Rivers

A river is a physical landform that conducts (primarily fresh) water, sediment, and nutrients downslope under gravity from topographically higher land toward depressions at a lower elevation, most often into bodies of water. The total volume of water in the global hydrosphere that can be found in rivers is miniscule (0.0001%), yet rivers are a key part of the hydrologic cycle, conducting water delivered to the landscape via precipitation, springs (groundwater), or glacial meltwater to oceanic environments. The river channel, or depression of varying width and depth, contains water at least sporadically, if not constantly, depending on its geography. A river's physical character is a direct result of its local [Page 2472]climatic and geologic conditions, which may vary with distance along its course. As both erosional and depositional agents, rivers are highly dynamic and adjust their forms over time as a response to the active processes that influence them. Rivers are recognized as one of the most influential physical forces on Earth, each year reworking about 19 trillion tons of suspended and dissolved solids. Rivers are responsible for shaping a majority of continental and coastal landscape features and provide unique ecosystem environments that foster the development of biological diversity rivaling that of both rain forests and oceans.

Most rivers across the globe have been diverted, dammed, or drained as a result of human activity. Rivers are also a hazard to the global population, with floods destroying human infrastructure and taking thousands of lives each year. With the effects of global climate change beginning to unfold, rivers are responding in concert. Flow regimes are changing to reflect both human influences and climatic shifts. Changes in flow cause geomorphic responses such as aggradation or degradation of the channel bed, reduced interactions between the river and its floodplain, and altered floral and faunal cues that affect the health of riparian ecosystems. This entry outlines the fundamentals of river science, including river geography, the impact of geology and climate on rivers, drainage networks within watersheds, and river processes and the resulting landforms (fluvial geomorphology). It then examines both the impacts of rivers on humans and the impacts of humans on rivers.