River

A river is a natural waterway that transits water through a setting from higher to lower elevations. A river may have its basis in a spring, lake, from damp, boggy landscapes where the soil is waterlogged, from glacial melt, or from surface runoff of precipitation. Almost each and every one river is joined by other rivers and streams termed tributaries the highest of which are known as headwaters. Water may also begin from groundwater sources. Throughout the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a important contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain. For many rivers in large valleys, this unseen component of flow may greatly go above the visible flow.

From their source, all rivers flow downhill, typically terminating in the sea or in a lake, through a confluence. In arid areas rivers sometimes end by losing water to desertion. River water may also infiltrate into the soil or pervious rock, where it becomes groundwater. Excessive abstraction of water for use in commerce, irrigation, etc., can also cause a river to dry before success its natural terminus.

Human interrelationship

Although humans currently comprise only about one-half of one percent of the total living biomass on Earth, the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what we regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is presently diminishing at an increasingly rapid pace, or, according to some, has already disappeared.

The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human civilization remains closely linked to changes in the environment. There exists a highly complex feedback-loop between the use of advanced technology and changes to the environment that are only slowly becoming understood. Manmade threats to the Earth's natural environment include pollution, deforestation, and disasters such as oil spills. Humans have contributed to the extinction of many plants and animals.