A plethora of geologic, climatic and biological factors affect drainage basins.
A drainage basin describes an area flushed by surface water draining to a single outlet, whether a larger stream, an ocean, a lake or some other land form. Drainage divides, which can be dramatic serrate ridges or subtle swells on a gentle plain, mark the boundaries of these watersheds. Beyond their geomorphic significance, drainage basins are immensely important from an ecological standpoint.
Geologic Character
Conical volcanoes tend to support radial drainage networks.
The type and layout of rock in a drainage basin influences its nature. Certain kinds of rock are more resistant to erosion by moving water than others. The configuration of drainages depends on the underlying geology and the "age" of the stream system involved --- the amount of time it has been functioning under the same geological and climatic regimes. Streams dropping off a conical volcano often assume a radial drainage pattern, resembling spokes of a bicycle wheel; while in gentler terrain the system may be dendritic, like a network of veins.
Patterns of Runoff
A river at high flow abrades the landscape with large rocks and boulders.
While a tropical basin might experience relatively stable patterns of precipitation and runoff throughout the year, other parts of the world have more erratic or seasonal climates; adesert basin may be dry much of the year, laced with waterless drainages that fill only after occasional deluges. Typically the most extensive erosion and sculpting of canyons, alluvial fans and other landforms occurs during sporadic high-flow events, such as sparked by rapid snowmelt or heavy rains. At these instances, streams run powerfully, gaining the strength to transport large rocks and boulders. Knocking together and grinding against the stream bed and banks, this debris scours the stream channel.
Ecology
A beaver's industrious work can have big impacts on streams and rivers.
Vegetation patterns also impact drainage basins. In an arid basin such a tract of badlands, slopes are often lightly vegetated or entirely barren, making them highly susceptible to severe erosion during periods of high runoff. A forested drainage has a different regime: Trees and other vegetation more firmly secure the soil and significantly slow rates of runoff by absorbing water and diverting flow. They also buffer the land somewhat against the micro-erosion of pelting raindrops. An urbanized watershed will experience high rates of runoff if its impervious surfaces of asphalt and concrete are not contrasted with greenspace.
While less influential than vegetation, animals can affect drainage basins. Beavers are notable examples: These aquatic mammals fell trees and construct lodges out of timber and debris, damming streams and creating backwater swamps, ponds or small lakes.
Human Modifications
Human impoundments and diversions change drainage regimes.
The alterations of drainage regimes by humans can be important. Like beavers, humans dam watercourses --- only on the scale of huge, main-stem rivers to generate hydroelectric power and control patterns of flow. Such impoundments change a river's character, promoting sedimentation and other effects. Humans frequently diminish floodplain ecosystems --- riverine swamps, canebrakes and sloughs --- either through outright draining or starving them off by attempting to suppress the occasional flooding that builds and sustains them.
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