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The Hydrologic Cycle
The hydrologic cycle is the name given to the process by which earth's supply of water is transported from bodies of water, into the air and back to the ground again. In its simplest form, the first stage of the hydrologic cycle features water vapor carried into the atmosphere by air current traveling upward. This primarily comes from ground-level water evaporating from the sun's warmth. Next, the water vapor in the atmosphere cools and condenses to form clouds of water droplets. When a great deal of water has collected in clouds, it falls back to the surface of the earth through precipitation. This water then travels back into bodies of water, such as rivers, ready for the cycle to begin again.
Precipitation
Once the tiny droplets of water that make up clouds have gained enough size and weight to fall to the ground, the water travels back down to the surface of the earth in the form of precipitation. Precipitation can take on a variety of forms, including rain, hail and snow. Some of this precipitation may fall directly into rivers, but a great deal will end up in rivers after traveling a longer route, over or through the ground. Some will fall on land as snow and not travel any further for months.
Surface Runoff
After water in the atmosphere has fallen to the ground as precipitation, some of it enters the river as surface runoff, traveling there along downward-sloping ground, particularly in valleys. Not all surface runoff makes it to the river, though. While water runs straight over impermeable surfaces, such as roads, other areas, like earthy fields, will absorb it. Surface runoff can also meet other obstacles, such as becoming drinking water for animals.
Groundwater
Some water that travels over the ground, after precipitation, is absorbed by the ground. Once absorbed, this water filters downward until it reaches the water table. Below the level of the water table, the ground is completely saturated with water. Groundwater aquifers lie below the water table. These vast amounts of water that are stored in these areas often travel back into the river through inlets in the sides of the river bank.
Snow Melt
In colder regions, snow melt is a major influence on the level of water in rivers. In winter, precipitation is collected in mountainous areas and stored as snow and ice. When temperatures become warm enough to melt this precipitation in spring, this melting ice travels over or through the ground into nearby rivers, accounting for a large proportion of the total water held by a river.