Elements of Geology — Ch. III: Rivers and Valleys, pt 1

RIVERS AND VALLEYS

THE RUN-OFF.

We have traced the history of that portion of the rainfall which soaks into the ground; let us now return to that part which washes along the surface and is known as the RUN-OFF.

Fed by rains and melting snows, the run-off gathers into courses, perhaps but faintly marked at first, which join more definite and deeply cut channels, as twigs their stems.

In a humid climate the larger ravines through which the run-off flows soon descend below the ground-water surface.

Here springs discharge along the sides of the little valleys and permanent streams begin.

The water supplied by the run-off here joins that part of the rainfall which had soaked into the soil, and both now proceed together by way of the stream to the sea.

RIVER FLOODS.

Streams vary greatly in volume during the year.

At stages of flood they fill their immediate banks, or overrun them and inundate any low lands adjacent to the channel; at stages of low water they diminish to but a fraction of their volume when at flood.

At times of flood, rivers are fed chiefly by the run-off; at times of low water, largely or even wholly by springs.

How, then, will the water of streams differ at these times in turbidity and in the relative amount of solids carried in solution?

In parts of England streams have been known to continue flowing after eighteen months of local drought, so great is the volume of water which in humid climates is stored in the rocks above the drainage level, and so slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter and a "spring freshet" follows the melting of the winter's snows.

A "June rise" is produced by the heavy rains of early summer.

Low water follows in July and August, and streams are again swollen to a moderate degree under the rains of autumn.

THE DISCHARGE OF STREAMS.

The per cent of rainfall discharged by rivers varies with the amount of rainfall, the slope of the drainage area, the texture of the rocks, and other factors.

With an annual rainfall of fifty inches in an open country, about fifty per cent is discharged; while with a rainfall of twenty inches only fifteen per cent is discharged, part of the remainder being evaporated and part passing underground beyond the drainage area.

Thus the Ohio discharges thirty per cent of the rainfall of its basin, while the Missouri carries away but fifteen per cent.

A

number of the streams of the semi-arid lands of the West do not discharge more than five per cent of the rainfall.

Other things being equal, which will afford the larger proportion of run-off, a region underlain with granite rock or with coarse sandstone?

grass land or forest?

steep slopes or level land?

a well-drained region or one abounding in marshes and ponds?

frozen or unfrozen ground?

Will there be a larger proportion of run-off after long rains or after a season of drought?

after long and gentle rains, or after the same amount of precipitation in a violent rain?

during the months of growing vegetation, from June to August, or during the autumn months?

DESERT STREAMS.

In arid regions the ground-water surface lies so low that for the most part stream ways do not intersect it.

Streams therefore are not fed by springs, but instead lose volume as their waters soak into the thirsty rocks over which they flow.

They contribute to the ground water of the region instead of being increased by it.

Being supplied chiefly by the run-off, they wither at times of drought to a mere trickle of water, to a chain of pools, or go wholly dry, while at long intervals rains fill their dusty beds with sudden raging torrents.

Desert rivers therefore periodically shorten and lengthen their courses, withering back at times of drought for scores of miles, or even for a hundred miles from the point reached by their waters during seasons of rain.

THE GEOLOGICAL WORK OF STREAMS.

The work of streams is of three kinds,--transportation, erosion, and deposition.

Streams TRANSPORT the waste of the land; they wear, or ERODE, their channels both on bed and banks; and they DEPOSIT portions of their load from time to time along their courses, finally laying it down in the sea.

Most of the work of streams is done at times of flood.

TRANSPORTATION

THE INVISIBLE LOAD OF STREAMS.

Of the waste which a river transports we may consider first the invisible load which it carries in solution, supplied chiefly by springs but also in part by the run-off and from the solution of the rocks of its bed.

More than half the dissolved solids in the water of the average river consists of the carbonates of lime and magnesia; other substances are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate (Epsom salts), sodium chloride (common salt), and even silica, the least soluble of the common rock-making minerals.

The amount of this invisible load is surprisingly large.

The Mississippi, for example, transports each year 113,000,000 tons of dissolved rock to the Gulf.

THE VISIBLE LOAD OF STREAMS.

This consists of the silt which the stream carries in suspension, and the sand and gravel and larger stones which it pushes along its bed.

Especially in times of flood one may note the muddy water, its silt being kept from settling by the rolling, eddying currents; and often by placing his ear close to the bottom of a boat one may hear the clatter of pebbles as they are hurried along.

In mountain torrents the rumble of bowlders as they clash together may be heard some distance away.

The amount of the load which a stream can transport depends on its velocity.

A current of two thirds of a mile per hour can move fine sand, while one of four miles per hour sweeps along pebbles as large as hen's eggs.

The transporting power of a stream varies as the sixth power of its velocity.

If its velocity is multiplied by two, its transporting power is multiplied by the sixth power of two: it can now move stones sixty-four times as large as it could before.

Stones weigh from two to three times as much as water, and in water lose the weight of the volume of water which they displace.

What proportion, then, of their weight in air do stones lose when submerged?

MEASUREMENT OF STREAM LOADS.

To obtain the total amount of waste transported by a river is an important but difficult matter.

The amount of water discharged must first be found by multiplying the number of square feet in the average cross section of the stream by its velocity per second, giving the discharge per second in cubic feet.

The amount of silt to a cubic foot of water is found by filtering samples of the water taken from different parts of the stream and at different times in the year, and drying and weighing the residues.

The average amount of silt to the cubic foot of water, multiplied by the number of cubic feet of water discharged per year, gives the total load carried in suspension during that time.

Adding to this the estimated amount of sand and gravel rolled along the bed, which in many swift rivers greatly exceeds the lighter material held in suspension, and adding also the total amount of dissolved solids, we reach the exceedingly important result of the total load of waste discharged by the river.

Dividing the volume of this load by the area of the river basin gives another result of the greatest geological interest,-- the rate at which the region is being lowered by the combined action of weathering and erosion, or the rate of denudation.

THE RATE OF DENUDATION OF RIVER BASINS.

This rate varies widely.

The Mississippi basin may be taken as a representative land surface because of the varieties of surface, altitude and slope, climate, and underlying rocks which are included in its great extent.

Careful measurements show that the Mississippi basin is now being lowered at a rate of one four-thousandth of a foot a year, or one foot in four thousand years.

Taking this as the average rate of denudation for the land surfaces of the globe, estimates have been made of the length of time required at this rate to wash and wear the continents to the level of the sea.

As the average elevation of the lands of the globe is reckoned at 2411 feet, this result would occur in nine or ten million years, if the present rate of denudation should remain unchanged.

But even if no movements of the earth's crust should lift or depress the continents, the rate of wear and the removal of waste from their surfaces will not remain the same.

It must constantly decrease as the lands are worn nearer to sea level and their slopes become more gentle.

The length of time required to wear them away is therefore far in excess of that just stated.

The drainage area of the Potomac is 11,000 square miles.

The silt brought down in suspension in a year would cover a square mile to the depth of four feet.

At what rate is the Potomac basin being lowered from this cause alone?

It is estimated that the Upper Ganges is lowering its basin at the rate of one foot in 823 years, and the Po one foot in 720 years.

Why so much faster than the Potomac and the Mississippi?

HOW STREAMS GET THEIR LOADS.

The load of streams is derived from a number of sources, the larger part being supplied by the weathering of valley slopes.

We have noticed how the mantle of waste creeps and washes to the stream ways.

Watching the run-off during a rain, as it hurries muddy with waste along the gutter or washes down the hillside, we may see the beginning of the route by which the larger part of their load is delivered to rivers.

Streams also secure some of their load by wearing it from their beds and banks,--a process called erosion.

EROSION

Streams erode their beds chiefly by means of their bottom load,-- the stones of various sizes and the sand and even the fine mud which they sweep along.

With these tools they smooth, grind, and rasp the rock of their beds, using them in much the fashion of sandpaper or a file.

WEATHERING OF RIVER BEDS.

The erosion of stream beds is greatly helped by the work of the weather.

Especially at low water more or less of the bed is exposed to the action of frost and heat and cold, joints are opened, rocks are pried loose and broken up and made ready to be swept away by the stream at time of flood.

POTHOLES.

In rapids streams also drill out their rocky beds.

Where some slight depression gives rise to an eddy, the pebbles which gather in it are whirled round and round, and, acting like the bit of an auger, bore out a cylindrical pit called a pothole.

Potholes sometimes reach a depth of a score of feet.

Where they are numerous they aid materially in deepening the channel, as the walls between them are worn away and they coalesce.

WATERFALLS.

One of the most effective means of erosion which the river possesses is the waterfall.

The plunging water dislodges stones from the face of the ledge over which it pours, and often undermines it by excavating a deep pit at its base.

Slice after slice is thus thrown down from the front of the cliff, and the cataract cuts its way upstream leaving a gorge behind it.

NIAGARA FALLS.

The Niagara River flows from Lake Erie at Buffalo in a broad channel which it has cut but a few feet below the level of the region.

Some thirteen miles from the outlet it plunges over a ledge one hundred and seventy feet high into the head of a narrow gorge which extends for seven miles to the escarpment of the upland in which the gorge is cut.

The strata which compose the upland dip gently upstream and consist at top of a massive limestone, at the Falls about eighty feet thick, and below of soft and easily weathered shale.

Beneath the Falls the underlying shale is cut and washed away by the descending water and retreats also because of weathering, while the overhanging limestone breaks down in huge blocks from time to time.

Niagara is divided by Goat Island into the Horseshoe Falls and the American Falls.

The former is supplied by the main current of the river, and from the semicircular sweep of its rim a sheet of water in places at least fifteen or twenty feet deep plunges into a pool a little less than two hundred feet in depth.

Here the force of the falling water is sufficient to move about the fallen blocks of limestone and use them in the excavation of the shale of the bed.

At the American Falls the lesser branch of the river, which flows along the American side of Goat Island, pours over the side of the gorge and breaks upon a high talus of limestone blocks which its smaller volume of water is unable to grind to pieces and remove.

A series of surveys have determined that from 1842 to 1890 the Horseshoe Falls retreated at the rate of 2. 18 feet per year, while the American Falls retreated at the rate of 0. 64 feet in the same period.

We cannot doubt that the same agency which is now lengthening the gorge at this rapid rate has cut it back its entire length of seven miles.

While Niagara Falls have been cutting back a gorge seven miles long and from two hundred to three hundred feet deep, the river above the Falls has eroded its bed scarcely below the level of the upland on which it flows.

Like all streams which are the outlets of lakes, the Niagara flows out of Lake Erie clear of sediment, as from a settling basin, and carries no tools with which to abrade its bed.

We may infer from this instance how slight is the erosive power of clear water on hard rock.

Assuming that the rate of recession of the combined volumes of the American and Horseshoe Falls was three feet a year below Goat Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST, how long is it since the Niagara River fell over the edge of the escarpment where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39) shows alternating deeps and shallows which cannot be accounted for, except in a single instance, by the relative hardness of the rocks of the river bed.

The deeps do not exceed that at the foot of the Horseshoe Falls at the present time.

When the gorge was being cut along the shallows, how did the Falls compare in excavating power, in force, and volume with the Niagara of to-day?

How did the rate of recession at those times compare with the present rate?

Is the assumption made above that the rate of recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous rapid impossible to sound.

Its depth has been estimated at thirty- five feet.

From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat Island the sooner, the American or the Horseshoe Falls?

What will be the fate of the Falls left behind when the other has passed beyond the head of the island?

The rate at which a stream erodes its bed depends in part upon the nature of the rocks over which it flows.

Will a stream deepen its channel more rapidly on massive or on thin-bedded and close- jointed rocks?

on horizontal strata or on strata steeply inclined?

DEPOSITION

While the river carries its invisible load of dissolved rock on without stop to the sea, its load of visible waste is subject to many delays en route.

Now and again it is laid aside, to be picked up later and carried some distance farther on its way.

One of the most striking features of the river therefore is the waste accumulated along its course, in bars and islands in the channel, beneath its bed, and in flood plains along its banks.

All this alluvium, to use a general term for river deposits, with which the valley is cumbered is really en route to the sea; it is only temporarily laid aside to resume its journey later on.

Constantly the river is destroying and rebuilding its alluvial deposits, here cutting and there depositing along its banks, here eroding and there building a bar, here excavating its bed and there filling it up, and at all times carrying the material picked up at one point some distance on downstream before depositing it at another.

These deposits are laid down by slackening currents where the velocity of the stream is checked, as on the inner side of curves, and where the slope of the bed is diminished, and in the lee of islands, bridge piers and projecting points of land.

How slight is the check required to cause a current to drop a large part of its load may be inferred from the law of the relation of the transporting power to the velocity.

If the velocity is decreased one half, the current can move fragments but one sixty-fourth the size of those which it could move before, and must drop all those of larger size.

Will a river deposit more at low water or at flood?

when rising or when falling?

STRATIFICATION.

River deposits are stratified, as may be seen in any fresh cut in banks or bars.

The waste of which they are built has been sorted and deposited in layers, one above another; some of finer and some of coarser material.

The sorting action of running water depends on the fact that its transporting power varies with the velocity.

A current whose diminishing velocity compels it to drop coarse gravel, for example, is still able to move all the finer waste of its load, and separating it from the gravel, carries it on downstream; while at a later time slower currents may deposit on the gravel bed layers of sand, and, still later, slack water may leave on these a layer of mud.

In case of materials lighter than water the transporting power does not depend on the velocity, and logs of wood, for instance, are floated on to the sea on the slowest as well as on the most rapid currents.

CROSS BEDDING.

A section of a bar exposed at low water may show that it is formed of layers of sand, or coarser stuff, inclined downstream as steeply often as the angle of repose of the material.

From a boat anchored over the lower end of a submerged sand bar we may observe the way in which this structure, called cross bedding, is produced.

Sand is continually pushed over the edge of the bar at b (Fig. 42) and comes to rest in successive layers on the sloping surface.

At the same time the bar may be worn away at the upper end, a, and thus slowly advance down stream.

While the deposit is thus cross bedded, it constitutes as a whole a stratum whose upper and lower surfaces are about horizontal.

In sections of river banks one may often see a vertical succession of cross-bedded strata, each built in the way described.

WATER WEAR.

The coarser material of river deposits, such as cobblestones, gravel, and the larger grains of sand, are WATER WORN, or rounded, except when near their source.

Rolling along the bottom they have been worn round by impact and friction as they rubbed against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose nearly half their weight and become well rounded after traveling fifteen miles in rotating cylinders partly filled with water.

Marbles are cheaply made in Germany out of small limestone cubes set revolving in a current of water between a rotating bed of stone and a block of oak, the process requiring but about fifteen minutes.

It has been found that in the upper reaches of mountain streams a descent of less than a mile is sufficient to round pebbles of granite.

LAND FORMS DUE TO RIVER EROSION

RIVER VALLEYS.

In their courses to the sea, rivers follow valleys of various forms, some shallow and some deep, some narrow and some wide.

Since rivers are known to erode their beds and banks, it is a fair presumption that, aided by the weather, they have excavated the valleys in which they flow.

Moreover, a bird's-eye view or a map of a region shows the significant fact that the valleys of a system unite with one another in a branch work, as twigs meet their stems and the branches of a tree its trunk.

Each valley, from that of the smallest rivulet to that of the master stream, is proportionate to the size of the stream which occupies it.

With a few explainable exceptions the valleys of tributaries join that of the trunk stream at a level; there is no sudden descent or break in the bed at the point of juncture.

These are the natural consequences which must follow if the land has long been worked upon by streams, and no other process has ever been suggested which is competent to produce them.

We must conclude that valley systems have been formed by the river systems which drain them, aided by the work of the weather; they are not gaping fissures in the earth's crust, as early observers imagined, but are the furrows which running water has drawn upon the land.

As valleys are made by the slow wear of streams and the action of the weather, they pass in their development through successive stages, each of which has its own characteristic features.

We may therefore classify rivers and valleys according to the stage which they have reached in their life history from infancy to old age.