vapor condenses to form clouds, which result in precipitation when the
conditions are suitable. Precipitation falls to the surface and infiltrates the soil [as
, where it
can recharge groundwater] or flows to the ocean as runoff. Surface water (e.g., lakes, streams, oceans, etc.),
evaporates, returning moisture to the atmosphere, while plants return water to the atmosphere by transpiration."
Image source: "The Hydrologic Cycle" (NASA Observatorium) 
At this time.
this online presentation on "The hydrologic cycle"
(WW2010). Pay particular attention to the module
Niagara Falls, Canada-US.