زمین شناسی رودخانه ها-River
River
A natural, fresh-water surface stream that
has considerable volume compared with its smaller tributaries. The tributaries
are known as brooks, creeks, branches, or forks. Rivers are usually the main
stems and larger tributaries of the drainage systems that convey surface runoff
from the land. Rivers flow from headwater areas of small tributaries to their
mouths, where they may discharge into the ocean, a major lake, or a desert
basin.
Rivers flowing to the ocean drain about 68%
of the Earth's land surface. The remainder of the land either is covered by ice
or drains to closed basins (common in desert regions). Regions draining to the
sea are termed exoreic, while those draining to interior closed basins are
endoreic. Areic regions are those which lack surface streams because of low
rainfall or lithoogic conditions.
Sixteen of the largest rivers account
for nearly half of the total world river flow of water. The
River
floods
River discharge varies over a broad range,
depending on many climatic and geologic factors. The low flows of the river
influence water supply and navigation. The high flows are a concern as threats
to life and property. However, floods are also beneficial. Indeed, the ancient
Egyptian civilization was dependent upon the
Floods are a natural consequence of the
spectrum of discharges exhibited by a river. Rivers in humid temperate regions
often exhibit less variable flood behavior than rivers in semiarid regions of
rugged terrain. Floods in the humid regions tend to occur on average once each
year. Rarer, larger floods are often no more than one or two times the magnitude
of more common annual floods. The semiarid floods, however, are usually very
small for common events, such as the average annual flood. However, rare,
high-magnitude floods may be catastrophic.
The greatest floods in the geologic record
occurred during the Pleistocene about 13,000 years ago in the northwestern
The
Fig. 1 Typical slope angles and associated
drainage patterns forming on relatively homogenous planar beds: (a) surface
gradient 1%, dendritic; (b) surface gradient 3%, subparallel; (c) surface
gradient 5%, parallel; and (d) surface gradient 5%, pinnate. Arrows indicate
direction of flow. (After L. F. Phillips and S. A. Schumm, Effects of regional
slope on drainage networks. Geology, 15:813–816,
1987)
Floods are but one attribute of rivers that
affect human society. Means of counteracting the vagaries of river flow have
concerned engineers for centuries. In modern times many of the world's rivers
are managed to conserve the natural flow for release at times required by human
activity, to confine flood flows to the channel and to planned areas of
floodwater storage, and to maintain water quality at optimum levels. See also:
Floodplain; River engineering
Fig. 2 Diagrams of broad categories of
structurally and topographically controlled drainage pattern forms. (a) Radial.
(b) Centripetal. (c) Deranged. (d) Dendritic. (e) Trellis. (f) Rectangular. (g)
Annular. The dendritic drainage (d) shows the pattern that evolves at the
deranged pattern (c) location after sufficient time has passed for erosion of
the divides between ponds and formation of a connected channel network. Broken
outlines in d show old pond locations in the deranged pattern (c). (After A. D.
Howard, Drainage analysis in geologic interpretation: A summation, Amer. Ass.
Petrol. Geol. Bull., 51:2246–2259, 1967)
Geologic
history
Some rivers possess a long heritage related
to their location along relatively stable continental cratons or their
correspondence to structural lows, such as rift valleys. More commonly, rivers
have been disrupted through geologic time by the Earth's active tectonic and
erosional processes. The most recent disruptions were caused by the glaciations
of the Pleistocene. Many rivers, like the
In tropical regions the effects of
glaciation were indirect. During full-glacial episodes of the Pleistocene,
tropical areas of the Amazon and
Victor R. Baker
Stream pattern
analysis
The patterns of intersecting stream channels
provide valuable information about the geology, topography, climate, and
hydrology of the region, and about some of the ways in which humans have altered
the land surface. The alignment of the streams (for example, branched,
rectangular, or parallel) is controlled by bedrock type, topography, and the
locations of faults, folds, and joint patterns. The density of the network (that
is, how closely spaced the channels are) is controlled by the climate,
vegetation, age, permeability, and slope of the surface. Stream patterns are
thus a product of the physical geography of a region and provide valuable clues
to that geography.
Geologists and geographers have long used
stream patterns for landscape analysis. An interesting application of pattern
analysis is in extraterrestrial studies; scientists have used stream patterns to
identify surficial processes on planets where on-site collection of data is
impossible.
Form, topography, and
geology
The general form of stream patterns reflects
the underlying topography and geology of a region. Dendritic, or branchlike,
drainage (Fig. 1a) is the most commonly occurring pattern; it indicates a lack
of strong structural controls (that is, faults, joints, or folds) or complex
topography. Dendritic patterns thus commonly occur on relatively horizontal
sediments or beveled plains of any rock type. Large drainage networks such as
the
If there are no structural controls or
significant variations in rock type and the surface gradient is increased to 3%
(that is, a rise of 3 m in 100 m of horizontal distance), the dendritic pattern
typically assumes a subparallel pattern (Fig. 1b). At slopes of 5% and higher on
planar surfaces, the subparallel pattern usually goes through a transition to a
parallel pattern (Fig. 1c) on most rock surfaces, or a pinnate pattern on silts
and clays (Fig. 1d).
The shape of the topography as well as the
gradient also plays a role in controlling the form of drainage patterns.
Volcanoes create radial drainage patterns (Fig. 2a). Topographic lows, such as
occur in the closed basins of Nevada, create the centripetal pattern shown in
Fig. 2b. The hummocky terrain of permafrost areas, regions of limestone
solution, and recently deposited materials from glaciers, volcanic explosions,
and slides creates a deranged pattern, which consists of many ponds and
unconnected tributaries (Fig. 2c). Deranged patterns may evolve to become
dendritic as streams erode the divides between ponds and eventually form an
interconnected network of tributaries (Fig. 2d).
Structural controls can also override the
natural tendency for drainages to assume a dendritic form, or a parallel form on
slopes with gradients greater than 5%. For example, parallel drainages can occur
on slopes of less than 5% in areas such as the south shore of Lake Superior,
where ancient glacial grooves restrict flow to parallel channels. Trellis
patterns (Fig. 2e) generally indicate parallel fractures or parallel dipping
rock beds (for example, old beaches that form resistant sandstone), which create
long reaches of parallel streams with smaller tributaries to either side. The
rectangular pattern shown in Fig. 2f reflects an underlying structure or joints
or faults at right angles. Annular patterns occur in areas where the rocks have
been bent to form domes and basins (Fig. 2g).
Pattern
density
The various stream patterns can occur at
different densities; in other words, channels may range from being closely to
widely spaced. Density can be controlled by climate, vegetation, bedrock and
soil permeability and strength, slope and shape of surface, and the age of a
surface and stream. In turn, human disruption of the landscape can affect all
these factors at a variety of scales, from a garbage dump to a river basin.
The evaluation of controls on channel
density is important because channel density provides key clues to the physical
geography of drainage basins. Furthermore, erosion and flash floods are
controlled by drainage density, because more channels mean more runoff and
excavation of sediment. Drainage densities therefore have been used by
hydrologists to estimate mean annual flows, annual flood sizes, low flows due to
ground-water discharge into streams, and sediment yields. Finally, by
understanding the controls on drainage density, engineers can design mine
tailing piles, landfills, and other artificial landscapes that help maintain
drainage densities that reduce erosion and flash floods. See also: Ground-water
hydrology; Hydrology
Fig. 3 Drainage pattern evolution from (a)
Initiation as a deranged pattern through (b)–(d) maximum channel development in
a dendritic pattern to (e, f) erosional lowering of the basin and loss of
channels from the dendritic pattern. (After W. S. Glock, The development of
drainage systems: A synoptic view, Geog. Rev., 21:475–482,
1931)
Climate and
vegetation
Drainage density is most strongly controlled
by the intensity of precipitation (the amount of precipitation falling per unit
time) at regional scales; other factors generally play a role at local scales.
The widest variation in drainage densities occurs in semiarid climates, where
local factors often override climatic controls.
Despite the impact of local factors,
climate-driven factors generally cause densities to be greatest in semiarid
areas such as the western Great Plains the United States, where occasional,
intense thunderstorm activity coupled with sparse vegetation cover promotes
erosion and the formation of many channels. Densities typically decrease in more
arid climates because of the absence of rainfall and runoff. Densities also
typically decrease in more humid climates, probably because of (1) increased
vegetation, which holds soil in place and reduces erosion, and (2) increased
soil cover, which acts in a spongelike manner to absorb and store water, thus
reducing the surface runoff that erodes channels.
Rock type and soil
permeability
Surfaces possessing high permeability, such
as sands or loose volcanic debris, generally produce low stream densities,
because precipitation can soak into the surface rather than generating runoff
and erosion. Similarly, surfaces such as clays or silts that have low
permeability produce high drainage densities, because water cannot sink into the
surface, and so runs off and erodes channels. Badlands, which are barren,
heavily eroded and dissected landscapes, almost invariably consist of shales or
silts that cannot absorb precipitation. On occasion, however, this relationship
can be confusing. Granites, for example, are impermeable to water, but are also
sufficiently strong to resist extensive erosion and channel formation. In
addition, the soils that form from granites are very porous and can absorb
significant quantities of rainfall.
Age of the
surface
The older the surface, the more time has
been available for erosion and network formation. Thus, if there are identical
surfaces that have experienced identical climates, the surface will progress
through stages of little erosion, tributary development and high drainage
density, and finally reduction in density as erosion reduces lower portions of
the basin to a relatively flat plane (Fig. 3). This process may be repeated if
the basin is uplifted or the base level (for example, sea level or the level of
a reservoir) drops, both events starting a new cycle of erosion.
At the scale of small areas (for example,
one hillside) in soft bedrock or soils, the drainage density may also change on
a seasonal basis. During the cold or dry season, channels may be destroyed as
frost action loosens dry sediment that fills in the channel, while during the
wet season, channels are reestablished by runoff.
Slope and shape of the
surface
If all other factors are equal, drainage
densities increase with gradient up to a material-dependent threshold value.
Densities increase because steeper slopes have less soil cover to absorb rain,
and runoff flows more quickly and has more erosive power on steeper gradients.
At lower gradients, upwardly convex slopes tend to produce slightly lower
drainage densities than planar or concave slopes, but this difference in
densities disappears as gradients increase to the threshold value.
W. Andrew Marcus
Bibliography
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W. L. Graf, Fluvial Processes in Dryland Rivers, 1988
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L. Leopold, M. G. Wolman, and J. P. Miller, Fluvial Processes in Geomorphology, 1995
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L. F. Phillips and S. A. Schumm, Effects of regional slope on drainage networks, Geology, 15:813–816, 1987
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S. A. Schumm, The Fluvial System, 1977
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S. A. Schumm, River Morphology, 1982
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S. A. Schumm, M. P. Mosley, and W. E. Weaver, Experimental Fluvial Geomorphology, 1987
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M. M. Smart et al. (eds.), Ecological Perspectives of the
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B. A. Whitton (ed.), Ecology of European Rivers, 1989
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alifazeli=egeology.blogfa.com
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