Rivers

The hydrologic cycle is very important to the existence of rivers, indeed, to all life on Earth. Without it, every stream and watercourse would dry up. The hydrological cycle is the continuous alternation between evaporation of surface water, precipitation, and streamflow. It is a cycle in which water evaporates from the oceans into the atmosphere and then falls as rain or snow on land. The water, then, is absorbed by the land and, after some period of time, makes its way back to the oceans to begin the cycle again. Scientists have found that the total amount of water on the earth has not changed in three billion years. Therefore, this cycle is said to be constant throughout time.

The water content of the atmosphere is estimated to be no greater than 0.001% of the total volume of water on the planet. Despite its seemingly insignificant amount, atmospheric water is essential in the hydrological cycle. As water falls as rain, three things can happen. First, usually some of the rain falls directly into rivers. Second, some of it is soaked up by ground, where it is either stored as moisture for the soil or where it seeps into ground water aquifers. Third, rainfall can freeze and become either ice or snow. Interestingly, water is sometimes stored outside the hydrological cycle for years in cavities as fossil ground water in continental glaciers. The next event, evaporation, is the most critical link in the cycle of water circulation. If rain water evaporates too rapidly, rivers cannot form. For example, in hot deserts, heavy downpours sometimes occur, but the water evaporates completely in a short period of time. However, as long as the evaporation is slower than the typical amount of rainfall, viable rivers can exist.

Rivers, like precipitation and evaporation, are a vital part of the hydrological cycle. Somewhat surprisingly, of all of the forms of water in nature, watercourses-rivers and streams-make up the smallest total amount of water on Earth, about 0.0001% of the total volume. However, when combined with the precipitation falling on the ocean and the run-off from melting ice in Antarctica and Greenland, rivers replace about the same amount of water as is evaporated by the oceans. In addition to this, because they carry water away from saturated soil, they prevent marshes and bogs from forming in many low-lying areas.

Although the hydrologic cycle is a constant phenomenon, it is not always evident in the same place, year after year. If it occurred consistently in all locations, floods and droughts would not exist. Thus, each year some places on Earth experience more than average rainfall, while other places endure droughts. It is not surprising, then, that people living near rivers often endure floods at some time or other.