The hydrologic cycle is the constant circulation of the earth's water through precipitation, evaporation, and transpiration (the release of water into the atmosphere by plants). It is the continuous exchange of water between the atmosphere, land, and ocean. Running water is the most active landscape‐transforming agent on the earth's surface. Waterways erode, transport, and deposit rock and sediment to produce landforms such as canyons, valleys, deltas, alluvial fans, and floodplains.
Streams (any flow of water within a natural channel regardless of size) are the most important kinds of channel flow that affect landscapes. A stream's headwaters are where the stream originates, usually in the higher elevations of mountainous terrain. The stream flows downhill and across lower elevations to its terminus, where it enters another stream, lake, or ocean. This terminus is called the mouth of the stream.
The stream is often flanked on both sides by a flat floodplain that is created when periodic flooding deposits mud and silt over extensive, low‐lying areas. Flooding results when a stream's flow is increased and exceeds the capacity of the stream channel. Water sometimes moves overland during heavy storms as a sheetwash, a thin layer of unchanneled water. Sheetwashes typically occur in arid climates or where the ground is saturated and cannot accept any more water. Eventually sheetwash flow forms small channels called rills; rills join to form larger temporary streams.
About 80 percent of all rainfall is soaked into the ground and becomes groundwater or is taken up by plants and returned to the atmosphere through transpiration. Very heavy amounts of rainfall over short periods create flash floods. The flooding is a result of the ground becoming saturated and not being able to absorb any more water or of the water just coming too fast to be entirely soaked into the ground. Flash floods are common in the southwestern United States where the terrain is dry, rocky, and sparsely vegetated.
A drainage basin is the land area that contributes water to a stream and its tributaries (the smaller streams that flow into it). The size of the drainage basin depends on the size of the stream—large river systems have drainage basins that cover thousands of square miles. A small tributary to the Mississippi River, on the other hand, might have a drainage basin of only a few square miles.
The line of highest elevation that separates one drainage basin from another is called a drainage divide. The Continental Divide is a north‐south line in the western United States and Canada that separates those streams that flow into the Pacific Ocean from those that empty into the Atlantic Ocean or the Gulf of Mexico.