A HIMALAYAN RIVER CARVES A TWIN PATHWAY THROUGH A SHELF OF SOLID ROCK ON ITS WAY TO THE BAY OF BENGAL
ON A HOT SEPTEMBER DAY in 1540, Garcia L6pez de CArdenas, scouting
what is now northern Arizona for the great Spanish explorer Coronado,
crossed a level plateau that seemed to stretch unbroken for mile upon
mile. Suddenly, with no warning, he found himself at the brink of an
abyss-an astounding gash in the surface of the earth, in places a mile
in depth, but so abrupt that it could not be seen by a man standing
more than a few feet from its edge. Cardenas was the first European to
look into the Grand Canyon. But its desolate grandeur-the great steep
cliffs gaudily banded like a many-colored layer cake-has since dazzled
thousands upon thousands of observers. It is perhaps the most spectac-
ular sight among the world's landscapes. At the very bottom, hidden
from the rim, flows the Colorado River, as muddy as is implied by its
name, which is the Spanish word for "red." It was the river that made
the gorge, slowly grinding, a fraction of an inch at a time, through the
red and buff sandstone, green shale and gray and red limestone, until
at last it had gouged its present channel into the tough, billion-year-old
black gneiss at the bottom.
Water shaped not only this grandiose landscape but, in one way or
another, all the world's landscapes. The gross features of the earth-
the ocean basins, continental masses, mountain ranges-were estab-
lished by movements of the earth's crust. But the fine details-the con-
tours of hill and valley, the spreads of plain-are entirely owing to the
action of water, with some assistance from wind. Everywhere on this
globe is evident the work of water.
All the physical and chemical properties of water aided in this task of
transformation. Solid water in the great ice sheets planed and furrowed
much of North America and Europe into their characteristic rolling hills
dotted with lakes. Glaciers honed the sharp faces of high mountains
such as the Matterhorn. The expansion of ice, freezing in crevices and
pores, shattered big rocks into little ones. Liquid water dissolved eerie
caverns and freakish valleys out of the solid earth. Water-lubricated
particles of soil slid slowly down over one another in the "creep" that
helps round the tops of hills. Water in the form of ocean waves broke off
shorelines, created and destroyed beaches, crushed rock into sand.
But it is river water, tumbling turbulently here, flowing gently there,
that has been the foremost sculptor of landscape. It has achieved this
role chiefly by pulling weathered solids from one place to deposit them
in another. In the process it has created upstream valleys that, depend-
ing on the surrounding earth, are narrow or broad, straight or winding.
Downstream the silt suspended in the river acts as an abrasive, grinding
watercourses into gorges. Where the silt settles, the river spreads its bur-
den more widely, generating broad valleys, floodplains and, at the river
mouth, often a delta. The majestic scale of this movement of earth is
illustrated by the delta of the Tigris and Euphrates, which in 4,50tf
years has shifted the shore of the Persian Gulf about 180 miles.
All these processes continue now as they have since the hydrologic
cycle began. Rainfall and glacier ice, flowing inexorably toward the sea,
GEOLOGIC "CREEP" occurs when boulders
are moved downhill by natural processes, such
as the alternate freezing and thawing of ground
(above). The original position of the boulder
above is shown in blue. When water in the
topsoil freezes it expands, raising the surface
and everything on it, including the boulder
(dotted outline). At the spring thaw the ground
melts, lowering the rock. But instead of
returning to the same spot, the boulder
slides downhill to a new position (solid outline).
drag along the land, bit by bit. The high points are steadily leveled
down, old hollows filled in, new ones created. If the leveling-down were
to continue without interruption, water would eventually smooth the
crust of the earth into a round ball entirely covered by ocean. Interrup-
tions, however, always come. Periodically the crust readjusts, raising
new mountains and dropping new lowlands. Less often, the climate
changes. Either event causes a drastic shift in the distribution of the
earth's water and starts the cycle of landscape-sculpturing anew.
A chamois hunter's theory
The process begins with ice. The idea that ice helps create the land-
scape is only about 150 years old. One of the first men to suggest it was
an untutored guide in the canton of Valais, Switzerland, J. P. Perraudin.
He is described merely as "a skilled hunter of chamois, and an amateur
in these types of observations," but he studied the spectacular scenery
about him with a discerning eye. He noted, high up on the sides of
the local valleys, distinctive piles of boulders-rounded, of all sizes and
quite unlike the adjacent mountain rocks. These, he reasoned, must
have been brought there and left by glaciers that had retreated.
Perraudin's idea won some converts among the Swiss geologists, but
encountered even more skeptics, the most important being the brilliant
young naturalist at the University in Neuchatel, Jean Louis Rodolphe
Agassiz. Setting out to prove that glaciers could not have moved as far
as Perraudin suggested, Agassiz measured over a period of time the
location of marker stakes in a mountain glacier-only to discover that
the guide was right. This discovery prompted Agassiz to fit together the
observations and ideas of many other men into a unified theory; the
result was his great concept of climatic change and the ice age, which
he popularized first in Europe and later, during a long and colorful ca-
reer at Harvard, in the United States.
Geologists are now convinced that several times during the history
of the earth its generally mild and equable climate has been interrupted
by glacial epochs. During these times immense sheets of ice repeatedly
advanced and retreated. Even a small decrease in the average summer
temperature-a drop of as little as 5 or 10" F.-would prevent winter
snow from melting over broad areas of the earth, there to mass into
thick ice sheets. The evidence can be seen in the ground: gravel made
up of rocks that otherwise occur only in far-distant places, as well as
deposits of unstratified rocks, sand and clay, which must have been
pushed into their present positions not by water but by ice.
What causes the climate to change is a matter of argument. The heat
received from the sun could diminish for any of several reasons-a wob-
ble in the axis of the earth, thinning of the atmosphere's heat-trapping
carbon dioxide, or a variation in the sun's output of energy. Once the ice
begins to accumulate, of course, its very presence serves to perpetuate
Precisely when the Pleistocene ice epoch began we do not know as yet.
Some evidence suggests that the first ice sheets began to move 1.5 mil-
lion years ago. Other evidence indicates that it began at an even earlier
time. Whenever the ice age began, it was marked by recurrent advances
and retreats of the glaciers. At least four major stages-and possibly
more-have occurred. These times of glaciers were separated by longer
periods of mild climate, with weather probably milder than that which
we enjoy today.
The most recent ice age reached its climax some 50,000 years ago. It
covered much of northern Europe, all of Canada, and much of the north-
ern half of the United States with packs often a mile and more thick. In
areas now lush and green, the landscape then was a bleak abstraction
of white on white, flat and colorless save where some mountain peaks
rose up as barren islands above the frozen waste. The world map, too,
was far different from that of today. Millions of cubic miles of formerly
liquid water, withdrawn from the oceans, was frozen into stupendous
ice masses, lowering the general sea level between 300 and 400 feet. This
ice did not leave the United States until about 10,000 years ago.
The ice sheets originated in the north and shaped the face of the
land by flowing over it, like a solid flood. When they retreated, they did
not flow back, but simply melted back from their southern edges, gradu-
ally dropping material that they had picked up or swept ahead of them.
In Illinois, for example, are found chunks of copper that were torn from
the Upper Peninsula of Michigan, 600 miles away.
Glaciers flow downhill, like rivers. They also flow outward, just as
honey dripping on the center of a table flows outward from the pressure
of the "pile" at the center. Exactly how this movement is accomplished,
however, is still debated. To some extent glaciers slide bodily across the
ground, but they also ooze forward like toothpaste squeezed out of a
tube, perhaps as layers of the flat ice crystals slide over one another.
The power of a glacier
This ponderous tide picks up rock and sand in several ways as it
grinds along. Some is embedded within the ice sheet by an alternation
of melting and freezing. The pressures at the bottom of a great mass of
ice; particularly when it is being shoved against an unyielding piece of
rock, can be enormous-enough to break down the hydrogen bonds in
individual ice crystals and turn them into liquid water. This water may
trickle down around the rock and, as the pressure eases, refreeze. This
alternate thawing and freezing due to changes in pressure can gradually
work rocks of considerable size up into the ice sheet, which takes them
along with it on its slow travels. Melted water can also seep into cracks
in bedrock, breaking loose and plucking out large chunks as the water
freezes and expands. One such boulder, left behind 12,000 years ago near
Conway, New Hampshire, is 90 feet across and weighs close to 10,000
tons. Most glaciers move their debris, however, by simply bulldozing it
ahead of them, or by dragging it along underneath.
A slowly moving, rock-loaded glacier acts like a rasp on the earth. Its
LANDSLIDES AND AVALANCHES are
examples of erosion in its most spectacular
form. A rockfall (top), the most sudden
of landslides, occurs when a process of
weathering fractures the edge of a cliff, causing
it to drop to the ground below. A block glide
happens when a large chunk of earth becomes
waterlogged at its base and falls of its own
weight. A debris avalanche occurs when loose
earth on a steep slope becomes wet and slides
to the bottom. A debris flow is a river of mud
that runs down a valley, usually after a storm.
rough bottom and the loose abrasive material that it drags and pushes
shape the sides of mountains and cut U-shaped valleys. If several gla-
ciers work on different sides of a mountain, they sharpen its crest into
a point, or "horn." Two glaciers extending from opposite sides of a ridge
may join at their heads to cut a narrow, sharp-edged pass. A glacier
flowing down a valley plucks rocks most easily from shattered zones
and may dig a succession of basins which later fill with water, forming a
string of lakes. In central New York State, the Finger Lakes were scooped
out as the last continental ice sheet piled up against the barrier of the
Appalachian Plateau to the south. Where bedrock is exposed, the glacier
may polish the surface smooth-and occasionally, like an inept sculptor,
leave scratches. There are grooves of this kind two feet deep and three
feet wide in the bedrock at Kelleys Island, in Lake Erie north of San-
dusky, Ohio, and others 50 feet deep, 150 feet wide and a mile long in
the Mackenzie Valley west of Great Bear Lake, Canada.
In the wake of melting ice
The residue left behind when glaciers melt creates other features of
the landscape. Some broad plains in the Northwest are composed of gla-
cial silt and sand that settled out smoothly as the melting water of the
glacier spread over level areas. The boulders which litter New England
and thus make farming so laborious are "drift" that a glacier pushed and
rode over. Long low ridges may be moraines--debris that was either left
behind in the sides of valleys along the edges of long-departed glaciers,
or gradually assembled at the point of a glacier's farthest advance. As it
melts, it leaves its accumulated pile of sand, gravel and boulders. Some
moraines are many hundreds of feet high and contain millions of tons
The isolated lakes often found in Canada were also formed by glaciers.
These are known as "kettles," and are depressions left by large blocks of
ice that remained frozen long after the rest of the surrounding glacier
Most mountain glaciers are now in retreat the world over. Whether
they will continue to shrink or will grow again to freeze the earth in an-
other ice age is an open question. But meanwhile water in liquid form
continues to mold the land.
It is the water's energy, its ability to slam and break off things and
then to move them around, that does the job. The things that water
moves-the sand and rocks-are abrasives that wear away at the surface
of the earth.
The effect of this abrasive-in-liquid is perhaps most obvious at the
shores of the land, where coastlines are trimmed by waves and currents.
The cutting power of their loads of sand and rock can actually be seen
A RIVER'S HISTORY unfolds in a
well-defined pattern from the moment of its
origin upon a newly upthrust land surface.
In its early stage (left), it tends to cut a deep
and narrow valley, where the flow is small,
but on a flat slope downstream where the flow
is large, deepening of the channel proceeds
more slowly: instead the channel widens
the valley. In maturity the river continues to
widen its valley, and develops a flat floor, I
the river floodplain. On wide floodplains the
river usually develops a meandering pattern.
and felt and even heard in action. The surging seas tear down coastlines,
cutting back headlands and moving tons of sand. They may wash away
a beach completely, only to build a new one somewhere else. This chip-
ping at the contours of the continents is immediate and noticeable, work-
ing changes that can be appreciated within the span of a human lifetime.
Entire beaches occasionally disappear in a single great storm. New Eng-
land's famous sandbar peninsula, Cape Cod, a relatively young offspring
of the last ice age, has lost a two-mile-wide strip of land to the waves. At
the present rate of erosion the outer Cape will be gone entirely in 4,000
to 5,000 years.
Cliffs on the south shore of Nantucket Island, off Cape Cod, where the
waves hurl rocks and rock fragments at the land, lose as much as six feet
a year as the abrasive stream undercuts them and causes huge sections
to fall into the sea. The grinding and smashing of rock fragments in such
a surf fills the air with a distinctive rumble that is easily recognized and,
once heard, never forgotten.
Waves often carve a rocky coast into weird forms. They may excavate
horizontally under a cliff, then curve upward to hollow out a cave. The
enclosed space sometimes directs the force of the inrushing water up-
ward to dig at the roof until a hole is torn through. The sea then spouts
through the hole like a geyser. A narrow promontory sticking into the
ocean may be eroded until a cave breaks through the other side, creating
a natural bridge. When, after years of wear, the arch of the bridge falls,
the outermost column of rock stands alone, surrounded by water, in the
unusual formation called a "stack."
Water, subterranean sculptor
Even more spectacular effects are created on land, by water that acts
not as an immense pounding weight but as an inconspicuous yet steady
solvent. Perhaps the most dramatic of these are the great caverns which
seepage dissolves out of underground limestone. This material, paradox-
ically, is not soluble in pure water. Rainfall, however, contains some car-
bon dioxide, absorbed first while dropping through the atmosphere and
later while seeping among plant roots. As a result, groundwater becomes
dilute carbonic acid and enters into a chemical reaction with limestone,
which is mostly calcium carbonate. This reaction converts the insoluble
calcium carbonate into calcium bicarbonate, which is quite soluble in
water. In submerged crevices, the mild acid eats away the limestone and
enlarges the cracks into large rooms and passageways. Then water drip-
ping from the ceiling begins to ornament it with beautiful, icicle-like sta-
lactites, and with stalagmites rising up from the floor. Their formation,
too, depends on the calcium carbonate-calcium bicarbonate reaction:
when a solution of bicarbonate evaporates, it releases carbon dioxide
and the residue reverts to the carbonate-limestone. The drops which
cling to a cavern ceiling are rich solutions of calcium bicarbonate; when
they evaporate they deposit on the ceiling a microscopic quantity of lime-
stone. Similarly, when a drop that falls to the floor evaporates, it adds a
speck of limestone there. And so the stalactites grow down, the stalag-
mites grow up.
Caverns formed and decorated in this way are to be found in nearly
all the great limestone districts of the world. They are common in the
Shenandoah Valley of Virginia, and in Tennessee, Kentucky, southern
Indiana and northern Florida. The Carlsbad labyrinth in southeastern
New Mexico, which has still not been completely explored, is deeper
than 1,100 feet and contains one irregular chamber that is 280 feet high
and contains 14 acres of floor area.
When underground caverns collapse they often make surface hollows,
or "sinks." The earth's surface, when dotted with such sinks, forms a
strangely topsy-turvy landscape called karst topography, after a region
in Yugoslavia where such sinks are particularly common. The valleys
widen and narrow abruptly, walled at one end, and sometimes at both,
by vertical cliffs; sizable rivers suddenly disappear into the earth; deep
depressions are arched by natural bridges of stone; branch valleys hang
unfinished, having been robbed of the water which was cutting them.
Force in falling raindrops
Water usually forms landscape not so much by dissolving earth as by
carrying it as waterborne debris. Rain tears away at the earth's surface
with a power that is astonishing. The power of the drops striking one
acre in a heavy rain of one inch per hour is equivalent to that of a
100 horsepower automobile engine running at top speed. A very heavy
rain-two inches an hour-expends enough energy in one hour to lift a
seven-inch layer of that acre 258 feet into the air.
Each raindrop is a miniature hammer, breaking off minute fragments
from the hardest rock. Striking loose dirt, it gouges holes and splashes
particles into the air. The water collects other particles within itself,
forming a muddy suspension that often clogs the surface pores of the
ground. This puddling prevents the rain that follows from sinking into
the ground and causes it to accumulate in sheets over the surface. In a
pelting rain, the sheets of water are churned, greatly increasing the ca-
pacity of the subsequent runoff to dislodge, lift and transport soil and
rock materials. These materials further augment erosion by acting as
abrasives that scour and cut the ground. The runoff trickles its bit of the
land into the nearest streams. The streams tear at their courses and dump
their burden into the river. The river continues the process. From the
stuff of mountains and foothills the river constructs the lowlands-the
floodplains, the broad alluvial valleys, the delta thrusting into the sea.
Despite the importance of the rivers to mankind, only now is a mathe-
matical theory of river science being developed. New concepts, based on
energy relations, reveal a remarkable unity underlying all the world's
river systems, and are leading to a deeper understanding of such ancient
problems as floods, silting, erosion and channel-shifting.
A long-lasting view of landscape sculpture was developed in the 1880s
by the American William Morris Davis, an ebullient geologist of force-
ful personality. Davis saw rivers and their valleys as living organisms
that grew from infancy through youth and maturity to old age. Accord-
ing to Davis, the life cycle of a river begins when new land is raised
above the sea. Rain runs down and gullies it; the gullies run together to
form river channels. Lakes collect in the low spots of the uneven surface
but soon disappear as shallow streams deepen their channels and cut
downward to form steep-sided, V-shaped valleys. Tributaries then grow
from the main trunk of the river like branches from a tree. As the river
matures, its valleys deepen and the tributaries lengthen.
From vigorous maturity to sluggish age
During this period, Davis said, when the greatest amount of sloping
surface is exposed to erosion, the river carries away its greatest load of
earth. The lower trunk stream, flowing down a gentle slope, may not be
able to transport so much and therefore lays some of it aside in a flood-
plain. Almost imperceptibly, the river passes from vigorous maturity
into old age. All the valley slopes have been much reduced; the once
steeply concave river basin becomes a shallow dish, and the now slug-
gish river, its surface only slightly above sea level, carries less sediment.
Davis' biological metaphor was persuasive and continues to be used
today. But to it has been added the detailed understanding which came
from field measurement. One pioneer in statistical analysis of such
measurements was an American engineer, very different from Davis,
named Robert E. Horton. A tall, lanky man with a tremendous shock of
white hair that stood up from the top of his head as if pulled by light-
ning, Horton enjoyed a long and highly remunerative career as a con-
sulting hydraulic engineer. His statistical description of river systems
was one of the first indications of the elementary mathematics that re-
lates all rivers everywhere in the world. He classified rivers as to size
according to the complexity of their tributaries. A stream with no trib-
utaries is designated as one of the first order. A river with one or more
first-order tributaries is a stream of the second order. The river be-
comes third order only when it acquires at least one second-order trib-
utary, and so on. The Mississippi is about 10th order, and the Amazon
and Congo, largest rivers in the world, are variously classified as 12th
or 13th order.
As might be expected, nearly every river system on earth includes
fewer high-order than low-order streams. As the order increases, so does
the length of the river, and so do both the total number of streams that
feed into it and the area of the watershed that they drain. More sur-
prising is the mathematical relationship that determines how many
tributaries of each order a stream will possess. It turns out that the
average stream has three or four tributaries of the next smaller order.
A DRAINAGE BASIN, the region which
feeds a river system, is part of a geologic
hierarchy that ranks all rivers according
to the kinds of streams feeding them. In this
ranking, a first-order stream is one that has
no tributaries, rising entirely from a
spring or from precipitation. A second-order
stream is fed by first-order streams.
Shown here is a third-order stream; its drainage
basin consists solely of second- and
first-order streams (a second-order drainage
basin is shown in blue). This method of ranking
streams was developed during the 1940s by
Robert Horton, an American hydraulic engineer.
That is, a second-order stream is fed by three or four first-order creeks,
and a 10th-order river is fed by three or four ninth-order rivers. The
Potomac is estimated to be seventh-order; its tributaries of approxi-
mately sixth order are the Shenandoah, the northern and southern
branches of the Potomac, and the Monocacy.
When these broad relationships became apparent, they suggested how
basic physical laws govern the operation of rivers. This led Walter B.
Langbein and his colleagues of the U.S. Geological Survey to compare
a river to an energy system. A river contains energy gained when its
water was evaporated from the sea and lifted into the atmosphere. This
is energy of position-the same as that gained by a weight which is
lifted off the floor. If the weight is dropped, the energy of position is
converted into energy of motion-that is, into the velocity of the falling
weight. And finally the energy of motion is dissipated in the impact
when the falling weight hits the floor. In exactly the same way, some of
the position energy of atmospheric moisture is converted into motion
energy and then dissipated on impact when raindrops fall. But many
of the fallen raindrops still retain position energy because they land
at elevations above sea level; they have not dropped all the way to the
"floor." This remaining position energy passes into the water of streams
and rivers. It is spent gradually-converted into heat by various kinds
of friction-as the water courses downhill, until finally all the position
energy is used up when the river reaches the sea. The slope of the
downward-flowing water surface is a measure of the energy loss along
the stream length.
The thermodynamics of flowing rivers
A similar dissipation of energy occurs in a heat cycle, such as that
taking place in a steam engine, and this has led Langbein to apply the
mathematical equations of thermodynamics to rivers. These equations
specify two facts involving the rates at which energy is lost: first, the
rate of loss must be fairly evenly distributed along the channel so that
every unit of bed area assumes an equal share in the energy loss. That
is, the river rubs about as hard on one spot as it does on the next.
Although exactly equal distribution is rarely achieved, a concentration
of energy loss is also rare-it occurs, for example, when a fire hose kinks
and the full force of the stream of water is exerted at that single point.
Second, these rates of loss of energy, when added up for all parts of
the river system, must be as low as possible. This simply means that the
river, taken as a whole, expends its energy as slowly as it can. When it
is found to expend energy at a great rate, as at a waterfall, it attempts
to redress the balance by wearing down the waterfall.
These two requirements of the theory-minimum rates of loss and
uniform distribution of loss-conflict with each other. The slope of the
river course that best suits one will not suit the other. There has to
be an adjustment. As a result, the river never achieves either uniform
distribution or minimum rate of loss anywhere, but trades one require-
ment off against the other, like a housewife balancing a skimpy budget.
The practical outcome of these theoretical considerations can be seen
in familiar situations. Because of the uniform-distribution require-
ment, the kinked fire hose will not stay kinked: the concentration of
energy at the kink forces the hose to untwist itself into a gentle curve.
A waterfall quickly wears its precipice down into a sloping valley be-
cause of the combination of requirements: the sudden drop at the falls
constitutes both a high rate of loss and a concentrated loss; erosion
tends to provide a more uniform distribution, exemplified by the usually
gentle slope of the valley.
A tendency to wander
A more complex demonstration of these effects is provided by rivers'
tendency to meander. Unless they are hemmed in by the hand of man,
all streams will flow in curves: natural channels are seldom straight for
a distance of more than 10 channel-widths. Thus, a stream 100 feet wide
will have straight stretches no longer than about a thousand feet.
Meandering seems to be a characteristic of all streams. Its immediate
cause, it has been suggested, is the undulating surface of the channel
bottom. A riverbed is not a flat trough but an alternating series of pools
and riffles established by incoming sediment deposits. The sediment
flows in at random but the particles quickly sort themselves into bunches
-like cars forming platoons on a busy highway. This bunching action
creates the riffles, with pools in between.
Water flows steeply downward over a riffle, expending more energy
per unit length than could be dissipated in the level pool behind it-
unless balancing effects intervene. To compensate for the lack of slope
in the pool and increase the loss per unit length there, the water is
forced to change its direction and travel around a curve. Thus the river
balances its accounts, keeping energy loss uniform and minimum. The
mathematics of the theory explains a fact visible to any observer: me-
andering rivers curve at their pools and run straight over their riffles.
Meandering is more noticeable in flat valleys, where banks erode
easily, than in rocky, steep mountains. The curves follow a pattern of
their own, a series of arcs extending over about five to seven channel-
widths. That is, the curved sections of a river 100 feet wide will contain
an average of eight to 10 curves per mile. Their shape is also character-
istic, not a circular arc but a specialized trigonometric curve, which
keeps total bending to a minimum for a given length-precisely the curve
assumed by an unkinking fire hose.
Eventually the rivers-small or large, famous or obscure-reach the
sea, their energy spent. They deposit in their deltas the last of the land
they have torn away. And finally they mix into the currents of the ocean
and help to shape its basins until, once more, some of their waters
are beckoned by the sun to rise as vapor-to return again to the land-
forms that are never quite the same as they were a million years ago,
last month or even yesterday.
SHIFTING ITS COURSE, a river moves
sideways where it curves because of the way its
water erodes and deposits bed material.
Water rounding the curve is subjected to
centrifugal force. The water at the bottom,
retarded by friction, moves more slowly than
the surface water. These influences combine to
give a corkscrew movement to the water.
As a result, silt picked up at the outer
bank is deposited on the inner bank,
slowly causing an increase in the curve's arc.
Ever since the earth's crust began forming, about four bil-
lion years ago, water has been wearing it away. Erosion has
scarred and sculpted the earth's surface with mountains,
chasms, deserts and deltas. Rivers and rainfall, glaciers
and waves, dew and frost, all participate in the process. Yet,
despite the variety of erosive agents, certain patterns of
erosion are visible everywhere in the earth's features. In
any given environment the slopes of hills will always be
about the same: the slopes of the Swiss Alps are almost
uniform; so are the desert slopes of the U.S. Southwest. All
rivers share common profiles, cutting mountain gorges, me-
andering over floodplains and building deltas. In the same
way, the pattern of drainage channels cut into the earth by
rainfall remains basically constant. These similarities oc-
cur because whenever water works on the land, it always
tries to expend its energy at a constant rate. For this rea-
son it abhors obstacles, wearing them down in much the
same fashion wherever it finds them. If water had its way,
all landscapes would be the same, and all streams would
flow at an even rate on an unobstructed journey to the sea.
The ridges, ribbed slopes and canyons of the Hen- the land where the mountains now stand. The
ry Mountains in Utah have been cut by the run- mountains thrust up through this plain were orig-
off of rainwater over many eons. About 80 mil- finally dome-shaped, but erosive streams have
lion years ago a barren shale plain stretched over cut the shale caps down to the present pattern.
90^. m N
0 S -'~t~
rr------ l r -.- -._ __
The Changing Face
of the Land
No crag or cranny of the earth es-
capes the irresistible force of water
as it flows from mountains to sea.
Erosion works on all principal geo-
logic forms. as illustrated in the fan-
ciful landscape at right. Flowing gla-
ciers, meandering rivers and crashing
waves combine with subtler weather-
ing processes to carve valleys. flatten
plains, straighten coastlines.
Where water meets the most re-
sistance. it works the hardest. On
mountain slopes, torrential streams
land occasionally glaciers, cut down
through rock. the earth's most re-
sistant form. On more gently sloping
plains, water tends to meander in
broad swaths over soft soil. deposit-
ing as much material as it erodes. If
not for this balance of erosion and
resistance, the earth might rapidly
be reduced to an inhospitable terrain
of sheer precipices and deep chasms.
Over great geologic ages, another
sort of balance is struck-between
erosion and rebuilding of mountains.
The Appalachians of the eastern U.S.
have had seven face-liftings in the
past 211( million years, upraised by
the earth's intense churnings and
then cut down by millennia of rain-
fall. Were it not for this erosion, the
chain's tallest peaks would tower
a mile higher than Mount Everest.
THE FEATURES OF EROSION
Thr many inlerjacionI, l0 walei and lanrdform
are snown .n ih-, comrcros.le landscape Across
the loregrouna waves weiar away ai a sandy
CoasI in ihe mounlaira at rear glaciers scooD
ou! vallevs wnlle hill, lair right are slr'oi-d or
Iheir co,,er.ng soil iby drainage Sirsam; and
rivers icentrrl carry ,ed.n er.i r IC e sea in
de;err regions imlddle rQhtr waler *:arvei oul
rocky canyon; anrd c.:i away D1aleBaus 01 sofer
clay Cllils are cui back tiy the flooding ol nearby
rivers The ,hanagel iThal m.aght be rougnl in
10000 ears of SuCh we 1arIher.ni are Fhovwn
*n a matining lliuuralion or. pages 100-101
e A'4 Y
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of Frozen Water
Glaciers, colossal masses of ice capa-
ble of grinding up mountains, form
where freezing temperatures permit
the packing-down of the snows of
many years. In Greenland, 10,000
centuries of snow have formed an
icecap 10,000 feet high. Under so
much snow, internal pressure may
reach seven tons to the square foot.
The bottom layers of ice become so
compressed that they begin to flow,
as illustrated in the diagram below.
At right, a photographic detail of
the larger picture on the preceding
pages shows a glacier (white square
in diagram at left) thus set in mo-
tion. It flows in much the same fash-
ion as a river, although it may take
centuries to creep down a mountain
valley. Tremendous amounts of rock
are ground to a fine flour as the gla-
cier gouges the land. Streams melt-
ing out of the lower reaches of gla-
ciers on Mont Blanc in France carry
off some 80,000 tons of debris a year.
PRESSURE THAT MAKES SOLIDS FLOW
Glaciers flow, according to the current accepted
theory, because their great weight alters the
molecular structure of the ice. Each ice crystal
consists of 60 water molecules grouped in hex-
agonal layers, seen near the top. Internal pres-
sure distorts the crystals, causing layers to slip;
at the base of the glacier, the pressure be-
comes so great that the crystal layers flow freely.
VAST RIVERS OF ICE
The long ribbon of Kaskawulsh Glacier in the
St. Elias Mountains in the Yukon flows like a
winding highway between rocky peaks. Debris
cut from the edges of outcrops is carried away,
forming the long dark lines of crushed rock.
called moraines, visible in the picture above.
-. -* -.-
- -* 9
T.~, b- rt~F
Drops and Crystals
That Split Rock
Some of the most dramatic changes
effected by water are accomplished
by processes too minute to be seen
with the unaided eye. A single drop
of rain may pound the soil with a
force of 2.3 pounds per square inch
(in an average downpour, some five
million drops fall on every acre every
second). A single crystal of ice lodged
in a rock pore, formed as moisture is
frozen, expands with a pressure of
400 pounds per square inch; multi-
plied by a great number of crystals,
this is enough to split the rock. A
crystal of salt left behind by evap-
orating water grows in crevices of
rock, expanding until the rock is frac-
tured. Water also exerts a profound
chemical force on the earth. Certain
elements abundant in groundwater,
rain and dew react on minerals in
rock, dissolving it into the soil, and
in some cases creating plant foods.
If water can destroy rock in many
ways, it can also rebuild it. Three
fourths of the surface of the earth is
now covered with layers of sedimen-
tary rock, which are built out of com-
pressed material washed away from
one place and redeposited in another.
A raindrop pounds the wet soil, loosening particles. This photograph, taken at 1/1,000 of a second, has been greatly magnified.
THE BEGINNINGS OF EROSION
Many common features of the land. like those
shown in these photographs, reveal erosion in
progress. At upper left, a slab of basalt is en-
crusted with salt crystals formed as moisture
evaporated. Growing in crevices, the salt crys-
tals can split the rock. At upper nght. a snow-
bank is caked with soil it has picked up from
the ground. As the snow melts, the soil is
washed away. At middle left. a granite boulder
is streaked with trickling dew. The granite de-
composes as carbon dioxide dissolved in the
dew reacts with the mineral feldspar in the
rock. changing the feldspar to clay At middle
right, the face of a granite cliff, shattered with
irregular cracks, shows the effect of frost on
solid rock. The cracks are caused by expanding
ice crystals lodged in pores of the rock. The
bottom pictures show two different ways in
which flowing water erodes rock. At left, a
stream digs a deep pothole into granite rock as
it swirls against the surface: small pebbles,
called grinders, caught up In the current scrape
against the rock. milting it down. At right, water
flows through a channel in limestone, cut into
the rock by chemical action. Carbon dioxide
dissolved in the water reacts with the mineral
calcite in limestone to decompose the rock.
B/L) DA -"
-'"^ __?__ "~_"7---
,,I,, -DR IN.
Wherever rainfall runs off the land,
it dredges its own drainage system-
and the pattern of rills and channels
it leaves behind is always the same.
Smaller channels drain into larger
ones in a treelike arrangement of
branches until all the surface water
empties into a main trunk. On a
small scale this pattern can be seen
in the gullies that drain an acre or
two; on a large scale, the same pat-
tern applies to the tributaries that
empty into a great river. In this way
the Mississippi and its extensive
branching network of tributaries
drain a region of 1,250,000 square
miles, emptying 724 billion cubic feet
of water a year into the Gulf of Mexico.
Although the pattern is constant,
the number of drainage channels in
a region varies with the amount of
rainfall and the nature of the soil.
In one New Mexico arroyo, 43 small
streams and channels drain 3.7 acres;
on the Appalachian slopes, 43 chan-
nels are enough to drain 4,224 acres.
This basic pattern is constant be-
cause the treelike arrangement of
channels or tributaries is the most
efficient: any other system would re-
quire a greater total length of chan-
nel (i.e., the combined length of all
the branches) to drain the same area.
Thus, in constructing its own drain-
age ditches, water works everywhere
to make the least work for itself.
CHANNELS IN DRIED SILT
Cracks in a silt bed occur as the sun dries out
the surface moisture. With each rainfall, water
running down from the upturned edges of the
cracks carves a series of small, tree-shaped rills
and channels-a pattern seen on a larger scale
on the sides of rocks below (right, foreground).
CHANNELS IN ANCIENT ROCK
The easily erodable clays and shales of the
Painted Desert in Arizona are extensively sculp-
tured with drainage rills and channels. On the
entire Colorado Plateau, of which this area is a
part, an estimated quadrillion tons of rock has
been eroded during the last 13 million years.
'i --; '- -, -." "' "
WATERFALL AND CANYON
Waterfalls are, in a sense, great acci-
dents of erosion. Where a river abrupt-
ly crosses from hard to soft rock, the
soft rock-an ancient lava flow, for
example-is rapidly worn down,
leaving a lip. Niagara was created in
this fashion. Where a river cuts down
into its bed faster than does a tribu-
tary, the tributary is left hanging,
with a waterfall connecting the two.
Occasionally, a river flows over an un-
derground cavern and cuts it open,
creating a waterfall at the site.
However it may be formed, a wa-
terfall is an aberration. Ordinarily, a
river expends its energy more or less
steadily, not too much at any one
point along its course. But at a wa-
terfall, great quantities of energy are
dissipated at an extravagant rate. As
soon as a waterfall is formed, how-
ever, water power goes to work to
erode the falls and restore the river's
original, less precipitous bed. The
water dropping over Niagara digs
great plunge pools at the base, un-
dermining the shale cliff and causing
the hard, limestone cap to cave in.
Niagara has eaten itself seven miles
upstream since it was formed 10,000
years ago. At this rate, it will dis-
appear into Lake Erie in 22,800 years.
WASHING AWAY THE ROCK
The IguazO River plunges off 180-million-year-
old lava beds at the Iguaz6 Falls on the Brazil-
Argentina border. A hard-rock cap at the top
is undermined by the churning water (an aver-
age of 44.000 cubic feet per second). With
occasional cave-ins, the falls recede upstream.
WIDENING A GORGE
The 20-mile canyon gouged 1,200 feet deep
through soft, volcanic rock of the Yellowstone
Plateau has been left behind by Yellowstone
Falls (at the canyon's head) as it has slowly
inched its way upstream. Since the canyon was
carved out, smaller drainage streams have con-
tinued to strip covering rock from its sides.
slowly increasing the width of the entire gorge.
In its wanderings, Alabama's Black Warrior River has curved so far that it has looped back on itself; when this happens, the river flow bypasses
CUTTING SHARPER CURVES
As a river cuts a meandering course across a
plain, the water at its surface, represented by
the arrows, crosses in a diagonal path from one
bank to the next. Some of this water collides
with the bank and erodes it; some is forced
down and erodes the bed. Material from the
eroded area (dark blue) is deposited downstream
(beige) on the same side. Thus, at each curve,
one bank is cut away, one built up, and the
net result is that the river curves more sharply.
Deltas Built on
M N R
MEA DE IN R V -'AND-- L-T--
,;^' -/ ".r ^^ "* .^-'-----
MEANDERING RIVER AND DELTA
THE CONSTRUCTION OF A DELTA
The Colorado River carries its load of silt down-
stream into Lake Havasu in Colorado (above),
building a fan-shaped area of new land extend-
ing out from the river mouth. Such deltas are
formed (left) wherever flowing water meets a
body of still water: when this occurs, deposited
On a level floodplain, a river often
flows over a bed of its own creation.
The plain is built up with layers of
silt deposited in the course of the riv-
er's restless meanderings, or during
its floods. A single grain of silt may
take thousands of years to move from
mountain peak to sea. Eroded mate-
rial is washed along in a series of
short hops-a grain of silt may be
swept downstream for several days,
then sit lodged in a bank for several
thousand years before its next hop.
Where a river empties into a lake or
the sea, its velocity is checked and it
dumps a heavy load of silt to the bot-
tom. As the silt accumulates, a river
mouth becomes a new delta of land.
sediment builds up, forcing the main flow of the
river or creek to a new channel. The Mississippi
drops two million tons of sediment a day into
the Gulf of Mexico. a load so great its weight is
actually deforming the crust of the earth-caus-
ing it to sag at the rate of three feet a century.
-, 'C r
FLATTENING A PROJECTION
A coastal headland creates the conditions for
its own erosion by causing the waves to bend
around it. The force a wave exerts (arrows)
is always focused at right angles to the wave
Smashing against the shore with tre-
mendous energy, ocean waves slow-
ly erode the most jagged coast to a
smooth, straight shoreline. A wave
10 feet high hits the coast with 1,675
pounds of pressure per square foot.
A storm wave 18 feet high can move
10-ton blocks of stone. As the wind
whips a wave toward the coast, it
moves in a straight front. Where a
beach is uneven, however, a wave is
bent around the jutting headland-
and most of its force is brought to
bear on that point (below and right).
Just as a beach is straightened, the
promontory of a cliff, receiving the
brunt of a wave's erosive force (far
right), is eventually worn smooth.
Thus a wave erodes any coastline at
the point of most resistance. It is
the same principle by which water
and leveling, eroding and rebuilding,
until it need expend as small amount
of energy in one place as in another.
front (blue lines). As the wave bends around a
point, its force is concentrated, or bunched, at
the point, thus causing the most erosion there.
Where the shore is recessed, forces are diffused.
EVIDENCE OF AGE-OLD SHORES
Gentle waves lap against a central California
beach (right), as the breakers in the background
erode the headland. The rocky ledges or marine
terraces on the hill above the beach are actual-
ly old shorelines, raised as the rock was thrust
up from sea level over succeeding centuries.
Waves splash against an outcropping of volcan-
ic rock in the British West Indies. The water
erodes the rock at sea level; eventually, this
rocky obstacle will be entirely washed away.
I I I I
Although a rampaging flood or a dis-
astrous drought, striking within a
relatively brief time, may leave a lo- RECEDING
cal blemish on the earth's features, GLACIER
most of the massive changes wrought
by water's erosion can be measured
only over millennia. Part of the rea-
son for this is that only a small frac-
tion of water's energy is expended
on effectively eroding soil and rock.
If all the potential energy of the wa-
ter in the United States were put to
work, it would provide power equiv-
alent to five million bulldozers work-
ing around the clock.
The landscape at right, showing
the same geologic formations as in
the matching illustration on pages
86-87, depicts the changes that might
actually occur after 10,000 years--a
span of time equal to that from the
Stone Age to the present. This is a
short period in geologic terms-the
Rocky Mountains, for example, took
more than 70 million years to form.
But even in 10,000 years a great deal
of water-engineering goes on. For one
thing, in 100 centuries enough water
will have run down the Mississippi
River to fill eight Gulfs of Mexico,
carrying enough sediment to build a
mountain taller than Mount Everest.
THE WEATHERED LANDSCAPE
Compared with the way the same landscape
looked 10.000 years earlier, the terrain at right
has undergone considerable change. Mountain
glaciers (background) have retreated as the cli-
mate has warmed: the coastline (foreground),
straightened at center, has been pushed out at
right by the formation of a delta; the moun-
tain peaks (right background) are somewhat re-
duced. the valleys deepened. The meandering
course of rivers (center) has created a variety of
oxbow lakes. On the plain, one lake has evapo-
rated into a salt bed, another has been absorbed
into a gorge, and the badlands have receded.
~~~ '~ -''.*" .l