The water cycle. The saturated zone is the part of the subsurface in which all the spaces between rock and soil grains are filled with groundwater. The water table is the boundary between the saturated and unsaturated zones. A river is a natural stream of water of fairly large size that flows over the land surface in a distinct course or channel. Water flows because of the pull of gravity, and thus rivers start in areas of high elevation, and flow downhill toward lower and lower elevations until eventually reaching the lowest elevation possible, the ocean.
Rivers occupy the lowest areas of a landscape, such as the low parts of valleys.
The area of land that captures precipitation and directs it into a river or lake is the watershed of that river or lake. Because small streams drain into larger rivers, most watersheds are part of a larger watershed. Watersheds for different lakes and rivers are separated from one another by drainage divides, which are ridges of higher elevation. A famous example of a drainage divide is the Continental Divide, which separates land draining to rivers that feed into the Pacific Ocean from land draining to rivers that flow to the Gulf of Mexico. Most drainage divides are not as dramatic, but separate different watersheds nonetheless.