larger ones descend close to baselevel. If local contrasts of hardness
allow a quick deepening of the down-stream part of the channel, while
the part next up-stream resists erosion, a cascade or waterfall
results; but like the lakes of earlier youth, it is evanescent, and
endures but a small part of the whole cycle of growth; but the falls on
the small headwater streams of a large river may last into its
maturity, just as there are young twigs on the branches of a large
tree. With the deepening of the channels, there comes an increase in
the number of gulleys on the slopes of the channel; the gulleys grow
into ravines and these into side valleys, joining their master streams
at right angles (La Noë and Margerie). With their continued
development, the maturity of the system is reached; it is marked by an
almost complete acquisition of every part of the original
constructional surface by erosion under the guidance of the streams, so
that every drop of rain that falls finds a way prepared to lead it to a
stream and then to the ocean, its goal. The lakes of initial
imperfection have long since disappeared; the waterfalls of adolescence
have been worn back, unless on the still young headwaters. With the
increase of the number of side-streams, ramifying into all parts of the
drainage basin, there is a proportionate increase in the surface of the
valley slopes, and with this comes an increase in the rate of waste
under atmospheric forces; hence it is at maturity that the river
receives and carries the greatest load; indeed, the increase may be
carried so far that the lower trunk-stream, of gentle slope in its
early maturity, is unable to carry the load brought to it by the upper
branches, and therefore resorts to the temporary expedient of laying it
aside in a flood-plain. The level of the flood-plain is sometimes built
up faster than the small side-streams of the lower course can fill
their valleys, and hence they are converted for a little distance above
their mouths into shallow lakes. The growth of the flood-plain also
results in carrying the point of junction of tributaries farther and
farther down stream, and at last in turning lateral streams aside from
the main stream, sometimes forcing them to follow independent courses
to the sea (Lombardini). But although thus separated from the main
trunk, it would be no more rational to regard such streams as
independent rivers than it would be to regard the branch of an old
tree, now fallen to the ground in the decay of advancing age, as an
independent plant; both are detached portions of a single individual,
from which they have been separated in the normal processes of growth
and decay.
In the later and quieter old age of a river system, the waste of the
land is yielded slower by reason of the diminishing slopes of the
valley sides; then the headwater streams deliver less detritus to the
main channel, which, thus relieved, turns to its postponed task of
carrying its former excess of load to the sea, and cuts terraces in its
flood-plain, preparatory to sweeping it away. It does not always find
the buried channel again, and perhaps settling down on a low spur a
little to one side of its old line, produces a rapid or a low fall on
the lower slope of such an obstruction (Penck). Such courses may be
called locally superimposed.
It is only during maturity and for a time before and afterwards that
the three divisions of a river, commonly recognized, appear most
distinctly; the torrent portion being the still young headwater
branches, growing by gnawing backwards at their sources; the valley
portion proper, where longer time of work has enabled the valley to
obtain a greater depth and width; and the lower flood-plain portion,
where the temporary deposition of the excess of load is made until the
activity of middle life is past.
Maturity seems to be a proper term to apply to this long enduring
stage; for as in organic forms, where the term first came into use, it
here also signifies the highest development of all functions between a
youth of endeavor towards better work and an old age of relinquishment
of fullest powers. It is the mature river in which the rainfall is best
lead away to the sea, and which carries with it the greatest load of
land waste; it is at maturity that the regular descent and steady flow
of the river is best developed, being the least delayed in lakes and
least overhurried in impetuous falls.
Maturity past, and the power of the river is on the decay. The relief
of the land diminishes, for the streams no longer deepen their valleys
although the hill tops are degraded; and with the general loss of
elevation, there is a failure of rainfall to a certain extent; for it
is well known that up to certain considerable altitudes rainfall
increases with height. A hyetographic and a hypsometric map of a
country for this reason show a marked correspondence. The slopes of the
headwaters decrease and the valley sides widen so far that the land
waste descends from them slower than before. Later, what with failure
of rainfall and decrease of slope, there is perhaps a return to the
early imperfection of drainage, and the number of side streams
diminishes as branches fall from a dying tree. The flood-plains of
maturity are carried down to the sea, and at last the river settles
down to an old age of well-earned rest with gentle flow and light load,
little work remaining to be done. The great task that the river entered
upon is completed.
16. _Mutual adjustment of river courses_.--In certain structures,
chiefly those of mountainous disorder on which the streams are at first
high above baselevel, there is a process of adjustment extremely
characteristic of quiet river development, by which the down-hill
courses that were chosen in early life, and as we may say unadvisedly
and with the heedlessness and little foresight of youth, are given up
for others better fitted for the work of the mature river system. A
change of this kind happens when the young stream taking the lowest
line for its guide happens to flow on a hard bed at a considerable
height above baselevel, while its branches on one side or the other
have opened channels on softer beds: a part of the main channel may
then be deserted by the withdrawal of its upper waters to a lower
course by way of a side stream. The change to better adjustment also
happens when the initial course of the main stream is much longer than
a course that may be offered to its upper portion by the backward
gnawing of an adjacent stream (Löwl, Penck). Sometimes the lateral
cutting or planation that characterizes the main trunk of a mature
river gives it possession of an adjacent smaller stream whose bed is at
a higher level (Gilbert). A general account of these processes may be
found in Phillippson's serviceable "Studien über Wasserscheiden"
(Leipzig, 1886). This whole matter is of much importance and deserves
deliberate examination. It should be remembered that changes in river
courses of the kind now referred to are unconnected with any external
disturbance of the river basin, and are purely normal spontaneous acts
during advancing development. Two examples, pertinent to our special
study, will be considered.
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
Let AB, fig. 9, be a stream whose initial consequent course led it down
the gently sloping axial trough of a syncline. The constructional
surface of the syncline is shown by contours. Let the succession of
beds to be discovered by erosion be indicated in a section, laid in
proper position on the several diagrams, but revolved into the
horizontal plane, the harder beds being dotted and the baselevel
standing at OO. Small side streams will soon be developed on the slopes
of the syncline, in positions determined by cross-fractures or more
often by what we call accident; the action of streams in similar
synclines on the outside of the enclosing anticlines will be omitted
for the sake of simplicity. In time, the side streams will cut through
the harder upper bed M and enter the softer bed N, on which
longitudinal channels, indicated by hachures, will be extended along
the strike, fig. 10 (La Noë and Margerie). Let these be called
"subsequent" streams. Consider two side streams of this kind, C and D,
heading against each other at E, one joining the main stream lower down
the axis of the syncline than the other. The headwaters of C will rob
the headwaters of D, because the deepening of the channel of D is
retarded by its having to join the main stream at a point where the
hard bed in the axis of the fold holds the main channel well above
baselevel. The notch cut by D will then be changed from a water-gap to
a wind-gap and the upper portion of D will find exit through the notch
cut by C, as in fig. 11. As other subsequent headwaters make capture of
C, the greater depth to which the lateral valley is cut on the soft
rock causes a slow migration of the divides in the abandoned gaps
towards the main stream, and before long the upper part of the main
stream itself will be led out of the synclinal axis to follow the
monoclinal valley at one side for a distance, fig. 12, until the axis
can be rejoined through the gap where the axial portion of the
controlling hard bed is near or at baselevel. The upper part of the
synclinal trough will then be attacked by undercutting on the slope of
the quickly deepened channels of the lateral streams, and the hard bed
will be worn away in the higher part of the axis before it is consumed
in the lower part. The location of the successful lateral stream on one
or the other side of the syncline may be determined by the dip of the
beds, gaps being cut quicker on steep than on gentle dips. If another
hard bed is encountered below the soft one, the process will be
repeated; and the mature arrangement of the streams will be as in fig.
13 (on a smaller scale than the preceding), running obliquely off the
axis of the fold where a hard bed of the syncline rises above
baselevel, and returning to the axis where the hard bed is below or at
baselevel; a monoclinal stream wandering gradually from the axis along
the strike of the soft bed, AE, by which the side-valley is located and
returning abruptly to the axis by a cataclinal[17] stream in a
transverse gap, EB, in the next higher hard bed, and there rejoining
the diminished representative or survivor of the original axial or
synclinal stream, GB.
[Footnote 17: See the terminology suggested by Powell. Expl. Col. R. of
the West, 1875, 160. This terminology is applicable only to the most
detailed study of our rivers, by reason of their crossing so many
folds, and changing so often from longitudinal to transverse courses.]
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
17. _Terminology of rivers changed by adjustment_.--A special
terminology is needed for easy reference to the several parts of the
streams concerned in such an adjustment. Let AB and CD, fig. 14, be
streams of unequal size cutting gaps, H and G, in a ridge that lies
transverse to their course. CD being larger than AB will deepen its gap
faster. Of two subsequent streams, JE and JF, growing on the up-stream
side of the ridge, JE will have the steeper slope, because it joins the
deeper master-stream. The divide, J, will therefore be driven towards
AB, and if all the conditions concerned conspire favorably, JE will at
last tap AB at F, and lead the upper part, AF, out by the line FEGD,
fig. 15, through the deeper gap, G. We may then say that JE becomes the
_divertor_ of AF, which is _diverted_; and when the process is
completed, by the transfer of the divide from J, on the soft rocks, to
a stable location, H, on the hard rocks, there will be a short
_inverted_ stream, HF; while HB is the remaining _beheaded_ portion of
the original stream, AB, and the water-gap of AB becomes a wind-gap, H.
It is very desirable that geographic exploration should discover
examples of the process of adjustment in its several stages. The
preparatory stage is easily recognized by the difference in the size of
the two main streams, the difference in the depth of their gaps, and
the unsymmetrical position of the divide, J. The very brief stage of
transition gives us the rare examples of bifurcating streams. For a
short time after capture of the diverted stream by the divertor, the
new divide will lie between F and H, in an unstable position, the
duration of this time depending on the energy of the process of
capture.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
The consequences resulting from readjustments of this kind by which
their recent occurrence can be detected are: a relatively sudden
increase of volume of the divertor and hence a rapid deepening of the
course of the diverting stream, FE, and of the diverted, AF, near the
point of capture; small side-streams of these two being unable to keep
pace with this change will join their masters in local rapids, which
work up stream gradually and fade away (Löwl, Penck, McGee). The
expanded portion, ED, of the larger stream, CD, already of faint slope,
may be locally overcome for a time with the increase of detritus that
will be thus delivered to it at the entrance, E, of the divertor; while
the beheaded stream, HB, will find itself embarrassed to live up to the
habits of its large valley [Heim]. Geographic exploration with these
matters in mind offers opportunity for the most attractive discoveries.
[Illustration: FIG. 16.]
[Illustration: FIG. 17.]
[Illustration: FIG. 18.]
18. _Examples of adjustment_.--Another case is roughly figured in the
next three diagrams, figs. 16, 17, 18. Two adjacent synclinal streams,
EA and HB, join a transverse master stream, C, but the synclines are of
different forms; the surface axis of one, EA, stands at some altitude
above baselevel until it nearly reaches the place of the transverse
stream; while the axis of the other, HB, descends near baselevel at a
considerable distance from the transverse stream. As lateral valleys, E
and D, are opened on the anticline between the synclines by a process
similar to that already described, the divide separating them will
shift towards the stream of fainter slope, that is, towards the
syncline, EA, whose axis holds its hard beds above baselevel; and in
time the upper part of the main stream will be withdrawn from this
syncline to follow an easier course by crossing to the other, as in
fig. 17. If the elevation of the synclinal axis, AES, take the shape of
a long flat arch, descending at the further end into a synclinal lake
basin, S, whose outlet is along the arching axis, SA, then the mature
arrangement of stream courses will lead the lake outlet away from the
axis by some gap in the nearer ascending part of the arch where the
controlling hard bed falls near to baselevel, as at F, fig. 18,[18] and
will take it by some subsequent course, FD, across the lowland that is
opened on the soft beds between the synclines, and carry it into the
lower syncline, HB, at D where the hard beds descend below baselevel.
[Footnote 18: This figure would be improved if a greater amount of
wasting around the margin of the hard bed were indicated in comparison
with the preceding figure.]
The variety of adjustments following the general principle here
indicated is infinite. Changes of greater or less value are thus
introduced in the initial drainage areas, until, after attaining an
attitude of equilibrium, further change is arrested, or if occurring,
is relatively insignificant. It should be noticed that the new stream
courses thus chosen are not named by any of the terms now current to
express the relation of stream and land history; they are neither
consequent, antecedent nor superimposed. The stream is truly still an
original stream, although no longer young; but its channel is not in
all parts strictly consequent on the initial constructional form of the
land that it drains. Streams thus re-arranged may therefore be named
original streams of mature adjustment.
It should be clearly recognized that the process of adjustment is a
very slow one, unless measured in the extremely long units of a river's
life. It progresses no faster than the weathering away of the slopes of
a divide, and here as a rule weathering is deliberate to say the least,
unless accelerated by a fortunate combination of favoring conditions.
Among these conditions, great altitude of the mass exposed to erosion
stands first, and deep channeling of streams below the surface--that
is, the adolescent stage of drainage development--stands second. The
opportunity for the lateral migration of a divide will depend on the
inequality of the slopes on its two sides, and here the most important
factors are length of the two opposite stream courses from the water
parting to the common baselevel of the two, and inequality of structure
by which one stream may have an easy course and the other a hard one.
It is manifest that all these conditions for active shifting of divides
are best united in young and high mountain ranges, and hence it is that
river adjustments have been found and studied more in the Alps than
elsewhere.
19. _Revival of rivers by elevation and drowning by depression_.--I
make no contention that any river in the world ever passed through a
simple uninterrupted cycle of the orderly kind here described. But by
examining many rivers, some young and some old, I do not doubt that
this portrayal of the ideal would be found to be fairly correct if
opportunity were offered for its development. The intention of the
sketch is simply to prepare the way for the better understanding of our
actual rivers of more complicated history.
At the close or at any time during the passage of an initial cycle such
as the one just considered, the drainage area of a river system may be
bodily elevated. The river is then turned back to a new youth and
enters a new cycle of development. This is an extremely common
occurrence with rivers, whose life is so long that they commonly
outlive the duration of a quiescent stage in the history of the land.
Such rivers may be called revived. Examples may be given in which
streams are now in their second or third period of revival, the
elevations that separate their cycles following so soon that but little
work was accomplished in the quiescent intervals.
The antithesis of this is the effect of depression, by which the lower
course may be drowned, flooded or fjorded. This change is, if slow,
favorable to the development of flood-plains in the lower course; but
it is not essential to their production. If the change is more rapid,
open estuaries are formed, to be transformed to delta-lowlands later
on.
20. _Opportunity for new adjustments with revival_.--One of the most
common effects of the revival of a river by general elevation is a new
adjustment of its course to a greater or less extent, as a result of
the new relation of baselevel to the hard and soft beds on which the
streams had adjusted themselves in the previous cycle. Synclinal
mountains are most easily explained as results of drainage changes of
this kind [Science, Dec. 21st, 1888]. Streams thus rearranged may be
said to be adjusted through elevation or revival. It is to be hoped
that, as our study advances, single names of brief and appropriate form
may replace these paraphrases; but at present it seems advisable to
keep the desired idea before the mind by a descriptive phrase, even at
the sacrifice of brevity. A significant example may be described.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
Let it be supposed that an originally consequent river system has lived
into advanced maturity on a surface whose structure is, like that of
Pennsylvania, composed of closely adjacent anticlinal and synclinal
folds with rising and falling axes, and that a series of particularly
resistant beds composes the upper members of the folded mass. The
master stream, A, fig. 19, at maturity still resides where the original
folds were lowest, but the side streams have departed more less from
the axes of the synclinals that they first followed, in accordance with
the principles of adjustment presented above. The relief of the surface
is moderate, except around the synclinal troughs, where the rising
margins of the hard beds still appear as ridges of more or less
prominence. The minute hachures in figure 19 are drawn on the outcrop
side of these ridges. Now suppose a general elevation of the region,
lifting the synclinal troughs of the hard beds up to baselevel or even
somewhat above it. The deepening of the revived master-stream will be
greatly retarded by reason of its having to cross so many outcrops of
the hard beds, and thus excellent opportunity will be given for
readjustment by the growth of some diverting stream, B, whose beginning
on adjacent softer rocks was already made in the previous cycle. This
will capture the main river at some up-stream point, and draw it nearly
all away from its hard path across the synclinal troughs to an easier
path across the lowlands that had been opened on the underlying softer
beds, leaving only a small beheaded remnant in the lower course. The
final re-arrangement may be indicated in fig. 20. It should be noted
that every capture of branches of the initial main stream made by the
diverting stream adds to its ability for further encroachments, for
with increase of volume the channel is deepened and a flatter slope is
assumed, and the whole process of pushing away the divides is thereby
accelerated. In general it may be said that the larger the stream and
the less its elevation above baselevel, the less likely is it to be
diverted, for with large volume and small elevation it will early cut
down its channel so close to baselevel that no other stream can offer
it a better course to the sea; it may also be said that, as a rule, of
two equal streams, the headwaters of the one having a longer or a
harder course will be diverted by a branch of the stream on the shorter
or easier course. Every case must therefore be examined for itself
before the kind of re-arrangement that may be expected or that may have
already taken place can be discovered.
21. _Antecedent and superimposed rivers_.--It not infrequently
happens that the surface, on which a drainage system is more or
less fully developed, suffers deformation by tilting, folding or
faulting. Then, in accordance with the rate of disturbance, and
dependent on the size and slope of the streams and the resistance of
the rocks, the streams will be more or less re-arranged, some of the
larger ones persisting in their courses and cutting their channels down
almost as fast as the mass below them is raised and offered to their
action. It is manifest that streams of large volume and considerable
slope are the ones most likely to persevere in this way, while small
streams and large ones of moderate slope may be turned from their
former courses to new courses consequent on the new constructional form
of the land. Hence, after a disturbance, we may expect to find the
smaller streams of the former cycle pretty completely destroyed, while
some of the larger ones may still persist; these would then be called
antecedent streams in accordance with the nomenclature introduced by
Powell.[19] A fuller acquaintance with the development of our rivers
will probably give us examples of river systems of all degrees of
extinction or persistence at times of disturbance.
[Footnote 19: Exploration of the Colorada River of the West, 1875, 153,
163-166.]