THE EPHEMERAL EXISTENCE OF LAKES
=Lakes as settling basins.=—Of all the processes which conspire to
blot out the lakes with which our northern landscapes are dotted,
the one of greatest importance is in most cases a process of filling
by the sediments brought in by tributary streams. The carrying of
sediment in suspension depends, as we know, upon the velocity of the
current, and as this is checked where it reaches the lake margin, all
coarser material is at once deposited to form a delta, while the finer
sediments are held longer in suspension and finally settle in thin
layers over the entire bottom of the lake. Clay deposits surrounded by
coarser sediments are thus characteristic of filled lake basins.
[Illustration: FIG. 462.—Map of the Arve and the upper Rhone to show
the importance of Lake Geneva as a settling basin of the larger stream.]
How waters are clarified by their passage through a lake is indicated
by a comparison of a river system such as the St. Lawrence, with
a river like the Missouri and Mississippi. Not only are the lower
stretches of the St. Lawrence in striking contrast with the muddy
floods of the Missouri and Mississippi; but the delta, which is so
remarkable a feature in the Mississippi, has no counterpart in the
northern river.
The most noteworthy examples of settling are, however, furnished by
the lakes of Switzerland, for the reason that SWISS rivers are heavily
charged with rock flour produced beneath the numerous glaciers at the
valley heads, and, further, because these rivers descend with turbulent
currents to near the borders of the larger lakes. To look out upon the
murky waters of the upper Rhone, where they enter Lake Geneva near
Villeneuve, and then to watch the flood of crystal water which issues
from the lake and passes under the bridge at Geneva, is an object
lesson which no traveling student should miss (Fig. 462). Yet even more
instructive is a visit to the _Bois de la Bâtie_ at the junction of
this clear stream with the Arve, a half hour’s walk only below Geneva.
The waters of the Arve have come on a steep descent directly from the
glaciers of the Mont Blanc district, and as they meet the cleared
waters of the Rhone, they flow beside them down the common valley
without mingling. Dull and opaque, the Arve waters can be discerned for
a long distance as a white belt against the left bank of the river,
sharply defined against the blue reflecting surface of the Rhone waters
(Fig. 463). Upon the banks of the Arve, just above its junction, a
cement manufactory has been established to utilize the clays which are
here deposited.
[Illustration:
FIG. 463.—View looking upstream across the opaque waters of the Arve
to the clear reflecting surface of the Rhone. To the right across the
Arve is seen the cement works for recovering the Arve sediments.]
Wherever lakes are contained in long and narrow valleys, the greater
part of the tributary drainage enters at the upper end, and the delta
which there forms extends from bank to bank. As it continues to advance
into the lake, the earlier water basin is gradually transformed into a
level plain of delta deposit, a feature so common as to be deserving
of a special name. The Scottish lochs, which are lakes of this type,
are each extended in a longer or shorter delta plain described as a
_strath_, and this local term may well be given a general application
(frontispiece). The city of Ithaca, the seat of Cornell University, is
built upon a strath at the head of Lake Cayuga, and numberless Scottish
and Swiss hamlets have been located upon such fertile plains (Fig. 464).
[Illustration:
FIG. 464.—The village of Poschiavo in eastern Switzerland, built upon
a strath at the head of Lake Poschiavo.]
=Drawing off of water by erosion of outlet.=—Next in importance to
the filling up of lake basins as a factor in their early extinction is
the cutting down of their channels of outflow. Whenever the walls of
the outlet are cut in rock, this draining process is apt to be slow,
for the reason that the outlet stream is of filtered water and so
lacks the necessary cutting tools. By far the larger number of lakes
are, however, held back by dams of loose drift deposits laid down by
the earlier continental glaciers; and so the very clarity of the water
promotes the erosion of the outlet by allowing the stream’s full burden
of sediment to be lifted and then removed from the channel.
=The pulling in of headlands and the cutting off of bays.=—The removal
of projecting headlands by wave action, though it increases the area of
the lake, yet it decreases directly the volume of lake water through
formation of the built terrace, and indirectly in far larger measure
through the transformation of bays into quiet lagoons within which the
extinguishing process of peat growth is set in operation.
=Lake extinction by peat growth.=—The first condition for the growth
of lake vegetation is quiet water. Within small lakes, such as the
kettle basins upon moraines, aquatic vegetation develops rapidly,
and bogs of peat might almost be included among the most important
distinguishing marks of a glaciated country. Within larger lakes it is
only after barrier beaches have been thrown across the mouths of the
bays to form natural breakwaters for the waves that this process of
lake extinction by peat growth can become effective.
[Illustration:
FIG. 465.—View of the floating bog and surrounding zones of vegetation
in a small glacial lake of the Yellowstone National Park (after a
photograph by Fairbanks).]
Many erroneous notions are still held concerning the prime importance
of sphagnum in peat formation, owing to the peculiar local conditions
under which the early studies were made. Within the glaciated districts
of the United States, the formation of peat involves the successive
growths of a number of zones of vegetation and the formation of a
floating bog which advances into the lake from the shores, followed
in turn by belts of low shrubs, tamaracks, and lastly deciduous trees
(Fig. 465).
In most cases the first plants to develop in a quiet lake are the water
lilies, though these are sometimes preceded by chara and floating
bladderwort. Next behind the water lilies come the sedges, which form a
mat of floating bog by their grasslike stems sinking down in the water
and being there interwoven with the rhizomes below. This mat of sedge
is often so firm that cattle may advance upon it to the water’s edge,
but it is separated by a layer of water from the bed of growing peat
at the bottom of the lake (Fig. 466). This bed of peat appears to grow
upward toward the surface and become joined to the shore end of the
floating bog by decaying vegetation which is dropped from the bottom of
the mat above.
[Illustration: FIG. 466.—Diagram to show how small lakes are
transformed into peat bogs (after C. A. Davis).]
In order behind the floating bog come the advanced plants of the
conifer group, with sphagnum and low shrub here upon a peat base
extending to the lake bottom. Behind the belt of shrubs arise the
tamaracks and spruces, and lastly, toward the shore, come the deciduous
trees and especially poplars, maples, and marginal willows. Upon the
margin of the basin there is usually a low trench, or “fosse”, filled
with water during wet seasons, as a result, no doubt, of seasonal
inwash that does not reach the residual lake toward the center of the
basin.
=Extinction of lakes in desert regions.=—In arid regions there are
special causes of lake extinction. Thus the blowing in of sand and dust
carried for long distances in the air, a by no means negligible factor
even in humid regions, here assumes large importance. The now exposed
basins of extinct desert lakes afford the evidence, however, of an even
greater factor of extinction, in climatic change. The clouds, which
at one time found their way into the drainage basin of a lake, may
later through the rise of a mountain barrier be cut off, and so with
reduced water supply a period of lake desiccation is begun. When, in
this process of drying up, the lake level has fallen below that of the
outlet, the saline content of the waters begins to increase, and later
a stage is reached, as in Great Salt Lake, when the sodium salts are
precipitated. When the lake has become extinct, these deposits remain
as a witness to the changed climatic condition.
=The rôle of lakes in the economy of nature.=—It is natural, in
considering the extinction of lakes, to give some attention to the
rôle which they play in the economy of nature. That lakes filter the
water of rivers, and prevent the formation of important delta deposits,
has already been noticed. A curious exception to this general rule is
furnished by the great delta at the head of Lake St. Clair, just below
the outlet of Lake Huron. This anomaly is, however, explained by the
peculiar currents of Lake Huron, which are so directed as to sweep the
beach sand into the swift current of the outlet, to be deposited in the
quiet waters of Lake St. Clair (Fig. 467).
[Illustration:
FIG. 467.—Map to show anomalous position of the delta in Lake St.
Clair, due to the peculiar currents in Lake Huron (after maps by Cole).]
As regulators of the flow of rivers, lakes perform an important
function. Such disastrous floods as are characteristic of the spring
season within the basin of the lower Mississippi could not occur in the
lower St. Lawrence, for the reason that the great basins of the lakes
serve as distributing reservoirs. The annual floods, upon which the
agriculture of Egypt depends, are explained by the flood waters from
the high mountains of Abyssinia entering the Nile _below_ the lakes of
its upper basin.
In one further respect large inland bodies of water have an important
function as regulators. It is the property of water to respond but
slowly to the variations in the quantity of heat which reaches the
earth’s surface from the sun. A larger quantity of heat must be
added to or abstracted from a body of water, in order to change its
temperature by one degree, than would be required for a like change in
the same bulk of earth or rock. Thus bodies of water by more slowly
acquiring the summer’s heat retard the coming spring, and by storing
up this energy and carrying it over into the autumn the warm season is
prolonged and early frosts prevented. The fruit belts about the lower
Great Lakes are thus dependent upon this regulating property of the
lake waters. The discomfort of the long spring of raw weather is thus
compensated by an unusually salubrious harvest season.
=Ice ramparts on lake shores.=—Small ridges known as ice ramparts are
formed upon lake shores by the action of lake ice, though subject to
so many qualifying conditions that the range of their occurrence is
somewhat limited. Within districts where a winter ice cover of some
thickness is formed, the shores of lakes are apt to present ridges of
bowlders parallel to and near the water’s edge, and such lakes have
sometimes become known as “wall lakes” (Fig. 468).
[Illustration:
FIG. 468.—A bowlder wall upon the shore of a small lake in the
Adirondacks of New York.]
In many cases these small ridges have been formed at the time of the
spring “break up” of the ice; for the ice cover, when once loosened,
is drifted in great rafts first against one shore, and later, with a
change of wind direction, against another. Under the impact of such
heavy rafts, the half-submerged bowlders near the shore are forced up
the beach until they lie in a ridge or bowlder wall.
At other times such bowlder walls, and far more interesting ridges as
well, result from a kind of ice shove independent of the wind, but
caused by expansion within the ice itself during a sudden rise of
temperature of the surrounding air. Such ice ramparts require for their
explanation a consideration of the sequence of events from the time the
ice cover closes the lakes.
[Illustration: FIG. 469.—Diagrams to show the effect of ice shove in
producing ice ramparts upon the shores of lakes (after Buckley with a
slight modification).]
The first lake ice of early winter forms in most cases with air
temperatures a few degrees only below the freezing point of the water.
When later a severe “cold wave” arrives, the ice cover is contracted
and becomes too small for the lake surface. To this contraction it
yields and opens cracks up which the water rises, and in the prevailing
low temperature this water is quickly frozen and the lake cover again
made complete. Skaters are familiar with the opening of these cracks
and the loud “roaring” which accompanies it on cold mornings, the sharp
skate runners sometimes starting a crack in the strained ice, as does a
light scratch upon glass that is in a similar strained condition.
[Illustration: FIG. 470.—Various forms of ice ramparts (after
Buckley).]
The original ice cover of the lake, which was formed at near-freezing
temperatures, has now received a number of inserted wedges of new ice
at a time when its contracted volume has made this possible. If now
a “warm wave” succeeds to the “cold wave” in the air, the ice cover
expands at a rate corresponding to its rate of contraction, so that a
strong pressure is exerted against the shore (Fig. 469). Sliding up
the sloping surface of the cut and built terrace, the force of this
shove may be deflected upward against the cliff, and if this is of
loose materials, the effect may be to ram bowlders into the bank, to
push up ramparts or ridges, to overturn trees, etc. (Fig. 470). In
marsh land the frozen surface layer may slide over its unfrozen base
and be forced up into broken folds (lower diagram of Figs. 469 and 470).
[Illustration:
FIG. 471.—Map of Lake Mendota at Madison, Wisconsin, showing the
position of the ridge which forms from ice expansion, and the ice
ramparts about the shores of the bays (based on Buckley’s map).]
In order that ice ramparts may be formed, it is necessary that
the winter climate of the district be severe and characterized by
alternating cold and warm waves, involving considerable range of air
temperature below the freezing point. If the lake is small, the push of
the ice will be through so small a distance as not to yield appreciable
ramparts. If, on the other hand, the lake is too large, the ice cover
is not rigid enough to transmit the push to the distant shore, but,
like a long beam employed in the same manner to transmit a compressive
stress, it is bent out of a straight line and later broken. Thus in a
broad lake, with the coming of a “warm wave”, the ice cover opens in a
crack from shore to shore and finds relief from the stress by pushing
up a ridge above the crack. On such lakes ice ramparts are found only
about the shores of bays whose expanse does not greatly exceed a mile
(Fig. 471).
When there is heavy snowfall, ice ramparts either do not form or are
of smaller dimensions, probably in part because the ice is blanketed
by the snow and so prevented from sudden elevation of temperature
during the “warm wave”, but even more because the ice cover is sensibly
bowed down under its load and so rendered incompetent to transmit the
developed stresses to the shores.
READING REFERENCES FOR CHAPTER XXX
Lake extinction by peat growth:
C. A. DAVIS. Peat, Essays on its Origin, Uses, and Distribution in
Michigan, Ann. Rept. Mich. Geol. Surv. for 1906, 1907, pp. 105-182;
Peat Deposits as Geological Records, 10th Rept. Mich. Acad. Sci.,
1908, pp. 107-112.
G. P. BURNS. Bog Studies. Ann Arbor, 1906, pp. 13.
Ice ramparts:
C. H. HITCHCOCK. Shore Ramparts in Vermont, Proc. Am. Assoc. Adv.
Sci., vol. 13, 1869, pp. 335-337.
G. K. GILBERT. Lake Bonneville, Mon. 1, U. S. Geol. Surv., 1890, pp.
71-72.
E. R. BUCKLEY. Ice Ramparts, Trans. Wis. Acad. Sci., etc., vol. 13,
1900, pp. 141-162, pls. 1-18.
WILLIAM H. HOBBS. Requisite Conditions for the Formation of Ice
Ramparts, Jour. Geol., vol. 19, 1911, pp. 157-160.